ABSTRACT
We present a new statistical method for constructing background subtracted measurements from event list data gathered by X-ray and gamma-ray observatories. This method was initially ...developed specifically to construct images that account for the high background fraction and low overall count rates observed in survey data from the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum Röntgen Gamma (SRG) mission, although the mathematical underpinnings are valid for data taken with other imaging missions and analysis applications. This method fully accounts for the expected Poisson fluctuations in both the sky photon and non-X-ray background count rates in a manner that does not result in unphysical negative counts. We derive the formulae for arbitrary confidence intervals for the source counts and show that our new measurement converges exactly to the standard background subtraction calculation in the high signal limit. Utilizing these results, we discuss several variants of images designed to optimize different science goals for both pointed and slewing telescopes. Using realistic simulated data of a galaxy cluster as observed by ART-XC, we show that our method provides a more significant and robust detection of the cluster emission as compared to a standard background subtraction. We also demonstrate its advantages using real observations of a point source from the ART-XC telescope. These calculations may have widespread applications for a number of source classes observed with high energy telescopes.
ABSTRACT We present the first direct measurements of the rest-frame 10-40 keV X-ray luminosity function (XLF) of active galactic nuclei (AGNs) based on a sample of 94 sources at 0.1 < z < 3, selected ...at 8-24 keV energies from sources in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic survey program. Our results are consistent with the strong evolution of the AGN population seen in prior, lower-energy studies of the XLF. However, different models of the intrinsic distribution of absorption, which are used to correct for selection biases, give significantly different predictions for the total number of sources in our sample, leading to small, systematic differences in our binned estimates of the XLF. Adopting a model with a lower intrinsic fraction of Compton-thick sources and a larger population of sources with column densities cm−2 or a model with stronger Compton reflection component (with a relative normalization of R ∼ 2 at all luminosities) can bring extrapolations of the XLF from 2-10 keV into agreement with our NuSTAR sample. Ultimately, X-ray spectral analysis of the NuSTAR sources is required to break this degeneracy between the distribution of absorbing column densities and the strength of the Compton reflection component and thus refine our measurements of the XLF. Furthermore, the models that successfully describe the high-redshift population seen by NuSTAR tend to over-predict previous, high-energy measurements of the local XLF, indicating that there is evolution of the AGN population that is not fully captured by the current models.
Hot dust-obscured galaxies (Hot DOGs) are among the most luminous galaxies in the universe. Powered by highly obscured, possibly Compton-thick, active galactic nuclei (AGNs), Hot DOGs are ...characterized by spectral energy distributions that are very red in the mid-infrared yet dominated by the host galaxy stellar emission in the UV and optical. An earlier study identified a subsample of Hot DOGs with significantly enhanced UV emission. One target, W0204-0506, was studied in detail and, based on Chandra observations, it was concluded that the enhanced emission was most likely due to either extreme unobscured star formation (star formation rate > 1000 M yr−1) or to light from the highly obscured AGN scattered by gas or dust into our line of sight. Here, we present a follow-up study of W0204-0506 as well as two more Hot DOGs with excess UV emission. For the two new objects we obtained Chandra/ACIS-S observations, and for all three targets we obtained Hubble Space Telescope/WFC3 F555W and F160W imaging. The analysis of these observations, combined with multiwavelength photometry and UV/optical spectroscopy suggests that UV emission is most likely dominated by light from the central highly obscured, hyperluminous AGN that has been scattered into our line of sight, by either gas or dust. We cannot decisively rule out, however, that star formation or a second AGN in the system may significantly contribute to the UV excess of these targets.
