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#### Energy Feedback from X-Ray Binaries in the Early Universe

Refereed Publication
Fragos, T.; Lehmer, B. D.; Naoz, S.; Zezas, A.; Basu-Zych, A.
The Astrophysical Journal Letters, Volume 776, Issue 2, article id. L31, 6 pp. (2013)

X-ray photons, because of their long mean-free paths, can easily escape the galactic environments where they are produced, and interact at long distances with the intergalactic medium, potentially having a significant contribution to the heating and reionization of the early universe. The two most important sources of X-ray photons in the universe are active galactic nuclei (AGNs) and X-ray binaries (XRBs). In this Letter we use results from detailed, large scale population synthesis simulations to study the energy feedback of XRBs, from the first galaxies (z ~ 20) until today. We estimate that X-ray emission from XRBs dominates over AGN at z >~ 6-8. The shape of the spectral energy distribution of the emission from XRBs shows little change with redshift, in contrast to its normalization which evolves by ~4 orders of magnitude, primarily due to the evolution of the cosmic star-formation rate. However, the metallicity and the mean stellar age of a given XRB population affect significantly its X-ray output. Specifically, the X-ray luminosity from high-mass XRBs per unit of star-formation rate varies an order of magnitude going from solar metallicity to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of stellar mass peaks at an age of ~300 Myr and then decreases gradually at later times, showing little variation for mean stellar ages >~ 3 Gyr. Finally, we provide analytical and tabulated prescriptions for the energy output of XRBs, that can be directly incorporated in cosmological simulations.

#### The Origin of Black Hole Spin in Galactic Low-Mass X-ray Binaries

Refereed Publication
Fragos, Tassos; McClintock, Jeffrey E.
eprint arXiv:1408.2661

Galactic field black hole (BH) low-mass X-ray binaries (LMXBs) are believed to form in situ via the evolution of isolated binaries. In the standard formation channel, these systems survived a common envelope phase, after which the remaining helium core of the primary star and the subsequently formed BH are not expected to be highly spinning. However, the measured spins of BHs in LMXBs cover the whole range of spin parameters from a*~0 to a*~1. We propose here that the BH spin in LMXBs is acquired through accretion onto the BH during their long and stable accretion phase. In order to test this hypothesis, we calculated extensive grids of binary evolutionary sequences in which a BH accretes matter from a close companion. For each evolutionary sequence, we examined whether, at any point in time, the calculated binary properties are in agreement with their observationally inferred counterparts of 16 observed Galactic LMXBs. Mass-transfer sequences that simultaneously satisfy all observational constraints represent possible progenitors of the considered LMXBs and thus give estimates of the mass that the BH has accreted since the onset of Roche-Lobe overflow. We find that in all Galactic LMXBs with measured BH spin, the origin of the spin can be accounted by the accreted matter, and we make predictions about the maximum BH spin in LMXBs where no measurement is yet available. Furthermore, based on this hypothesis, we derive limits on the maximum spin that a BH can have depending on the orbital period of the binary it resides in and the effective temperature of the companion star. Finally we discuss the implication that our findings have on the BH birth-mass distribution, which is shifted by ~1.3 Msolar towards lower masses, compared to the currently observed one.

#### Understanding Compact Object Formation and Natal Kicks. IV. The Case of IC 10 X-1

Refereed Publication
Wong, Tsing-Wai; Valsecchi, Francesca; Ansari, Asna; Fragos, Tassos; Glebbeek, Evert; Kalogera, Vassiliki; McClintock, Jeffrey
The Astrophysical Journal, Volume 790, Issue 2, article id. 119, 14 pp. (2014)

The extragalactic X-ray binary IC 10 X-1 has attracted attention as it is possibly the host of the most massive stellar-mass black-hole (BH) known to date. Here we consider all available observational constraints and construct its evolutionary history up to the instant just before the formation of the BH. Our analysis accounts for the simplest possible history, which includes three evolutionary phases: binary orbital dynamics at core collapse, common envelope (CE) evolution, and evolution of the BH-helium star binary progenitor of the observed system. We derive the complete set of constraints on the progenitor system at various evolutionary stages. Specifically, right before the core collapse event, we find the mass of the BH immediate progenitor to be >~ 31 M  (at 95% of confidence, same hereafter). The magnitude of the natal kick imparted to the BH is constrained to be <~ 130 km s-1. Furthermore, we find that the “enthalpy” formalism recently suggested by Ivanova & Chaichenets is able to explain the existence of IC 10 X-1 without the need to invoke unreasonably high CE efficiencies. With this physically motivated formalism, we find that the CE efficiency required to explain the system is in the range of ~= 0.6-1.

#### The X-Ray Luminosity Functions of Field Low-mass X-Ray Binaries in Early-type Galaxies: Evidence for a Stellar Age Dependence

Refereed Publication
Lehmer, B. D.; Berkeley, M.; Zezas, A.; Alexander, D. M.; Basu-Zych, A.; Bauer, F. E.; Brandt, W. N.; Fragos, T.; Hornschemeier, A. E.; Kalogera, V.; Ptak, A.; Sivakoff, G. R.; Tzanavaris, P.; Yukita, M.
The Astrophysical Journal, Volume 789, Issue 1, article id. 52, 15 pp. (2014)

We present direct constraints on how the formation of low-mass X-ray binary (LMXB) populations in galactic fields depends on stellar age. In this pilot study, we utilize Chandra and Hubble Space Telescope (HST) data to detect and characterize the X-ray point source populations of three nearby early-type galaxies: NGC 3115, 3379, and 3384. The luminosity-weighted stellar ages of our sample span ≈3-10 Gyr. X-ray binary population synthesis models predict that the field LMXBs associated with younger stellar populations should be more numerous and luminous per unit stellar mass than older populations due to the evolution of LMXB donor star masses. Crucially, the combination of deep Chandra and HST observations allows us to test directly this prediction by identifying and removing counterparts to X-ray point sources that are unrelated to the field LMXB populations, including LMXBs that are formed dynamically in globular clusters, Galactic stars, and background active galactic nuclei/galaxies. We find that the “young” early-type galaxy NGC 3384 (≈2-5 Gyr) has an excess of luminous field LMXBs (L X >~ (5-10) × 1037 erg s-1) per unit K-band luminosity (LK ; a proxy for stellar mass) than the “old” early-type galaxies NGC 3115 and 3379 (≈8-10 Gyr), which results in a factor of ≈2-3 excess of L X/LK for NGC 3384. This result is consistent with the X-ray binary population synthesis model predictions; however, our small galaxy sample size does not allow us to draw definitive conclusions on the evolution field LMXBs in general. We discuss how future surveys of larger galaxy samples that combine deep Chandra and HST data could provide a powerful new benchmark for calibrating X-ray binary population synthesis models.

#### The Two-dimensional Spatial Distributions of the Globular Clusters and Low-mass X-Ray Binaries of NGC 4649

Refereed Publication
D'Abrusco, R.; Fabbiano, G.; Mineo, S.; Strader, J.; Fragos, T.; Kim, D.-W.; Luo, B.; Zezas, A.
The Astrophysical Journal, Volume 783, Issue 1, article id. 18, 14 pp. (2014)

We report significant anisotropies in the projected two-dimensional (2D) spatial distributions of globular clusters (GCs) of the giant Virgo elliptical galaxy NGC 4649 (M60). Similar features are found in the 2D distribution of low-mass X-ray binaries (LMXBs), both associated with GCs and in the stellar field. Deviations from azimuthal symmetry suggest an arc-like excess of GCs extending north at 4-15 kpc galactocentric radii in the eastern side of major axis of NGC 4649. This feature is more prominent for red GCs, but still persists in the 2D distribution of blue GCs. High- and low-luminosity GCs also show some segregation along this arc, with high-luminosity GCs preferentially located in the southern end and low-luminosity GCs in the northern section of the arc. GC-LMXBs follow the anisotropy of red GCs, where most of them reside; however, a significant overdensity of (high-luminosity) field LMXBs is present to the south of the GC arc. These results suggest that NGC 4649 has experienced mergers and/or multiple accretions of less massive satellite galaxies during its evolution, of which the GCs in the arc may be the fossil remnant. We speculate that the observed anisotropy in the field LMXB spatial distribution indicates that these X-ray binaries may be the remnants of a star formation event connected with the merger, or maybe be ejected from the parent red GCs, if the bulk motion of these clusters is significantly affected by dynamical friction. We also detect a luminosity enhancement in the X-ray source population of the companion spiral galaxy NGC 4647. We suggest that these may be younger high mass X-ray binaries formed as a result of the tidal interaction of this galaxy with NGC 4649.

