Tassos Fragos

X-ray Binary Evolution Across Cosmic Time

High redshift galaxies permit the study of the formation and evolution of X-ray binary populations on cosmological timescales, probing a wide range of metallicitie s and star-formation rates. In this paper, we present results from a large scale population synthesis study that m odels the X-ray binary 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. (2011) to self-consistently account for the star formation history and metallicity evolution of the uni verse. Our modeling, which is constrained by the observed X-ray properties of local galaxies, gives predict ions about the global scaling of emission from X-ray binary populations with properties such as star-formation rate 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 X-ray binaries in our Universe today is dominated by low-mass X-ray binaries, and it is only at z & 2.5 that high-mass X-ray binaries become dominant. We also find t hat there is a delay of � 1.1 Gyr between the peak of X-ray emissivity from low-mass Xray binaries (at z � 2.1) and the peak of star-formation rate density (at z � 3.1). The peak of the X-ray luminosity from high-mass X-ray binaries (at z � 3.9), happens � 0.8 Gyr before the peak of the star-formation rate density, which is due to the metallicity evolution of the Universe. Subject headings:stars: binaries: close, stars: evolution, X-rays: binarie s, galaxies, diffuse background, galaxies: stellar content

THE X-RAY STAR FORMATION STORY AS TOLD BY LYMAN BREAK GALAXIES IN THE 4 Ms CDF-S

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 (CDF-S) 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 HST 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 LBGs are dominated by young stellar populations. The X-ray emission in LBGs, eliminating individually detected X-ray sources (potential AGN), 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 LX = 0.93log(1+z)+0.65logSFR+39.80. By comparing our observations with sophisticated X-ray binary population synthesis models, we interpret that the redshift evolution of LX/SFR is driven by metallicity evolution in HMXBs, 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 Mpc 3 ) 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 SMBH accretion history of the Universe around the epoch of reionization.

ENERGY FEEDBACK FROM X-RAY BINARIES IN THE EARLY UNIVERSE

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.

MODELS FOR LOW-MASS X-RAY BINARIES IN THE ELLIPTICAL GALAXIES NGC 3379 AND NGC 4278 : COMPARISON WITH OBSERVATIONS

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXB) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code (Belczynski et al. 2007), assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested

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

In black hole X-ray binaries, a misalignment between the spin axis of the black hole and the orbital angular momentum can occur during the supernova explosion that forms the compact object. In this letter we present population synthesis models of Galactic black hole X-ray binaries, 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 misalignment angle due to accretion of material onto the black hole during the X-ray binary phase. The major factor that sets the misalignment angle for X-ray binaries is the natal kick that the black hole 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 (>10 degrees) misalignment angles, while some systems may reach misalignment angles as high as ~90 degrees and even higher. This results is robust among all population synthesis models. The assumption of small small misalignment angles is extensively used to observationally estimate black hole spin magnitudes, and for the first time we are able to confirm this assumption using detailed population synthesis calculations.

Constraining Population Synthesis Models via the Binary Neutron Star Population

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.

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

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 (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.

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

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.

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

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 × 10 36 erg s −1 to 5.4 × 10 39 erg s −1 . The 90% detection completeness limit within the D25 ellipse is 5.5 × 10 37 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 10 39 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 × 10 38 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.

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

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, 15.65M⊙ black hole orbiting an underluminous, 70M⊙ main-sequence companion in a slightly eccentric 3.45-day orbit (M⊙, 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 85M⊙–99M⊙ and a secondary body of 28M⊙–32M⊙, 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 ∼16M⊙ 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.