ABSTRACT
We present the NuSTAR extragalactic survey of the James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-Domain Field. The survey covers a ∼0.16 deg2 area with a total exposure of ...681 ks acquired in a total of nine observations from three epochs. The survey sensitivities at 20 per cent of the area are 2.39, 1.14, 2.76, 1.52, and 5.20 × 10−14 erg cm−2 s−1 in the 3–24, 3–8, 8–24, 8–16, and 16–24 keV bands, respectively. The NEP survey is one of the most sensitive extragalactic surveys with NuSTAR so far. A total of 33 sources were detected above 95 per cent reliability in at least one of the five bands. We present the number counts, logN-logS, measured in the hard X-ray 8–24 and 8–16 keV bands, uniquely accessible by NuSTAR down to such faint fluxes. We performed source detection on the XMM–Newton and Chandra observations of the same field to search for soft X-ray counterparts of each NuSTAR detection. The soft band positions were used to identify optical and infrared associations. We present the X-ray properties (hardness ratio and luminosity) and optical-to-X-ray properties of the detected sources. The measured fraction of candidate Compton-thick (N$\rm _H\ge 10^{24}\, cm^{-2}$) active galactic nuclei, derived from the hardness ratio, is between 3 and 27 per cent. As this survey was designed to have variability as its primary focus, we present preliminary results on multiepoch flux variability in the 3–24 keV band.
We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray background (CXB) in the Chandra Deep Fields (CDFs) North and South, the longest observations with Chandra (2 and 1 Ms, ...respectively). We measure the unresolved CXB intensity by extracting spectra of the sky and removing all point and extended sources detected in the CDF. To model and subtract the instrumental background, we use observations obtained with ACIS in stowed position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band is dominated by diffuse Galactic and local thermal-like emission. We find unresolved intensites in the 0.5-1 keV band of (4.1 c 0.3) x 10 super(-12) ergs cm super(-2) s super(-1) deg super(-2) for CDF-N and (5.0 c 0.4) x 10 super(-12) for CDF-S. In the 1-8 keV band, the unresolved spectrum is adequately described by a power law with a photon index = 1.5 +0.5-0.4 and normalization 2.6 c 0.3 photons s super(-1) keV super(-1) sr super(-1) at 1 keV. We find unresolved CXB intensities of (1.04 c 0.14) x 10 super(-12) ergs cm super(-2) s super(-1) deg super(-2) for the 1-2 keV band and (3.4 c 1.7) x 10 super(-12) ergs cm super(-2) s super(-1) deg super(-2) for the 2-8 keV band. Our detected unresolved intensities in these bands significantly exceed the expected flux from sources below the CDF detection limits, if one extrapolates the log Nl log S curve to zero flux. Thus, these background intensities imply either a genuine diffuse component or a steepening of the log Nl log S curve at low fluxes, most significantly for energies <2 keV. Adding the unresolved intensity to the total contribution from sources detected in these fields and wider field surveys, we obtain a total intensity of the extragalactic CXB of (4.6 c 0.3) x 10 super(-12) ergs cm super(-2) s super(-1) deg super(-2) for 1-2 keV and (1.7 c 0.2) x 10 super(-11) ergs cm super(-2) s super(-1) deg super(-2) for 2-8 keV. These totals correspond to a CXB power-law normalization (for = 1.4) of 10.9 photons cm super(-2) s super(-1) keV super(-1) sr super(-1) at 1 keV. This corresponds to resolved fractions of 77% c 3% and 80% c 8% for 1-2 and 2-8 keV, respectively.
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 deg2 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 . 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; NH > 1024 cm−2) fraction. Taking into account the uncertainties on each NH 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.