#### The Radial Distribution of X-Ray Binaries and Globular Clusters in NGC 4649 and their Relation with the Local Stellar Mass Density

Refereed Publication
Mineo, S.; Fabbiano, G.; D'Abrusco, R.; Fragos, T.; Kim, D.-W.; Strader, J.; Brodie, J. P.; Gallagher, J. S.; Zezas, A.; Luo, B.
The Astrophysical Journal, Volume 780, Issue 2, article id. 132, 13 pp. (2014)

We investigate the radial distribution of the low-mass X-ray binary (LMXB) population in the elliptical galaxy NGC 4649, using Chandra and Hubble data to separate the field and globular cluster (GC) populations. GCs with LMXBs have the same radial distribution as the parent red and blue GCs. The radial profile of field LMXBs follows the V-band profile within the D25 of NGC 4649. Using the spatial information provided by our data, we find that the global galaxy-wide relations among cumulative number and luminosity of LMXBs and the integrated stellar mass hold on local scales within D25. An excess of field LMXBs with respect to the V-band light is observed in the galaxy’s outskirts, which may be partially due to unidentified GC sources or to a rejuvenated field LMXB population caused by past merging interactions.

#### Evidence for Elevated X-Ray Emission in Local Lyman Break Galaxy Analogs

Refereed Publication
Basu-Zych, Antara R.; Lehmer, Bret D.; Hornschemeier, Ann E.; Gonçalves, Thiago S.; Fragos, Tassos; Heckman, Timothy M.; Overzier, Roderik A.; Ptak, Andrew F.; Schiminovich, David
The Astrophysical Journal, Volume 774, Issue 2, article id. 152, 12 pp. (2013)

Our knowledge of how X-ray emission scales with star formation at the earliest times in the universe relies on studies of very distant Lyman break galaxies (LBGs). In this paper, we study the relationship between the 2-10 keV X-ray luminosity (L X), assumed to originate from X-ray binaries (XRBs), and star formation rate (SFR) in ultraviolet (UV) selected z < 0.1 Lyman break analogs (LBAs). We present Chandra observations for four new Galaxy Evolution Explorer selected LBAs. Including previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate L X/SFR ratios that are elevated by ~1.5σ compared to local galaxies, similar to the ratios found for stacked LBGs in the early universe (z > 2). Unlike some of the composite LBAs studied previously, we show that these LBAs are unlikely to harbor active galactic nuclei, based on their optical and X-ray spectra and the spatial distribution of the X-rays in three spatially extended cases. Instead, we expect that high-mass X-ray binaries (HMXBs) dominate the X-ray emission in these galaxies, based on their high specific SFRs (sSFRs ≡ SFR/M sstarf >= 10-9 yr-1), which suggest the prevalence of young stellar populations. Since both UV-selected populations (LBGs and LBAs) have lower dust attenuations and metallicities compared to similar samples of more typical local galaxies, we investigate the effects of dust extinction and metallicity on the L X/SFR for the broader population of galaxies with high sSFRs (>10-10 yr-1). The estimated dust extinctions (corresponding to column densities of N H < 1022 cm-2) are expected to have insignificant effects on observed L X/SFR ratio for the majority of galaxy samples. We find that the observed relationship between LX/SFR and metallicity appears consistent with theoretical expectations from XRB population synthesis models. Therefore, we conclude that lower metallicities, related to more luminous HMXBs such as ultraluminous X-ray sources, drive the elevated L X/SFR observed in our sample of z < 0.1 LBAs. The relatively metal-poor, active mode of star formation in LBAs and distant z > 2 LBGs may yield higher total HMXB luminosity than found in typical galaxies in the local universe.

#### Modeling X-Ray Binary Evolution in Normal Galaxies: Insights from SINGS

Refereed Publication
Tzanavaris, P.; Fragos, T.; Tremmel, M.; Jenkins, L.; Zezas, A.; Lehmer, B. D.; Hornschemeier, A.; Kalogera, V.; Ptak, A.; Basu-Zych, A. R.
The Astrophysical Journal, Volume 774, Issue 2, article id. 136, 17 pp. (2013)

We present the largest-scale comparison to date between observed extragalactic X-ray binary (XRB) populations and theoretical models of their production. We construct observational X-ray luminosity functions (oXLFs) using Chandra observations of 12 late-type galaxies from the Spitzer Infrared Nearby Galaxy Survey. For each galaxy, we obtain theoretical XLFs (tXLFs) by combining XRB synthetic models, constructed with the population synthesis code StarTrack, with observational star formation histories (SFHs). We identify highest-likelihood models both for individual galaxies and globally, averaged over the full galaxy sample. Individual tXLFs successfully reproduce about half of the oXLFs, but for some galaxies we are unable to find underlying source populations, indicating that galaxy SFHs and metallicities are not well matched and/or that XRB modeling requires calibration on larger observational samples. Given these limitations, we find that the best models are consistent with a product of common envelope ejection efficiency and central donor concentration ~= 0.1, and a 50% uniform-50% “twins” initial mass-ratio distribution. We present and discuss constituent subpopulations of tXLFs according to donor, accretor, and stellar population characteristics. The galaxy-wide X-ray luminosity due to low-mass and high-mass XRBs, estimated via our best global model tXLF, follows the general trend expected from the LX -star formation rate and LX -stellar mass relations of Lehmer et al. Our best models are also in agreement with modeling of the evolution of both XRBs over cosmic time and of the galaxy X-ray luminosity with redshift.

#### The Two-dimensional Projected Spatial Distribution of Globular Clusters. I. Method and Application to NGC 4261

Refereed Publication
D'Abrusco, R.; Fabbiano, G.; Strader, J.; Zezas, A.; Mineo, S.; Fragos, T.; Bonfini, P.; Luo, B.; Kim, D.-W.; King, A.
The Astrophysical Journal, Volume 773, Issue 2, article id. 87, 12 pp. (2013)

We present a new method for the determination of the two-dimensional (2D) projected spatial distribution of globular clusters (GCs) in external galaxies. This method is based on the K-Nearest Neighbor density estimator of Dressler, complemented by Monte-Carlo simulations to establish the statistical significance of the results. We apply this method to NGC 4261, a “test galaxy” where significant 2D anisotropy in the GC distribution has been reported. We confirm that the 2D distribution of GC is not azimuthally isotropic. Moreover, we demonstrate that the 2D distribution departures from the average GC radial distribution results in highly significant spiral-like or broken shell features. Overall, the same perturbations are found in “red” and “blue” GCs, but with some differences. In particular, we observe a central feature, roughly aligned with the minor axis of NGC 4261, composed of red and most luminous GCs. Blue and fainter GCs are more frequent at large radial distances and follow the spiral-like features of the overall density structure. These results suggest a complex merging history for NGC 4261.

#### Modeling the Redshift Evolution of the Normal Galaxy X-Ray Luminosity Function

Refereed Publication
Tremmel, M.; Fragos, T.; Lehmer, B. D.; Tzanavaris, P.; Belczynski, K.; Kalogera, V.; Basu-Zych, A. R.; Farr, W. M.; Hornschemeier, A.; Jenkins, L.; Ptak, A.; Zezas, A.
The Astrophysical Journal, Volume 766, Issue 1, article id. 19, 13 pp. (2013)

Emission from X-ray binaries (XRBs) is a major component of the total X-ray luminosity of normal galaxies, so X-ray studies of high-redshift galaxies allow us to probe the formation and evolution of XRBs on very long timescales (~10 Gyr). In this paper, we present results from large-scale population synthesis models of binary populations in galaxies from z = 0 to ~20. We use as input into our modeling the Millennium II Cosmological Simulation and the updated semi-analytic galaxy catalog by Guo et al. to self-consistently account for the star formation history (SFH) and metallicity evolution of each galaxy. We run a grid of 192 models, varying all the parameters known from previous studies to affect the evolution of XRBs. We use our models and observationally derived prescriptions for hot gas emission to create theoretical galaxy X-ray luminosity functions (XLFs) for several redshift bins. Models with low common envelope efficiencies, a 50% twins mass ratio distribution, a steeper initial mass function exponent, and high stellar wind mass-loss rates best match observational results from Tzanavaris & Georgantopoulos, though they significantly underproduce bright early-type and very bright (Lx > 1041) late-type galaxies. These discrepancies are likely caused by uncertainties in hot gas emission and SFHs, active galactic nucleus contamination, and a lack of dynamically formed low-mass XRBs. In our highest likelihood models, we find that hot gas emission dominates the emission for most bright galaxies. We also find that the evolution of the normal galaxy X-ray luminosity density out to z = 4 is driven largely by XRBs in galaxies with X-ray luminosities between 1040 and 1041 erg s-1.

