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Register a new userCosmic X-ray surveys of Active Galactic Nuclei: synergy between X-ray and infrared observations
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D. M. Alexander
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We briefly review the synergy between X-ray and infrared observations for Active Galactic Nuclei (AGNs) detected in cosmic X-ray surveys, primarily with XMM-Newton, Chandra, and NuSTAR. We focus on two complementary aspects of this X-ray-infrared synergy (1) the identification of the most heavily obscured AGNs and (2) the connection between star formation and AGN activity. We also briefly discuss future prospects for X-ray-infrared studies over the next decade.
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The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Though they play a key role in cultivating the cosmological environment and/or enabling our studies of it, there is still much we do not know about AGNs and GRBs, particularly the avenue in which and through which they supply radiation and energetic particles, namely their jets. This White Paper is the second of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. The focus of this white paper is active galactic nuclei and gamma-ray bursts.
We review results from cosmic X-ray surveys of active galactic nuclei (AGNs) over the past ~ 15 yr that have dramatically improved our understanding of growing supermassive black holes in the distant universe. First, we discuss the utility of such surveys for AGN investigations and the capabilities of the missions making these surveys, emphasizing Chandra, XMM-Newton, and NuSTAR. Second, we briefly describe the main cosmic X-ray surveys, the essential roles of complementary multiwavelength data, and how AGNs are selected from these surveys. We then review key results from these surveys on the AGN population and its evolution (demographics), the physical processes operating in AGNs (physics), and the interactions between AGNs and their environments (ecology). We conclude by describing some significant unresolved questions and prospects for advancing the field.
We present an investigation into how well the properties of the accretion flow onto a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure the characteristic spectral index, Gamma, of a power-law energy distribution, over an energy range of 2 to 10 keV, for X-ray selected, broad-lined radio-quiet AGN up to z~2 in COSMOS and E-CDF-S. We test the previously reported dependence between Gamma and black hole mass, FWHM and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg ii and H-alpha emission lines with the later afforded by recent near infrared spectroscopic observations using Subaru/FMOS. We calculate the Eddington ratios, lambda_Edd, for sources where a bolometric luminosity (L_Bol) has been presented in the literature, based on SED fitting, or, for sources where these data do not exist, we calculate L_Bol using a bolometric correction to the LX, derived from a relationship between the bolometric correction, and LX/L3000. From a sample of 69 X-ray bright sources (>250 counts), where Gamma can be measured with greatest precision, with an estimate of L_Bol, we find a statistically significant correlation between Gamma and lambda_Edd, which is highly significant with a chance probability of 6.59x10^-8. A statistically significant correlation between Gamma and the FWHM of the optical lines is confirmed, but at lower significance than with lambda_Edd indicating that lambda_Edd is the key parameter driving conditions in the corona. Linear regression analysis reveals that Gamma=(0.32+/-0.05)log10 lambda_Edd+(2.27+/-0.06) and Gamma=(-0.69+/-0.11)log10(FWHM/km/s)+(4.44+/-0.42). Our results on Gamma-lambda_Edd are in very good agreement with previous results. (ABRIDGED)
X-ray variation is a ubiquitous feature of active galactic nuclei (AGNs), however, its origin is not well understood. In this paper, we show that the X-ray flux variations in some AGNs, and correspondingly the power spectral densities (PSDs) of the variations, may be interpreted as being caused by absorptions of eclipsing clouds or clumps in the broad line region (BLR) and the dusty torus. By performing Monte-Carlo simulations for a number of plausible cloud models, we systematically investigate the statistics of the X-ray variations resulting from the cloud eclipsing and the PSDs of the variations. For these models, we show that the number of eclipsing events can be significant and the absorption column densities due to those eclipsing clouds can be in the range from 10^{21} to 10^{24} cm^{-2}, leading to significant X-ray variations. We find that the PSDs obtained from the mock observations for the X-ray flux and the absorption column density resulting from these models can be described by a broken double power law, similar to those directly measured from observations of some AGNs. The shape of the PSDs depend strongly on the kinematic structures and the intrinsic properties of the clouds in AGNs. We demonstrate that the X-ray eclipsing model can naturally lead to a strong correlation between the break frequencies (and correspondingly the break timescales) of the PSDs and the masses of the massive black holes (MBHs) in the model AGNs, which can be well consistent with the one obtained from observations. Future studies of the PSDs of the AGN X-ray (and possibly also the optical-UV) flux and column density variations may provide a powerful tool to constrain the structure of the BLR and the torus and to estimate the MBH masses in AGNs.
Active galactic nuclei (AGN) are complex phenomena. At the heart of an AGN is a relativistic accretion disk around a spinning supermassive black hole (SMBH) with an X-ray emitting corona and, sometimes, a relativistic jet. On larger scales, the outer accretion disk and molecular torus act as the reservoirs of gas for the continuing AGN activity. And on all scales from the black hole outwards, powerful winds are seen that probably affect the evolution of the host galaxy as well as regulate the feeding of the AGN itself. In this review article, we discuss how X-ray spectroscopy can be used to study each of these components. We highlight how recent measurements of the high-energy cutoff in the X-ray continuum by NuSTAR are pushing us to conclude that X-ray coronae are radiatively-compact and have electron temperatures regulated by electron-positron pair production. We show that the predominance of rapidly-rotating objects in current surveys of SMBH spin is entirely unsurprising once one accounts for the observational selection bias resulting from the spin-dependence of the radiative efficiency. We review recent progress in our understanding of fast (v~0.1-0.3c), highly-ionized (mainly visible in FeXXV and FeXXVI lines), high-column density winds that may dominate quasar-mode galactic feedback. Finally, we end with a brief look forward to the promise of Astro-H and future X-ray spectropolarimeters.
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