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Register a new userNuSTAR measurement of the cosmic X-ray background in the 3-20 keV energy band
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We present measurements of the intensity of the Cosmic X-ray Background (CXB) with the NuSTAR telescope in the 3-20 keV energy range. Our method uses spatial modulation of the CXB signal on the NuSTAR detectors through the telescopes side aperture. Based on the NuSTAR observations of selected extragalactic fields with a total exposure of 7 Ms, we have estimated the CXB 3-20 keV flux to be 2.8E-11 erg/s/cm^2/deg^2, which is ~8 higher than measured with HEAO-1 and consistent with the INTEGRAL measurement. The inferred CXB spectral shape in the 3-20 keV energy band is consistent with the canonical model of Gruber et al. We demonstrate that the spatially modulated CXB signal measured by NuSTAR is not contaminated by systematic noise and is limited by photon statistics. The measured relative scatter of the CXB intensity between different sky directions is compatible with cosmic variance, which opens new possibilities for studying CXB anisotropy over the whole sky with NuSTAR.
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We report on the first ten identifications of sources serendipitously detected by the NuSTAR to provide the first sensitive census of the cosmic X-ray background (CXB) source population at >10 keV. We find that these NuSTAR-detected sources are ~100x fainter than those previously detected at >10 keV and have a broad range in redshift and luminosity (z=0.020-2.923 and L_10-40 keV~4x10^{41}-5x10^{45} erg/s); the median redshift and luminosity are z~0.7 and L_10-40 keV~3x10^{44} erg/s, respectively. We characterize these sources on the basis of broad-band ~0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared SED analyzes. We find that the dominant source population is quasars with L_10-40 keV>10^{44} erg/s, of which ~50% are obscured with N_H>10^{22} cm^{-2}. However, none of the ten NuSTAR sources are Compton thick (N_H>10^{24} cm^{-2}) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L_10-40 keV>10^{44} erg/s) selected at >10 keV of ~33% over the redshift range z=0.5-1.1. We jointly fitted the rest-frame ~10-40 keV data for all of the non-beamed sources with L_10-40 keV>10^{43} erg/s to constrain the average strength of reflection; we find R<1.4 for Gamma=1.8, broadly consistent with that found for local AGNs observed at >10 keV. We also constrain the host galaxy masses and find a median stellar mass of ~10^{11} M_sun, a factor ~5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass.
The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, is produced primarily by emission from accreting supermassive black holes (SMBHs). The CXB therefore serves as a constraint on the integrated SMBH growth in the Universe and the accretion physics and obscuration in active galactic nuclei (AGNs). This paper gives an overview of recent progress in understanding the high-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB, with an emphasis on results from NASAs NuSTAR hard X-ray mission. We then discuss remaining challenges and open questions regarding the nature of AGN obscuration and AGN physics. Finally, we highlight the exciting opportunities for a next-generation, high-resolution hard X-ray mission to achieve the long-standing goal of resolving and characterizing the vast majority of the accreting SMBHs that produce the CXB.
We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the E-CDFS, EGS and COSMOS fields. The sample includes 182 NuSTAR sources (64 detected at 8-24 keV), with 3-24 keV fluxes ranging between $f_{rm 3-24 keV}approx10^{-14}$ and $6times10^{-13}$ erg/cm$^2$/s ($f_{rm 8-24 keV}approx3times10^{-14}-3times10^{-13}$ erg/cm$^2$/s) and redshifts of $z=0.04-3.21$. We produce composite spectra from the Chandra+NuSTAR data ($Eapprox2-40$ keV, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) AGN, and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is $Gamma=1.65_{-0.03}^{+0.03}$, flatter than $Gammaapprox1.8$ typically found for AGN. While the spectral slope of BL and X-ray unabsorbed AGN is consistent with typical values ($Gamma=1.79_{-0.01}^{+0.01}$), a significant flattening is seen in NL AGN and heavily-absorbed sources ($Gamma=1.60_{-0.05}^{+0.08}$ and $Gamma=1.38_{-0.12}^{+0.12}$, respectively), likely due to the effect of absorption and to the contribution from Compton reflection to the high-energy flux (E>10 keV). We find that the typical reflection fraction in our spectra is $Rapprox0.5$ (for $Gamma=1.8$), with a tentative indication of an increase of the reflection strength with column density. While there is no significant evidence for a dependence of the photon index with X-ray luminosity in our sample, we find that $R$ decreases with luminosity, with relatively high levels of reflection ($Rapprox1.2$) for $L_{rm 10-40 keV}<10^{44}$ erg/s and $Rapprox0.3$ for $L_{rm 10-40 keV}>10^{44}$ erg/s AGN, assuming $Gamma=1.8$.
The Galactic diffuse X-ray emission (GDXE) is believed to arise from unresolved populations of numerous low-luminosity X-ray binary systems that trace stellar mass distribution of the Milky Way. Many dedicated studies carried out over the last decade suggest that a dominant contributor to GDXE is a population of accreting white dwarfs (WDs). The question arises about relative contribution of different subclasses of accreting WD population, namely non-magnetic WD binaries, magnetic intermediate polars (IPs) and polars, in different regions of the Galaxy: the Galactic center, bulge, and ridge. Recent low-energy (E$<10$ keV) studies indicate that non-magnetic WD binaries, in particular quiescent dwarf novae, provide a major contribution to the diffuse hard X-ray emission of the Galactic bulge. From the other side, previous high energy (E$>10$ keV) X-ray measurements of the bulge and ridge imply a dominant population of magnetic CVs, in particular intermediate polars. In this work we use side aperture of the NuSTAR to probe the diffuse continuum of the inner $sim1-3^{circ}$ of the Galactic bulge, which allows us to constrain possible mixture of soft and hard populations components of the spectrum. We found that GDXE spectrum is well-described by a single-temperature thermal plasma with $kT approx 8$ keV, which supports that the bulge is dominated by quiescent dwarf novae with no evidence of a significant intermediate polar population in the hard X-ray band. We also compare this result with previous NuSTAR measurements of the inner 10 pc and inner 100 pc of the Galactic center.
We present the 3-8 keV and 8-24 keV number counts of active galactic nuclei (AGN) identified in the NuSTAR extragalactic surveys. NuSTAR has now resolved 33-39% of the X-ray background in the 8-24 keV band, directly identifying AGN with obscuring columns up to approximately 1e25 /cm2. In the softer 3-8 keV band the number counts are in general agreement with those measured by XMM-Newton and Chandra over the flux range 5e-15 < S(3 - 8 keV)/(erg/cm2/s) < 1e-12 probed by NuSTAR. In the hard 8-24 keV band NuSTAR probes fluxes over the range 2e-14 < S(8-24 keV)/(erg/cm2/s) < 1e-12, a factor of approximately 100 fainter than previous measurements. The 8-24 keV number counts match predictions from AGN population synthesis models, directly confirming the existence of a population of obscured and/or hard X-ray sources inferred from the shape of the integrated cosmic X-ray background. The measured NuSTAR counts lie significantly above simple extrapolation with a Euclidian slope to low flux of the Swift/BAT 15-55 keV number counts measured at higher fluxes S(15-55 keV) > 1e-11 erg/cm2/s, reflecting the evolution of the AGN population between the Swift/BAT local (z<0.1) sample and NuSTARs z~1 sample. CXB synthesis models, which account for AGN evolution, lie above the Swift/BAT measurements, suggesting that they do not fully capture the evolution of obscured AGN at low redshifts.
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