#### Deep Chandra Monitoring Observations of NGC 4649. I. Catalog of Source Properties

Refereed Publication
Luo, B.; Fabbiano, G.; Strader, J.; Kim, D.-W.; Brodie, J. P.; Fragos, T.; Gallagher, J. S.; King, A.; Zezas, A.
The Astrophysical Journal Supplement, Volume 204, Issue 2, article id. 14, 15 pp. (2013)

We present the X-ray source catalog for the Chandra monitoring observations of the elliptical galaxy, NGC 4649. The galaxy has been observed with Chandra ACIS-S3 in six separate pointings, reaching a total exposure of 299 ks. There are 501 X-ray sources detected in the 0.3-8.0 keV band in the merged observation or in one of the six individual observations; 399 sources are located within the D25 ellipse. The observed 0.3-8.0 keV luminosities of these 501 sources range from 9.3 × 1036 erg s-1 to 5.4 × 1039 erg s-1. The 90% detection completeness limit within the D25 ellipse is 5.5 × 1037 erg s-1. Based on the surface density of background active galactic nuclei (AGNs) and detection completeness, we expect ≈45 background AGNs among the catalog sources (≈15 within the D25 ellipse). There are nine sources with luminosities greater than 1039 erg s-1, which are candidates for ultraluminous X-ray sources. The nuclear source of NGC 4649 is a low-luminosity AGN, with an intrinsic 2.0-8.0 keV X-ray luminosity of 1.5 × 1038 erg s-1. The X-ray colors suggest that the majority of the catalog sources are low-mass X-ray binaries (LMXBs). We find that 164 of the 501 X-ray sources show long-term variability, indicating that they are accreting compact objects. We discover four transient candidates and another four potential transients. We also identify 173 X-ray sources (141 within the D25 ellipse) that are associated with globular clusters (GCs) based on Hubble Space Telescope and ground-based data; these LMXBs tend to be hosted by red GCs. Although NGC 4649 has a much larger population of X-ray sources than the structurally similar early-type galaxies, NGC 3379 and NGC 4278, the X-ray source properties are comparable in all three systems.

#### Metallicity Effect on Low-mass X-Ray Binary Formation in Globular Clusters

Refereed Publication
Kim, D.-W.; Fabbiano, G.; Ivanova, N.; Fragos, T.; Jordán, A.; Sivakoff, G. R.; Voss, R.
The Astrophysical Journal, Volume 764, Issue 1, article id. 98, 8 pp. (2013)

We present comprehensive observational results of the metallicity effect on the fraction of globular clusters (GCs) that contain low-mass X-ray binaries (LMXB), by utilizing all available data obtained with Chandra for LMXBs and Hubble Space Telescope Advanced Camera for Surveys (ACS) for GCs. Our primary sample consists of old elliptical galaxies selected from the ACS Virgo and Fornax surveys. To improve statistics at both the lowest and highest X-ray luminosity, we also use previously reported results from other galaxies. It is well known that the fraction of GCs hosting LMXBs is considerably higher in red, metal-rich, GCs than in blue, metal-poor GCs. In this paper, we test whether this metallicity effect is X-ray luminosity-dependent and find that the effect holds uniformly in a wide luminosity range. This result is statistically significant (at >=3σ) in LMXBs with luminosities in the range L X = 2 × 1037 to 5 × 1038 erg s-1, where the ratio of GC-LMXB fractions in metal-rich to metal-poor GCs is R = 3.4 ± 0.5. A similar ratio is also found at lower (down to 1036 erg s-1) and higher luminosities (up to the ULX regime), but with less significance (~2σ confidence). Because different types of LMXBs dominate in different luminosities, our finding requires a new explanation for the metallicity effect in dynamically-formed LMXBs. We confirm that the metallicity effect is not affected by other factors such as stellar age, GC mass, stellar encounter rate, and galacto-centric distance.

#### X-Ray Binary Evolution Across Cosmic Time

Refereed Publication
Fragos, T.; Lehmer, B.; Tremmel, M.; Tzanavaris, P.; Basu-Zych, A.; Belczynski, K.; Hornschemeier, A.; Jenkins, L.; Kalogera, V.; Ptak, A.; Zezas, A.
The Astrophysical Journal, Volume 764, Issue 1, article id. 41, 13 pp. (2013)

High-redshift galaxies permit the study of the formation and evolution of X-ray binary (XRB) populations on cosmological timescales, probing a wide range of metallicities and star formation rates (SFRs). In this paper, we present results from a large-scale population synthesis study that models the XRB populations from the first galaxies of the universe until today. We use as input to our modeling the Millennium II cosmological simulation and the updated semi-analytic galaxy catalog by Guo et al. to self-consistently account for the star formation history and metallicity evolution of the universe. Our modeling, which is constrained by the observed X-ray properties of local galaxies, gives predictions about the global scaling of emission from XRB populations with properties such as SFR and stellar mass, and the evolution of these relations with redshift. Our simulations show that the X-ray luminosity density (X-ray luminosity per unit volume) from XRBs in our universe today is dominated by low-mass XRBs, and it is only at z >~ 2.5 that high-mass XRBs become dominant. We also find that there is a delay of ~1.1 Gyr between the peak of X-ray emissivity from low-mass XRBs (at z ~ 2.1) and the peak of SFR density (at z ~ 3.1). The peak of the X-ray luminosity from high-mass XRBs (at z ~ 3.9) happens ~0.8 Gyr before the peak of the SFR density, which is due to the metallicity evolution of the universe.

#### The X-Ray Star Formation Story as Told by Lyman Break Galaxies in the 4 Ms CDF-S

Refereed Publication
Basu-Zych, Antara R.; Lehmer, Bret D.; Hornschemeier, Ann E.; Bouwens, Rychard J.; Fragos, Tassos; Oesch, Pascal A.; Belczynski, Krzysztof; Brandt, W. N.; Kalogera, Vassiliki; Luo, Bin; Miller, Neal; Mullaney, James R.; Tzanavaris, Panayiotis; Xue, Yongquan; Zezas, Andreas
The Astrophysical Journal, Volume 762, Issue 1, article id. 45, 15 pp. (2013)

We present results from deep X-ray stacking of >4000 high-redshift galaxies from z ≈ 1 to 8 using the 4 Ms Chandra Deep Field-South data, the deepest X-ray survey of the extragalactic sky to date. The galaxy samples were selected using the Lyman break technique based primarily on recent Hubble Space Telescope ACS and WFC3 observations. Based on such high specific star formation rates (sSFRs): log SFR/M * > -8.7, we expect that the observed properties of these Lyman break galaxies (LBGs) are dominated by young stellar populations. The X-ray emission in LBGs, eliminating individually detected X-ray sources (potential active galactic nucleus), is expected to be powered by X-ray binaries and hot gas. We find, for the first time, evidence of evolution in the X-ray/SFR relation. Based on X-ray stacking analyses for z < 4 LBGs (covering ~90% of the universe’s history), we find that the 2-10 keV X-ray luminosity evolves weakly with redshift (z) and SFR as log L X = 0.93log (1 + z) + 0.65log SFR + 39.80. By comparing our observations with sophisticated X-ray binary population synthesis models, we interpret that the redshift evolution of L X/SFR is driven by metallicity evolution in high mass X-ray binaries, likely the dominant population in these high sSFR galaxies. We also compare these models with our observations of X-ray luminosity density (total 2-10 keV luminosity per Mpc3) and find excellent agreement. While there are no significant stacked detections at z >~ 5, we use our upper limits from 5 <~ z <~ 8 LBGs to constrain the supermassive black hole accretion history of the universe around the epoch of reionization.

#### On the Origin of the Metallicity Dependence in Dynamically formed Extragalactic Low-mass X-Ray Binaries

Refereed Publication
Ivanova, N.; Fragos, T.; Kim, D.-W.; Fabbiano, G.; Avendano Nandez, J. L.; Lombardi, J. C.; Sivakoff, G. R.; Voss, R.; Jordán, A.
The Astrophysical Journal Letters, Volume 760, Issue 2, article id. L24, 4 pp. (2012)

Globular clusters (GCs) effectively produce dynamically formed low-mass X-ray binaries (LMXBs). Observers detect ~100 times more LMXBs per stellar mass in GCs compared to stars in the fields of galaxies. Observationally, metal-rich GCs are about three times more likely to contain an X-ray source than their metal-poor counterparts. Recent observations have shown that this ratio holds in extragalactic GCs for all bright X-ray sources with L X between 2 × 1037 and 5 × 1038 erg s-1. In this Letter, we propose that the observed metallicity dependence of LMXBs in extragalactic GCs can be explained by the differences in the number densities and average masses of red giants in populations of different metallicities. Red giants serve as seeds for the dynamical production of bright LMXBs via two channels – binary exchanges and physical collisions – and the increase of the number densities and masses of red giants boost LMXB production, leading to the observed difference. We also discuss a possible effect of the age difference in stellar populations of different metallicities.