Abstract
We present a clustering analysis of 370 high-confidence Hα emitters (HAEs) at z = 2.23. The HAEs are detected in the Hi-Z Emission Line Survey (HiZELS), a large-area blank field 2.121 μm ...narrow-band survey using the United Kingdom Infrared Telescope Wide Field Camera (WFCAM). Averaging the two-point correlation function of HAEs in two ∼1° scale fields United Kingdom Infrared Deep Sky Survey/Ultra Deep Survey (UDS) and Cosmological Evolution Survey (COSMOS) fields we find a clustering amplitude equivalent to a correlation length of r
0 = 3.7 ± 0.3 h
−1 Mpc for galaxies with star formation rates of ≳7 M⊙ yr−1. The data are also well-fitted by the expected correlation function of cold dark matter (CDM), scaled by a bias factor: ωHAE = b
2ωDM where . The corresponding 'characteristic' mass for the haloes hosting HAEs is log (M
h/h
−1 M⊙) = 11.7 ± 0.1. Comparing to the latest semi-analytic galform predictions for the evolution of HAEs in a ΛCDM cosmology, we find broad agreement with the observations, with galform predicting an HAE correlation length of ∼4 h
−1 Mpc. Motivated by this agreement, we exploit the simulations to construct a parametric model of the halo occupation distribution (HOD) of HAEs, and use this to fit the observed clustering. Our best-fitting HOD can adequately reproduce the observed angular clustering of HAEs, yielding an effective halo mass and bias in agreement with that derived from the scaled ωDM fit, but with the relatively small sample size the current data provide a poor constraint on the HOD. However, we argue that this approach provides interesting hints into the nature of the relationship between star-forming galaxies and the matter field, including insights into the efficiency of star formation in massive haloes. Our results support the broad picture that 'typical' (≲L
⋆) star-forming galaxies have been hosted by dark matter haloes with M
h ≲ 1012 h
−1 M⊙ since z ≈ 2, but with a broad occupation distribution and clustering that is likely to be a strong function of luminosity.
We cross-correlate a cosmic microwave background (CMB) lensing map with the projected space densities of quasars to measure the bias and halo masses of a quasar sample split into obscured and ...unobscured populations, the first application of this method to distinct quasar subclasses. Several recent studies of the angular clustering of obscured quasars have shown that these objects likely reside in higher mass haloes compared to their unobscured counterparts. This has important implications for models of the structure and geometry of quasars, their role in growing supermassive black holes, and mutual quasar/host galaxy evolution. However, the magnitude and significance of this difference has varied from study to study. Using data from Planck, WISE, and Sloan Digital Sky Survey, we follow up on these results using the independent method of CMB lensing cross-correlations. The region and sample are identical to that used for recent angular clustering measurements, allowing for a direct comparison of the CMB-lensing and angular clustering methods. At z similar to 1, we find that the bias of obscured quasars is b sub(q) = 2.57 plus or minus 0.24, while that of unobscured quasars is b sub(q) = 1.89 plus or minus 0.19. This corresponds to halo masses of log(M sub(h) /M. h super(-1))=13.24 super(+0.14) sub(-0.15) (obscured) and log(M sub(h)/M. h super(-1))=12.71 super(+0.15) sub(-0.13) (unobscured). These results agree well with those from angular clustering (well within 1 sigma ), and confirm that obscured quasars reside in host haloes similar to 3 times as massive as haloes hosting unobscured quasars. This implies that quasars spend a significant portion of their lifetime in an obscured state, possibly more than one-half of the entire active phase.
Recent studies have found that obscured quasars cluster more strongly and are thus hosted by dark matter haloes of larger mass than their unobscured counterparts. These results pose a challenge for ...the simplest unification models, in which obscured objects are intrinsically the same as unobscured sources but seen through a dusty line of sight. There is general consensus that a structure like a ‘dusty torus’ exists, meaning that this intrinsic similarity is likely the case for at least some subset of obscured quasars. However, the larger host halo masses of obscured quasars imply that there is a second obscured population that has an even higher clustering amplitude and typical halo mass. Here, we use simple assumptions about the host halo mass distributions of quasars, along with analytical methods and cosmological N-body simulations to isolate the signal from this population. We provide values for the bias and halo mass as a function of the fraction of the ‘non-torus-obscured’ population. Adopting a reasonable value for this fraction of ∼25 per cent implies a non-torus-obscured-quasar bias that is much higher than the observed obscured quasar bias, because a large fraction of the obscured population shares the same clustering strength as the unobscured objects. For this non-torus-obscured population, we derive a bias of ∼3, and typical halo masses of ∼3 × 1013 M⊙ h
−1 at z = 1. These massive haloes are likely the descendants of high-mass unobscured quasars at high redshift, and will evolve into members of galaxy groups at z = 0.
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