Recently, keV) X-ray emission has been discovered within the central 10 pc of the Galaxy, possibly indicating a large population of intermediate polars (IPs). Chandra and XMM-Newton measurements in the surrounding ∼50 pc imply a much lighter population of IPs with M WD ≈ 0.5M ⊙ . Here we use broad-band NuSTAR observations of two IPs: TV Columbae, which has a fairly typical but widely varying reported mass of M WD ≈ 0.5-1.0M ⊙ , and IGR J17303-0601, with a heavy reported mass of M WD ≈ 1.0-1.2M ⊙ . We investigate how varying spectral models and observed energy ranges influence estimated white dwarf mass. Observations of the inner 10 pc can be accounted for by IPs with M WD ≈ 0.9M ⊙ , consistent with that of the CV population in general, and the X-ray observed field IPs in particular. The lower mass derived by Chandra and XMM-Newton appears to be an artifact of narrow energy band fitting. To explain the (unresolved) CHXE by IPs requires an X-ray (2-8 keV) luminosity function (XLF) extending down to at least 5 × 10 31 ergs s −1 . The CHXE XLF, if extended to the surrounding ∼50 pc observed by Chandra and XMM-Newton, requires at least ∼ 20-40% of the ∼9000 point sources are IPs. If the XLF extends just a factor of a few lower in luminosity, then the vast majority of these sources are IPs. This is in contrast to recent observations of the Galactic ridge, where the bulk of the 2-8 keV emission is ascribed to dwarf novae.

Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity (L X ) per star formation rate (SFR) with redshift due to the decrease of metallicity (Z) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z ∼ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ∼ 1500 galaxies in the CANDELS fields at 1.37 < z < 3.80. Using Chandra data from the AEGIS-X Deep, Deep Field North, and Deep Field South surveys, we stack the X-ray data at the galaxy locations in bins of redshift and Z because the galaxies are too faint to be individually detected. In agreement with previous studies, the average L X /SFR of our z ∼ 2 galaxy sample is enhanced by ≈ 0.4 − 0.8 dex relative to local HMXB L X -SFR scaling relations. Splitting our sample by Z, we find that L X /SFR and Z are anti-correlated with 97% confidence. This observed Z dependence for HMXB-dominated galaxies is consistent both with the local L X -SFR-Z relation and a subset of population synthesis models. Although the statistical significance of the observed trends is weak due to the low X-ray statistics, these results constitute the first direct evidence connecting the redshift evolution of L X /SFR and the Z dependence of HMXBs.

The integrated X-ray luminosity (LX) of high-mass X-ray binaries (HMXBs) in a galaxy is correlated with its star formation rate (SFR), and the normalization of this correlation increases with redshift. Population synthesis models suggest that the redshift evolution of LX/SFR is driven by the metallicity (Z) dependence of HMXBs, and the first direct evidence of this connection was recently presented using galaxies at z ∼ 2. To confirm this result with more robust measurements and better constrain the LX–SFR–Z relation, we have studied the Z dependence of LX/SFR at lower redshifts. Using samples of star-forming galaxies at z = 0.1–0.9 with optical spectra from the hCOSMOS and zCOSMOS surveys, we stacked Chandra data from the COSMOS Legacy survey to measure the average LX/SFR as a function of Z in three redshift ranges: z = 0.1–0.25, 0.25–0.4, and 0.5–0.9. We find no significant variation of the LX–SFR–Z relation with redshift. Our results provide further evidence that the Z dependence of HMXBs is responsible for the redshift evolution of LX/SFR. Combining all available z &gt; 0 measurements together, we derive a best-fitting LX–SFR–Z relation and assess how different population synthesis models describe the data. These results provide the strongest constraints to date on the LX–SFR–Z relation in the range of 8.0 &lt; 12 + log(O/H) &lt; 9.0.

We present the results and the source catalog of the NuSTAR survey in the UKIDSS Ultra Deep Survey (UDS) field, bridging the gap in depth and area between NuSTAR's ECDFS and COSMOS surveys. The survey covers a ∼0.6 deg 2 area of the field for a total observing time of ∼1.75 Ms, to a half-area depth of ∼155 ks corrected for vignetting at 3-24 keV, and reaching sensitivity limits at half-area in the full (3-24 keV), soft (3-8 keV), and hard (8-24 keV) bands of 2.2×10 −14 erg cm −2 s −1 , 1.0×10 −14 erg cm −2 s −1 , and 2.7×10 −14 erg cm −2 s −1 , respectively. A total of 67 sources are detected in at least one of the three bands, 56 of which have a robust optical redshift with a median of z 1.1 á ñ~. Through a broadband (0.5-24 keV) spectral analysis of the whole sample combined with the NuSTAR hardness ratios, we compute the observed Compton-thick (CT; N H >10 24 cm −2 ) fraction. Taking into account the uncertainties on each N H measurement, the final number of CT sources is 6.8±1.2. This corresponds to an observed CT fraction of 11.5%±2.0%, providing a robust lower limit to the intrinsic fraction of CT active galactic nuclei and placing constraints on cosmic X-ray background synthesis models.

We present an X-ray stacking analysis of ∼75,000 star-forming galaxies between 0.1 < z < 5.0 using the Chandra COSMOS Legacy survey to study the X-ray emission of low-luminosity active galactic nuclei (AGN) and its connection to host galaxy properties. The stacks at z < 0.9 have luminosity limits as low as 10 40 − 10 41 erg s −1 , a regime in which X-ray binaries (XRBs) can dominate the X-ray emission. Comparing the measured luminosities to established XRB scaling relations, we find that the redshift evolution of the luminosity per star formation rate (SFR) of XRBs depends sensitively on the assumed obscuration and may be weaker than previously found. The XRB scaling relation based on stacks from the Chandra Deep Field South overestimates the XRB contribution to the COSMOS high specific SFR (sSFR) stacks, possibly due to a bias affecting the CDF-S stacks because of their small galaxy samples. After subtracting the estimated XRB contribution from the stacks, we find that most stacks at z > 1.3 exhibit a significant X-ray excess indicating nuclear emission. The AGN emission is strongly correlated with stellar mass but does not exhibit an additional correlation with SFR. The hardness ratios of the high-redshift stacks indicate that the AGN are substantially obscured (N H ∼ 10 23 cm −2 ). These obscured AGN are not identified by IRAC color selection and have L X ∼ 10 41 − 10 43 erg s −1 , consistent with accretion at an Eddington rate of ∼ 10 −3 onto 10 7 − 10 8 M black holes. Combining our results with other X-ray studies suggests that AGN obscuration depends on stellar mass and an additional variable, possibly the Eddington rate.