#### A Variable Ultraluminous X-Ray Source in a Globular Cluster in NGC 4649

Refereed Publication
Roberts, T. P.; Fabbiano, G.; Luo, B.; Kim, D.-W.; Strader, J.; Middleton, M. J.; Brodie, J. P.; Fragos, T.; Gallagher, J. S.; Kalogera, V.; King, A. R.; Zezas, A.
The Astrophysical Journal, Volume 760, Issue 2, article id. 135, 7 pp. (2012)

We report the discovery of a new ultraluminous X-ray source associated with a globular cluster in the elliptical galaxy NGC 4649. The X-ray source was initially detected with a luminosity below 5 × 1038 erg s-1, but in subsequent observations 7 and 11 years later it had brightened substantially to 2-3 × 1039 erg s-1. Over the course of six separate observations it displayed significant spectral variability, in both continuum slope and absorption column. Short-term variability in the X-ray flux was also present in at least one observation. The properties of this object appear consistent with a stellar-mass black hole accreting at super-Eddington rates (i.e., in the ultraluminous accretion state), although a highly super-Eddington neutron star cannot be excluded. The coincidence of an increase in absorption column with a possible enhancement in short-term variability in at least one observation is suggestive of a clumpy, radiatively driven wind crossing our line of sight to the object.

#### Deep Chandra Monitoring Observations of NGC 4649. II. Wide-field Hubble Space Telescope Imaging of the Globular Clusters

Refereed Publication
Strader, Jay; Fabbiano, Giuseppina; Luo, Bin; Kim, Dong-Woo; Brodie, Jean P.; Fragos, Tassos; Gallagher, John S.; Kalogera, Vassiliki; King, Andrew; Zezas, Andreas
The Astrophysical Journal, Volume 760, Issue 1, article id. 87, 7 pp. (2012)

We present g and z photometry and size estimates for globular clusters (GCs) in the massive Virgo elliptical NGC 4649 (M60) using a five-pointing Hubble Space Telescope/Advanced Camera for Surveys mosaic. The metal-poor GCs show a monotonic negative metallicity gradient of -0.43 ± 0.10 dex per dex in radius over the full radial range of the data, out to ~24 kpc. There is evidence for substantial color substructure among the metal-rich GCs. The metal-poor GCs have typical sizes ~0.4 pc larger than the metal-rich GCs out to large galactocentric distances (gsim 20 kpc), favoring an intrinsic explanation for the size difference rather than projection effects. There is no clear relation between half-light radius and galactocentric distance beyond ~15 kpc, suggesting that the sizes of GCs are not generically set by tidal limitation. Finally, we identify ~20 candidate ultracompact dwarfs that extend down to surprisingly faint absolute magnitudes (Mz ~ -8.5), and may bridge the gap between this class and “extended clusters” in the Local Group. Three of the brighter candidates have published radial velocities and can be confirmed as bona fide ultracompact dwarfs; follow-up spectroscopy will determine the nature of the remainder of the candidates.

#### The Spectral and Temporal Properties of Transient Sources in Early-type Galaxies

Refereed Publication
Brassington, N. J.; Fabbiano, G.; Zezas, A.; Kundu, A.; Kim, D.-W.; Fragos, T.; King, A. R.; Pellegrini, S.; Trinchieri, G.; Zepf, S.; Wright, N. J.
The Astrophysical Journal, Volume 755, Issue 2, article id. 162, 16 pp. (2012)

We report the spectral and temporal variability properties of 18 candidate transient (TC) and potential transient (PTC) sources detected in deep multi-epoch Chandra observation of the nearby elliptical galaxies, NGC 3379, NGC 4278, and NGC 4697. Only one source can be identified with a background counterpart, leaving 17 TCs + PTCs in the galaxies. Of these, 14 are in the galaxy field, supporting the theoretical picture that the majority of field X-ray binaries (XRBs) will exhibit transient accretion for >75% of their lifetime. Three sources are coincident with globular clusters, including two high-luminosity candidate black hole (BH) XRBs, with L X = 5.4 × 1038 erg s-1and L X = 2.8 × 1039 erg s-1, respectively. The spectra, luminosities, and temporal behavior of these 17 sources suggest that the transient population is heterogeneous, including neutron star (NS) and BH XRBs in both normal and high-rate accretion modes, and super soft sources containing white dwarf binaries. Our TC and PTC detections are noticeably fewer than the number expected from the population synthesis (PS) models of Fragos et al., tailored to our new Chandra pointings of NGC 4278. We attribute this discrepancy to the PS assumption that the transient population is composed of NS XRBs, as well as differences between the statistical analysis and error estimates used in the model and our observations.

#### Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291

Refereed Publication
Luo, B.; Fabbiano, G.; Fragos, T.; Kim, D.-W.; Belczynski, K.; Brassington, N. J.; Pellegrini, S.; Tzanavaris, P.; Wang, Junfeng; Zezas, A.
The Astrophysical Journal, Volume 749, Issue 2, article id. 130, 14 pp. (2012)

We present Chandra studies of the X-ray binary (XRB) populations in the bulge and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available Chandra observations totaling an effective exposure of 179 ks. We report photometric properties of these sources in a point-source catalog. There are ≈40% of the bulge sources and ≈25% of the ring sources showing >3σ long-term variability in their X-ray count rate. The X-ray colors suggest that a significant fraction of the bulge (≈75%) and ring (≈65%) sources are likely low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate that it is a low-luminosity active galactic nucleus (AGN) with moderate obscuration; spectral variability is observed between individual observations. We construct 0.3-8.0 keV X-ray luminosity functions (XLFs) for the bulge and ring XRB populations, taking into account the detection incompleteness and background AGN contamination. We reach 90% completeness limits of ≈1.5 × 1037 and ≈2.2 × 1037 erg s-1 for the bulge and ring populations, respectively. Both XLFs can be fit with a broken power-law model, and the shapes are consistent with those expected for populations dominated by LMXBs. We perform detailed population synthesis modeling of the XRB populations in NGC 1291, which suggests that the observed combined XLF is dominated by an old LMXB population. We compare the bulge and ring XRB populations, and argue that the ring XRBs are associated with a younger stellar population than the bulge sources, based on the relative overdensity of X-ray sources in the ring, the generally harder X-ray color of the ring sources, the overabundance of luminous sources in the combined XLF, and the flatter shape of the ring XLF.

#### Understanding Compact Object Formation and Natal Kicks. III. The Case of Cygnus X-1

Refereed Publication
Wong, Tsing-Wai; Valsecchi, Francesca; Fragos, Tassos; Kalogera, Vassiliki
The Astrophysical Journal, Volume 747, Issue 2, article id. 111, 12 pp. (2012)

In recent years, accurate observational constraints have become available for an increasing number of Galactic X-ray binaries (XRBs). Together with proper-motion measurements, we could reconstruct the full evolutionary history of XRBs back to the time of compact object formation. In this paper, we present the first study of the persistent X-ray source Cygnus X-1 that takes into account all available observational constraints. Our analysis accounts for three evolutionary phases: orbital evolution and motion through the Galactic potential after the formation of a black hole (BH), and binary orbital dynamics at the time of core collapse. We find that the mass of the BH immediate progenitor is 15.0-20.0 M , and at the time of core collapse, the BH has potentially received a small kick velocity of <=77 km s-1 at 95% confidence. If the BH progenitor mass is less than ~17 M , a non-zero natal kick velocity is required to explain the currently observed properties of Cygnus X-1. Since the BH has only accreted mass from its companion’s stellar wind, the negligible amount of accreted mass does not explain the observationally inferred BH spin of a * > 0.95, and the origin of this extreme BH spin must be connected to the BH formation itself. Right after the BH formation, we find that the BH companion is a 19.8-22.6 M  main-sequence star, orbiting the BH at a period of 4.7-5.2 days. Furthermore, recent observations show that the BH companion is currently super-synchronized. This super-synchronism indicates that the strength of tides exerted on the BH companion should be weaker by a factor of at least two compared to the usually adopted strength.

#### Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system

Refereed Publication
Valsecchi, Francesca; Glebbeek, Evert; Farr, Will M.; Fragos, Tassos; Willems, Bart; Orosz, Jerome A.; Liu, Jifeng; Kalogera, Vassiliki
Nature, Volume 468, Issue 7320, pp. 77-79 (2010)

The X-ray source M33 X-7 in the nearby galaxy Messier 33 is among the most massive X-ray binary stellar systems known, hosting a rapidly spinning, 15.65Msolar black hole orbiting an underluminous, 70Msolar main-sequence companion in a slightly eccentric 3.45-day orbit (Msolar, solar mass). Although post-main-sequence mass transfer explains the masses and tight orbit, it leaves unexplained the observed X-ray luminosity, the star’s underluminosity, the black hole’s spin and the orbital eccentricity. A common envelope phase, or rotational mixing, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up. Here we report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85Msolar-99Msolar and a secondary body of 28Msolar-32Msolar, in a 2.8-3.1-d orbit, its observed properties can be consistently explained. In this model, the main-sequence primary transfers part of its envelope to the secondary and loses the rest in a wind; it ends its life as a ~16Msolar helium star with an iron-nickel core that collapses to a black hole (with or without an accompanying supernova). The release of binding energy, and possibly collapse asymmetries, kick’ the nascent black hole into an eccentric orbit. Wind accretion explains the X-ray luminosity, and the black-hole spin can be natal.

#### Black Hole Spin-Orbit Misalignment in Galactic X-ray Binaries

Refereed Publication
Fragos, T.; Tremmel, M.; Rantsiou, E.; Belczynski, K.
The Astrophysical Journal Letters, Volume 719, Issue 1, pp. L79-L83 (2010)

In black hole (BH) X-ray binaries (XRBs), a misalignment between the spin axis of the BH and the orbital angular momentum can occur during the supernova explosion that forms the compact object. In this Letter, we present population synthesis (PS) models of Galactic BH XRBs and study the probability density function of the misalignment angle and its dependence on our model parameters. In our modeling, we also take into account the evolution of the misalignment angle due to accretion of material onto the BH during the XRB phase. The major factor that sets the misalignment angle for XRBs is the natal kick that the BH may receive at its formation. However, large kicks tend to disrupt binaries, while small kicks allow the formation of XRBs and naturally select systems with small misalignment angles. Our calculations predict that the majority (>67%) of Galactic field BH XRBs have rather small (lsim10°) misalignment angles, while some systems may reach misalignment angles as high as ~90° and even higher. These results are robust among all PS models. The assumption of small misalignment angles is extensively used to observationally estimate BH spin magnitudes, and for the first time we are able to confirm this assumption using detailed PS calculations.

#### On the Formation of Ultraluminous X-ray Sources with Neutron Star Accretors: the Case of M82 X-2

Refereed Publication
Fragos, Tassos; Linden, Tim; Kalogera, Vicky; Sklias, Panos
The Astrophysical Journal Letters, Volume 802, Issue 1, article id. L5, 6 pp. (2015)

The recent discovery of a neutron star accretor in the ultra-luminous X-ray source M82 X-2 challenges our understanding of high-mass X-ray binary formation and evolution. By combining binary population synthesis and detailed mass-transfer models, however, we show that the binary parameters of M82 X-2 are not surprising provided non-conservative mass transfer is allowed. Specifically, the donor-mass lower limit and orbital period measured for M82 X-2 lie near the most probable values predicted by population synthesis models, and systems such as M82 X-2 should be observed in approximately 13% of the galaxies with a star-formation history similar to M82. We conclude that the binary system that formed M82 X-2 is most likely younger than 50 Myr old and contain a donor star which had an initial mass of approximately 8-10 M$_\odot$, while the NS’s progenitor star had an initial mass in the $8-25\,\rm M_{\odot}$ range. The donor star still currently resides on the main sequence, and is capable of continued MT on the thermal timescale, while in the ultra-luminous X-ray regime, for as long as 400,000 years.

#### Comparing GC and Field LMXBs in Elliptical Galaxies with Deep Chandra and Hubble Data

Refereed Publication
Kim, D.-W.; Fabbiano, G.; Brassington, N. J.; Fragos, T.; Kalogera, V.; Zezas, A.; Jordán, A.; Sivakoff, G. R.; Kundu, A.; Zepf, S. E.; Angelini, L.; Davies, R. L.; Gallagher, J. S.; Juett, A. M.; King, A. R.; Pellegrini, S.; Sarazin, C. L.; Trinchieri, G.
The Astrophysical Journal, Volume 703, Issue 1, article id. 829-844, pp. (2009)

We present a statistical study of the low-mass X-ray binary (LMXB) populations of three nearby, old elliptical galaxies: NGC 3379, NGC 4278, and NGC 4697. With a cumulative ~1 Ms Chandra ACIS observing time, we detect 90-170 LMXBs within the D25 ellipse of each galaxy. Cross-correlating Chandra X-ray sources and HST optical sources, we identify 75 globular cluster (GC) LMXBs and 112 field LMXBs with LX > 1036 erg s-1(detections of these populations are 90% complete down to luminosities in the range of 6 × 1036 to 1.5 × 1037 erg s-1). At the higher luminosities explored in previous studies, the statistics of this sample are consistent with the properties of GC-LMXBs reported in the literature. In the low-luminosity range allowed by our deeper data (LX < 5 × 1037 erg s-1), we find a significant relative lack of GC-LMXBs, when compared with field sources. Using the co-added sample from the three galaxies, we find that the incompleteness-corrected X-ray luminosity functions (XLFs) of GC and field LMXBs differ at ~4σ significance at LX < 5 × 1037 erg s-1. As previously reported, these XLFs are consistent at higher luminosities. The presently available theoretical models for LMXB formation and evolution in clusters are not sophisticated enough to provide a definite explanation for the shape of the observed GC-LMXB XLF. Our observations may indicate a potential predominance of GC-LMXBs with donors evolved beyond the main sequence, when compared to current models, but their efficient formation requires relatively high initial binary fractions in clusters. The field LMXB XLF can be fitted with either a single power-law model plus a localized excess at a luminosity of (5-6) × 1037 erg s-1, or a broken power law with a similar low-luminosity break. This XLF may be explained with NS-red-giant LMXBs, contributing to ~15% of total LMXBs population at ~5 × 1037 erg s-1. The difference in the GC and field XLFs is consistent with different origins and/or evolutionary paths between the two LMXB populations, although a fraction of the field sources are likely to have originated in GCs.

#### Transient Low-mass X-ray Binary Populations in Elliptical Galaxies NGC 3379 and NGC 4278

Refereed Publication
Fragos, T.; Kalogera, V.; Willems, B.; Belczynski, K.; Fabbiano, G.; Brassington, N. J.; Kim, D.-W.; Angelini, L.; Davies, R. L.; Gallagher, J. S.; King, A. R.; Pellegrini, S.; Trinchieri, G.; Zepf, S. E.; Zezas, A.
The Astrophysical Journal Letters, Volume 702, Issue 2, pp. L143-L147 (2009)

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration, and recurrence time. We apply this prescription to the population synthesis models of field LMXBs presented by Fragos et al., and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources. We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Finally, our comparison suggests that transient LMXBs are very rare in globular clusters (GCs), and thus the number of identified transient LMXBs may be used as a tracer of the relative contribution of field and GC LMXB populations.

#### Understanding Compact Object Formation and Natal Kicks. II. The Case of XTE J1118 + 480

Refereed Publication
Fragos, T.; Willems, B.; Kalogera, V.; Ivanova, N.; Rockefeller, G.; Fryer, C. L.; Young, P. A.
The Astrophysical Journal, Volume 697, Issue 2, pp. 1057-1070 (2009)

In recent years, an increasing number of proper motions have been measured for Galactic X-ray binaries. When supplemented with accurate determinations of the component masses, orbital period, and donor effective temperature, these kinematical constraints harbor a wealth of information on the system’s past evolution. Here, we consider all this available information to reconstruct the full evolutionary history of the black hole X-ray binary XTE J1118 + 480, assuming that the system originated in the Galactic disk and the donor has solar metallicity. This analysis accounts for four evolutionary phases: mass transfer through the ongoing X-ray phase, tidal evolution before the onset of Roche lobe overflow, motion through the Galactic potential after the formation of the black hole, and binary orbital dynamics due to explosive mass loss and possibly a black hole natal kick at the time of core collapse. We find that right after black hole formation, the system consists of a sime6.0-10.0 M sun black hole and a sime1.0-1.6 M sun main-sequence star. We also find that that an asymmetric natal kick is not only plausible but required for the formation of this system, and derive a lower and upper limit on the black hole natal kick velocity magnitude of 80 km s^{-1} and 310 km s^{-1}, respectively.

#### A multiphysics and multiscale software environment for modeling astrophysical systems

Refereed Publication
Portegies Zwart, Simon; McMillan, Steve; Harfst, Stefan; Groen, Derek; Fujii, Michiko; Nualláin, Breanndán Ó.; Glebbeek, Evert; Heggie, Douglas; Lombardi, James; Hut, Piet; Angelou, Vangelis; Banerjee, Sambaran; Belkus, Houria; Fragos, Tassos; Fregeau, John; Gaburov, Evghenii; Izzard, Rob; Jurić, Mario; Justham, Stephen; Sottoriva, Andrea; Teuben, Peter; van Bever, Joris; Yaron, Ofer; Zemp, Marcel
New Astronomy, Volume 14, Issue 4, p. 369-378

We present MUSE, a software framework for combining existing computational tools for different astrophysical domains into a single multiphysics, multiscale application. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for studying generalized stellar systems. We have now reached a “Noah’s Ark” milestone, with (at least) two available numerical solvers for each domain. MUSE can treat multiscale and multiphysics systems in which the time- and size-scales are well separated, like simulating the evolution of planetary systems, small stellar associations, dense stellar clusters, galaxies and galactic nuclei. In this paper we describe three examples calculated using MUSE: the merger of two galaxies, the merger of two evolving stars, and a hybrid N-body simulation. In addition, we demonstrate an implementation of MUSE on a distributed computer which may also include special-purpose hardware, such as GRAPEs or GPUs, to accelerate computations. The current MUSE code base is publicly available as open source at http://muse.li.

#### Deep Chandra Monitoring Observations of NGC 4278: Catalog of Source Properties

Refereed Publication
Brassington, N. J.; Fabbiano, G.; Kim, D.-W.; Zezas, A.; Zepf, S.; Kundu, A.; Angelini, L.; Davies, R. L.; Gallagher, J.; Kalogera, V.; Fragos, T.; King, A. R.; Pellegrini, S.; Trinchieri, G.
The Astrophysical Journal Supplement, Volume 181, Issue 2, pp. 605-626 (2009)

We present the properties of the discrete X-ray sources detected in our monitoring program of the globular cluster (GC) rich elliptical galaxy, NGC 4278, observed with Chandra ACIS-S in six separate pointings, resulting in a co-added exposure of 458 ks. From this deep observation, 236 sources have been detected within the region overlapped by all observations, 180 of which lie within the D 25 ellipse of the galaxy. These 236 sources range in L X from 3.5 × 1036 erg s-1 (with 3σ upper limit <=1 × 1037 erg s-1) to ~2 × 1040 erg s-1, including the central nuclear source which has been classified as a LINER. From optical data, 39 X-ray sources have been determined to be coincident with a GC, these sources tend to have high X-ray luminosity, with 10 of these sources exhibiting L X > 1 × 1038 erg s-1. From X-ray source photometry, it has been determined that the majority of the 236 point sources that have well-constrained colors have values that are consistent with typical low-mass X-ray binary spectra, with 29 of these sources expected to be background objects from the log N-log S relation. There are 103 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Three of these sources have been identified as transient candidates, with a further three possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.

#### Deep Chandra Monitoring Observations of NGC 3379: Catalog of Source Properties

Refereed Publication
Brassington, N. J.; Fabbiano, G.; Kim, D.-W.; Zezas, A.; Zepf, S.; Kundu, A.; Angelini, L.; Davies, R. L.; Gallagher, J.; Kalogera, V.; Fragos, T.; King, A. R.; Pellegrini, S.; Trinchieri, G.
The Astrophysical Journal Supplement Series, Volume 179, Issue 1, pp. 142-165

We present the properties of the discrete X-ray sources detected in our monitoring program of the typical’ elliptical galaxy, NGC 3379, observed with Chandra ACIS-S in five separate pointings, resulting in a co-added exposure of 324 ks. From this deep observation, 132 sources have been detected within the region overlapped by all observations, 98 of which lie within the D25 ellipse of the galaxy. These 132 sources range in LX from 6×1035 erg s-1 (with 3 σ upper limit <=4×1036 erg s-1) to ~2×1039 erg s-1, including one source with LX>1×1039 erg s-1, which has been classified as a ULX. From optical data, 10 X-ray sources have been determined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, with three of these sources exhibiting LX>1×1038 erg s-1. From X-ray source photometry, it has been determined that the majority of the 132 sources that have well constrained colors, have values that are consistent with typical LMXB spectra. In addition to this, a subpopulation of 10 sources has been found to exhibit very hard spectra and it is expected that most of these sources are absorbed background AGN. There are 64 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Five of these sources have been identified as transient candidates, with a further 3 possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.

#### Models for Low-Mass X-Ray Binaries in the Elliptical Galaxies NGC 3379 and NGC 4278: Comparison with Observations

Refereed Publication
Fragos, T.; Kalogera, V.; Belczynski, K.; Fabbiano, G.; Kim, D.-W.; Brassington, N. J.; Angelini, L.; Davies, R. L.; Gallagher, J. S.; King, A. R.; Pellegrini, S.; Trinchieri, G.; Zepf, S. E.; Kundu, A.; Zezas, A.
The Astrophysical Journal, Volume 683, Issue 1, pp. 346-356

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXBs) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code, assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested and calibrated using detailed binary star calculations and incorporates all the important physical processes of binary evolution. The simulations are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLFs) of point sources in these galaxies. For the first time we explore the population XLF in luminosities below 1037 ergs s-1, as probed by the most recent observational results. We consider models for the formation and evolution of LMXBs in galactic fields with different CE efficiencies, stellar wind prescriptions, magnetic braking laws, and IMFs. We identify models that produce XLFs consistent with the observations both in shape and absolute normalization, suggesting that a primordial galactic field LMXB population can make a significant contribution to the total population of an elliptical galaxy. We also find that the treatment of the outburst luminosity of transient systems remains a crucial factor for the determination of the XLF, since the modeled populations are dominated by transient X-ray systems.

#### A Multiphysics and Multiscale Software Environment for Modeling Astrophysical Systems

Refereed Publication
Portegies Zwart, Simon; McMillan, Steve; O'Nualláin, Breanndán; Heggie, Douglas; Lombardi, James; Hut, Piet; Banerjee, Sambaran; Belkus, Houria; Fragos, Tassos; Fregeau, John; Fuji, Michiko; Gaburov, Evghenii; Glebbeek, Evert; Groen, Derek; Harfst, Stefan; Izzard, Rob; Jurić, Mario; Justham, Stephen; Teuben, Peter; van Bever, Joris; Yaron, Ofer; Zemp, Marcel
Lecture Notes in Computer Science, vol. 5102, p. 207-216

We present MUSE, a software framework for tying together existing computational tools for different astrophysical domains into a single multiphysics, multiscale workload. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly-coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for a generalized stellar systems workload. MUSE has now reached a “Noah’s Ark” milestone, with two available numerical solvers for each domain. MUSE can treat small stellar associations, galaxies and everything in between, including planetary systems, dense stellar clusters and galactic nuclei. Here we demonstrate an examples calculated with MUSE: the merger of two galaxies. In addition we demonstrate the working of MUSE on a distributed computer. The current MUSE code base is publicly available as open source at http://muse.li.

#### The Modulated Emission of the Ultraluminous X-Ray Source in NGC 3379

Refereed Publication
Fabbiano, G.; Kim, D.-W.; Fragos, T.; Kalogera, V.; King, A. R.; Angelini, L.; Davies, R. L.; Gallagher, J. S.; Pellegrini, S.; Trinchieri, G.; Zepf, S. E.; Zezas, A.
he Astrophysical Journal, Volume 650, Issue 2, pp. 879-884

We report recent Chandra observations of the ULX in the elliptical galaxy NGC 3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ~5 years ago, we measure a flux modulation with a period of ~12.6 hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC 3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient; however, historical X-ray sampling suggests that the current “on” phase has lasted ~10 yr. We discuss our results in terms of ADC and wind-feedback models. If the flux modulation is orbital, we can constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4 Msolar and 12.5-17 hr, respectively. The duration of the mass transfer phase so far is probably ~1 Gyr, and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor.

#### Formation and progenitor of PSR J0737-3039: New constraints on the supernova explosion forming pulsar B

Refereed Publication
Willems, B.; Kaplan, J.; Fragos, T.; Kalogera, V.; Belczynski, K.
Physical Review D, vol. 74, Issue 4, id. 043003

We investigate the formation of the double pulsar PSR J0737-3039 and examine its most likely progenitors, taking into account the most recent and all currently available observational constraints. We show that the most likely kick velocity and progenitor parameters depend strongly on the consideration of the full five-dimensional probability distribution function for the magnitude and direction of the kick velocity imparted to pulsar B at birth, the mass of pulsar B’s presupernova helium star progenitor, and the presupernova orbital separation rather than marginalized one- or two-dimensional distributions for the kick velocity and progenitor mass. The priors that enter the analysis are the age of the system, the minimum helium star mass required to form a neutron star, the transverse systemic velocity, and the treatment of the unknown radial velocity. Since the latter cannot be measured observationally, we adopt a statistical approach and use theoretical radial-velocity distributions obtained from population synthesis calculations for coalescing double neutron stars. We find that when the minimum presupernova helium star mass required for neutron star formation is assumed to be 2.1M, the most likely kick velocity ranges from 70kms-1 to 180kms-1. When, on the other hand, masses lower than 2.1M are allowed as neutron star progenitors, the most likely kick velocity can reach very low values (as low as a few kms-1), although the majority of the models still yield most likely kick velocities of 50kms-1 to 170kms-1. Hence, we agree with Piran and Shaviv [T. Piran and N. J. Shaviv, Phys. Rev. Lett. 94, 051102 (2005).PRLTAO0031-900710.1103/PhysRevLett.94.051102] that the observed system properties, including the low transverse systemic velocity, can indeed be compatible with low progenitor masses and low kick velocities. Equally important though, we show that this is not the only likely formation path of pulsar B, due to the role of different prior assumptions that are necessary in the analysis. Moreover, in contrast to earlier claims in the literature, we show that the proximity of the double pulsar to the Galactic plane and the small proper motion do not pose stringent constraints on the kick velocity and progenitor mass of pulsar B at all. Instead, the constraints imposed by the current orbital semimajor axis and eccentricity and the orbital dynamics of asymmetric supernova explosions turn out to be much more restrictive. We conclude that without further knowledge of the priors, the currently available observational constraints cannot be used to unambiguously favor a specific core-collapse and neutron star-formation mechanism. Both electron capture and neutrino-driven supernovae therefore remain viable formation mechanisms for pulsar B.

#### Constraining Population Synthesis Models via the Binary Neutron Star Population

Refereed Publication
O'Shaughnessy, R.; Kim, C.; Fragos, T.; Kalogera, V.; Belczynski, K.
The Astrophysical Journal, Volume 633, Issue 2, pp. 1076-1084

The observed sample of double neutron-star (NS-NS) binaries presents a challenge to population-synthesis models of compact object formation: the input model parameters must be carefully chosen so the results match (1) the observed star formation rate and (2) the formation rate of NS-NS binaries, which can be estimated from the observed sample and the selection effects related to the discoveries with radio pulsar surveys. In this paper, we select from an extremely broad family of possible population synthesis models those few (2%) that are consistent with the rate implications of the observed sample of NS-NS binaries. To further sharpen the constraints the observed NS-NS population places on our understanding of compact object formation processes, we separate the observed NS-NS population into two channels: (1) merging NS-NS binaries, which will inspiral and merge through the action of gravitational waves within 10 Gyr and (2) wide NS-NS binaries, consisting of all the rest. With the subset of astrophysically consistent models, we explore the implications for the rates at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS binaries merge through the emission of gravitational waves.

#### On the distribution of magnetic energy storage in solar active regions

Refereed Publication
Fragos, T.; Rantsiou, E.; Vlahos, L.
Astronomy and Astrophysics, v.420, p.719-728 (2004)

A two-dimensional probabilistic Cellular Automaton is used to model the appearance of active regions at the solar surface. We assume that two main competing processes control the magnetic field evolution at the solar surface (1) the magnetic field is locally enhanced by the flux emergence and/or the coalescence of emerged magnetic flux and (2) it is diminished by flux cancellation or diffusion. The flux emergence follows a basic percolation rule; it is more probable at the points were magnetic flux already exists. The magnetic field is also enhanced when magnetic fields of the same polarity collide. The flux cancellation is due either to the gradual diffusion of the magnetic field, when it is isolated, or to the partial release of energy when opposite magnetic field lines collide. The percolation model proposed in this article is capable of reproducing the statistical properties of the evolving active regions. The evolving simulated magnetograms, derived from our model, are used to estimate the 3-D magnetic fields above the photosphere using constant α force-free extrapolation techniques. Based on the above analysis we are able to estimate a variety of observed statistical characteristics, e.g. the size and flux distribution of the magnetic fields at the solar surface, the fractal dimension of the magnetic structures formed at the photosphere, the energy release frequency distribution, the waiting time distribution of the sporadic energy releases and the statistical properties of the steep horizontal magnetic field gradients in the extrapolated coronal magnetic field. Our main conclusion is that the photospheric driver plays a crucial role in the observed flare statistics, and the solar magnetograms, when interpreted properly, carry important statistical information for the solar coronal activity (coronal heating, flares, CME etc.).

#### Statistical Properties of the Energy Release in Emerging and Evolving Active Regions

Refereed Publication
Vlahos, Loukas; Fragos, Tassos; Isliker, Heinz; Georgoulis, Manolis
The Astrophysical Journal, Volume 575, Issue 2, pp. L87-L90

The formation and evolution of active regions are inherently complex phenomena. Magnetic fields generated at the base of the convection zone follow a chaotic evolution before reaching the solar surface. In this article, we use a two-dimensional probabilistic cellular automaton to model the statistical properties of the magnetic patterns formed on the solar surface and to estimate the magnetic energy released in the interaction of opposite polarities. We assume that newly emerged magnetic flux tubes stimulate the emergence of new magnetic flux in their neighborhood. The flux tubes move randomly on the surface of the Sun, and they cancel and release their magnetic energy when they collide with magnetic flux of opposite polarity, or diffuse into the “empty” photosphere. We assume that cancellation of magnetic flux in collisions causes “flares” and determine the released energy as the difference in the square of the magnetic field flux (E~B2). The statistics of the simulated flares follow a power-law distribution in energy, f(E)~E-a, where a=2.2+/-0.1. The size distribution function of the simulated active regions exhibits a power-law behavior with index k~1.93+/-0.08, and the fractal dimension of the magnetized areas on the simulated solar surface is close to DF~1.42+/-0.12. Both quantities, DF and k, are inside the range of the observed values.

#### XSINGS: probing X-ray binaries in galaxies . Chandra observations and supercomputer simulations

Conference Proceedings
Tzanavaris, P.; Fragos, T.; Tremmel, M.; Jenkins, L.; Zezas, A.; Lehmer, B. D.; Hornschemeier, A.; Kalogera, V.; Ptak, A.; Basu-Zych, A. R.
Memorie della Societa Astronomica Italiana, v.84, p.768 (2013)

We present the largest population synthesis study of X-ray binaries (XRBs) in nearby galaxies to date. We consider an extensive set of physically motivated models for formation and evolution of binaries with the population synthesis code StarTrack. We convolve them with star formation histories obtained with SED modelling \citep{noll2009} and construct theoretical X-ray binary luminosity functions (XLFs) for a subset of nearby galaxies in the Spitzer Infrared Nearby Galaxy Sample (SINGS). We also construct observational XRB-XLFs for these galaxies from XSINGS, the extension of SINGS in the X-rays with Chandra. Using a likelihood approach, we identify models that produce XLFs consistent with the observations, providing constraints for XRB formation and evolution.

#### An Evolutionary Model for the Massive Black Hole X-Ray Binary M33 X-7.

Conference Proceedings
Valsecchi, F.; Glebbeek, E.; Farr, W. M.; Fragos, T.; Willems, B.; Orosz, J. A.; Liu, J.; Kalogera, V.
Evolution of compact binaries. Proceedings of a workshop held at Hotel San Martín, Viña del Mar, Chile 6-11 May 2011. Edited by Linda Schmidtobreick, Matthias R. Schreiber, and Claus Tappert. ASP Conference Proceedings, Vol. 447. San Francisco, CA: Astronomical Society of the Pacific, 2011., p.271

Black hole (BH) X-ray binaries (XRBs) are X-ray bright binary systems comprising a BH accreting matter from a companion star. Unravelling the evolutionary history of observed XRBs can shed light on how BHs form, evolve and interact with their host environment, processes that yet lack a conclusive physical explanation. Our focus here lies on M33 X-7, which is among the heaviest BH XRBs known to date, hosting a 15.65 M BH orbiting a 70 M companion star in a 3.45 day orbit. The high masses of the binary components and the tight orbit present a challenge for typically invoked BH-XRBs formation channels. The picture is further complicated by the measured underluminosity of the stellar component. Here we propose the first evolutionary scenario for M33 X-7 that is consistent with the complete set of current observational constraints. According to our model, the progenitors of M33 X-7 are constrained to host primaries and secondaries between 85-99 M and 28-32 M,
respectively, and initial orbital periods between 2.8-3.1 days. In order to form a BH of 15.65 M, the BH progenitor transferred part of its envelope to the companion star and lost the rest in a strong stellar wind. During this dynamically stable mass transfer phase the companion accreted matter, to become the presently underluminous 70 M star.

#### Modeling X-ray Binary Populations in Elliptical Galaxies

Conference Proceedings
Fragos, T.; Kalogera, V.
INTERNATIONAL CONFERENCE ON BINARIES: In celebration of Ron Webbink's 65th Birthday. AIP Conference Proceedings, Volume 1314, pp. 326-331 (2010).

We present theoretical models for populations of low-mass X-ray binaries in two elliptical galaxies NGC3379 and NGC4278. The models are calculated with the recently updated StarTrack code [1], and are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLF) in these galaxies. For the first time we explore the population XLF down to luminosities of 3×1036 erg s-1, as probed by the most recent observational results [2]. We consider models for the formation and evolution of low-mass X-ray binaries (LMXB) in galactic fields with different common envelope efficiencies, stellar wind prescriptions and initial mass functions. We identify models that produce an XLF in excellent agreement with the observations both in shape and number of systems at a specific luminosity. We also find that the treatment of the outburst luminosity of transient systems remains a crucial parameter for the determination of the XLF as the modeled populations are dominated by transient X-ray systems. Finally, we propose a physically motivated and self-consistent prescription for the modeling of transient neutron star (NS) LMXB properties, such as duty cycle (DC), outburst duration and recurrence time. We compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC3379 and NGC4278, which revealed several transient sources [3, 4]. Our comparison suggests that transient LMXBs are very rare in globular clusters (GC), and thus the number of identified transient LMXBs may be used as a tracer of the relative contribution of field and GC LMXB populations.

#### The Intriguing Evolutionary History of the Massive Black Hole X-ray Binary M33 X-7

Conference Proceedings
Valsecchi, Francesca; Glebbeek, Evert; Farr, Will M.; Fragos, Tassos; Willems, Bart; Orosz, Jerome A.; Liu, Jifeng; Kalogera, Vassiliki
INTERNATIONAL CONFERENCE ON BINARIES: In celebration of Ron Webbink's 65th Birthday. AIP Conference Proceedings, Volume 1314, pp. 285-290 (2010)

Black hole (BH) X-ray binaries (XRBs) are X-ray luminous binary systems comprising a BH accreting matter from a companion star. Understanding their origins sheds light on the still not well understood physics of BH formation. M33 X-7 hosts one of the most massive stellar-mass BH among all XRBs known to date, a 15.65Msolar BH orbiting a 70Msolar companion star in a 3.45 day orbit. The high masses of the two components and the tight orbit relative to the large H-rich stellar component challenge our understanding of the typically invoked BH-XRBs formation channels. The measured underluminosity of the optical component further complicates the picture. A solution to the evolutionary history of this system that can account for all its observed properties has yet to be presented, and here we propose the first scenario that is consistent with the complete set of current observational constraints. In our model, M33 X-7 started its life hosting a 85-99Msolar primary and a 28-32Msolar companion in a Keplerian orbit of 2.8-3.1 days. In order to form a BH of 15.65Msolar, the initially most massive component transferred part of its envelope to the companion star and lost the rest in a strong stellar wind. During this dynamically stable mass transfer phase the companion accreted matter, to become the presently underluminous 70Msolar star.

#### The Eclipsing Black Hole X-ray Binary M33 X-7: Understanding the Current Properties

Conference Proceedings
Valsecchi, F.; Willems, B.; Fragos, T.; Kalogera, V.
Hot and Cool: Bridging Gaps in Massive Star Evolution ASP Conference Series Vol. 425, Proceedings of a Workshop held at the California Institute of Technology, Pasadena, California, 10-12 November 2008. Edited by Claus Lietherer, Philip Bennett, Pat Morris, Jacco van Loon. San Francisco: Astronomical Society of the Pacific, 2010., p.227

We explore the formation and evolution of the black hole X-ray binary system M33 X-7. In particular, we examine whether accounting for systematic errors in the stellar parameters inherent to single star models, as well as the uncertainty in the distance to M33, can explain the discrepancy between the observed and expected luminosity of the ˜70 Msun companion star. Our analysis assumes no prior interactions between the companion star and the black hole progenitor. We use four different stellar evolution codes, modified to include a variety of current stellar wind prescriptions. For the models satisfying the observational constraints on the donor star’s effective temperature and luminosity, we recalculate the black hole mass, the orbital separation, and the mean X-ray luminosity. Our best model, satisfying simultaneously all observational constraints except the observationally inferred companion mass, consists of a ˜13 Msun black hole and a ˜54 Msun companion star. We conclude that a star with the observed mass and luminosity can not be explained via single star evolution models, and that a prior interaction between the companion star and the black hole progenitor should be taken into account.

#### Models for Low-Mass X-Ray Binaries in Elliptical Galaxies NGC3379 and NGC4278

Conference Proceedings
Kalogera, Vassiliki; Fragos, Tassos
A POPULATION EXPLOSION: The Nature & Evolution of X-ray Binaries in Diverse Environments. AIP Conference Proceedings, Volume 1010, pp. 373-377 (2008)

We present theoretical models for populations of low-mass X-ray binaries in two elliptical galaxies: NGC3379 and NGC4278. The models are calculated with the recently updated StarTrack code (Belczynski et al. 2007), and are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLF) in these galaxies. For the first time we explore the population XLF down to luminosities of 3×1036 erg s-1, as probed by the most recent observational results (Kim et al. 2006). We consider models for the formation and evolution of low-mass X-ray binaries (LMXB) in galactic fields with different common envelope efficiencies, stellar wind prescriptions and initial mass functions. We identify models that produce an XLF in excellent agreement with the observations both in shape and number of systems at a specific luminosity. We also find that the treatment of the outburst luminosity of transient systems remains a crucial parameter for the determination of the XLF as the modeled populations are dominated by transient X-ray systems.

#### Models for low-mass X-ray binaries in the elliptical galaxies NGC3379 and NGC4278: Comparsion with observations

Conference Proceedings
Fragos, T.; Kalogera, V.; Belczynski, K.; Fabbiano, G.; Kim, D. W.; Brassington, N.; Angelini, L.; Davies, R. L.; Gallagher, J. S.; King, A. R.; Pellegrini, S.; Trinchieri, G.; Zepf, S. E.; Zezas, A.
ESAC faculty workshop on x-rays from nearby galaxies : Proceedings of ESAC faculty workshop on x-rays from nearby galaxies, European Space Agency, ESAC, Madrid (Spain), September 5-7,2007, Max-Planck-Institut für extraterrestrische Physik, MPE Report 295, ISSN 0178-0719, p.108-111

We present theoretical models for popula- tions of low-mass X-ray binaries in two elliptical galaxies NGC3379 and NGC4278. The models are calculated with the recently updated StarTrack code (Belczynski et al. 2007), and are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLF) in these galaxies. For the first time we explore the population XLF down to luminosities of 3 × 1036 erg s−1, as probed by the most recent observational results (Kim et al. 2006). We consider models for the formation and evolution of low- mass X-ray binaries (LMXB) in galactic fields with different common envelope efficiencies, stellar wind prescrip- tions and initial mass functions. We identify models that produce an XLF in excellent agreement with the observa- tions both in shape and number of systems at a specific luminosity. We also find that the treatment of the outburst luminosity of transient systems remains a crucial parame- ter for the determination of the XLF as the modeled pop- ulations are dominated by transient X-ray systems.

#### Black Hole Formation in X-Ray Binaries: The Case of XTE J1118+480

Conference Proceedings
Fragos, T.; Willems, B.; Ivanova, N.; Kalogera, V.
THE MULTICOLORED LANDSCAPE OF COMPACT OBJECTS AND THEIR EXPLOSIVE ORIGINS. AIP Conference Proceedings, Volume 924, pp. 673-676 (2007)

In recent years, an increasing number of proper motions have been measured for Galactic X-ray binaries. When supplemented with accurate determinations of the component masses, orbital period, and donor luminosity and effective temperature, these kinematical constraints harbor a wealth of information on the systems’ past evolution. The constraints on compact object progenitors and kicks derived from this are of immense value for understanding compact object formation and exposing common threads and fundamental differences between black hole and neutron star formation. Here, we present the results of such an analysis for the black hole X-ray binary XTE J1118+480. We present results from modeling the mass transfer phase, following the motion in the Galaxy back to the birth site of the black hole, and examining the dynamics of symmetric and asymmetric core-collapses of the black hole progenitor.