The Ubiquity and Magnitude of Large FeKα\alpha Equivalent Widths in AGN Extended Regions

Narrow Fe K$\alpha$ fluorescent emission lines arising at $\sim$kpc-scale separations from the nucleus have only been detected in a few AGN. The detections require that the extended line emission be spatially resolved and sufficiently bright. Compared to narrow Fe K$\alpha$ lines arising closer to the nucleus, they have much lower fluxes but show substantially larger equivalent widths, EW$_{\rm Fe K\alpha}$. We show that, in the optically-thin limit, a purely analytical argument naturally predicts large, EW$_{\rm FeK\alpha}\sim$1 keV, values for such lines, regardless of the details of equivalent hydrogen column density, $N_H$, or reprocessor geometry. Monte Carlo simulations corroborate this result and show that the simple analytic EW$_{\rm FeK\alpha}$ prescription holds up to higher $N_H$ approaching the Compton-thick regime. We compare to $Chandra$ observations from the literature and discuss that our results are consistent with the large EW$_{\rm FeK\alpha}$ values reported for local AGN, for which the line is detected in extended, up to $\sim$kpc-scale, regions. We argue that large EW$_{\rm FeK\alpha}$ from kpc-scale regions in AGN should be ubiquitous, because they do not depend on the absolute luminosity of the central X-ray source, and are measured only against the scattered continuum. We predict values to be of the order of $\sim$1 keV or larger, even for covering factors $\ll$1, and for arbitrarily small column densities. We propose that the large-scale molecular material that is now routinely being detected with the Atacama Large Millimeter/Submillimeter Array (ALMA) may act as an extended X-ray scattering reprocessor giving rise to $\sim$kpc-scale Fe K$\alpha$ emission.Comment: Accepted for publication in Physical Review D. 9 pages, 6 figure

NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K Fluorescence Features

We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin Seyfert 2 galaxy NGC 4388. The spectrum above $\sim$2 keV can be described by a remarkably simple and rather mundane model, consisting of a uniform, neutral spherical distribution of matter, with a radial column density of $2.58 \pm 0.02 \times 10^{23}$ cm$^{-2}$, and an Fe abundance of $1.102^{+0.024}_{-0.021}$ relative to solar. The model does not require any phenomenological adjustments to self-consistently account for the low-energy extinction, the Fe K$\alpha$ and Fe K$\beta$ fluorescent emission lines, the Fe K edge, and the Compton-scattered continuum from the obscuring material. The spherical geometry is not a unique description, however, and the self-consistent, solar abundance MYTORUS model, applied with toroidal and non-toroidal geometries, gives equally good descriptions of the data. In all cases, the key features of the spectrum are so tightly locked together that for a wide range of parameters, a relativistic disk-reflection component contributes no more than $\sim$2% to the net spectrum in the 2-20 keV band. We show that the commonly invoked explanations for weak X-ray reflection features, namely a truncated and/or very highly ionized disk, do not work for NGC 4388. If relativistically-broadened Fe K$\alpha$ lines and reflection are ubiquitous in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2 galaxies. The case of NGC 4388 shows the need for similar studies of more Compton-thin AGN to ascertain whether this is true.Comment: MNRAS accepted. 21 pages, 7 figures, 4 tables; Appendix with historical notes and 1 table. This version: Corrected minor typo and affiliatio

Suprathermal electron isotropy in high-beta solar wind and its role in heat flux dropouts

[1] Time variations in plasma beta and a parameter which measures isotropy in suprathermal electron pitch angle distributions show a remarkably close correspondence throughout the solar wind. The finding implies that high-beta plasma, with its multiple magnetic holes and sharp field and plasma gradients, is conducive to electron pitch-angle scattering, which reduces heat flux from the Sun without field-line disconnection. Thus the finding impacts our understanding of signatures we use to determine magnetic topology in the heliosphere

Heliospheric plasma sheets

[1] As a high-beta feature on scales of hours or less, the heliospheric plasma sheet (HPS) encasing the heliospheric current sheet shows a high degree of variability. A study of 52 sector boundaries identified in electron pitch angle spectrograms in Wind data from 1995 reveals that only half concur with both high-beta plasma and current sheets, as required for an HPS. The remaining half lack either a plasma sheet or current sheet or both. A complementary study of 37 high-beta events reveals that only 5 contain sector boundaries while nearly all (34) contain local magnetic field reversals, however brief. We conclude that high-beta plasma sheets surround current sheets but that most of these current sheets are associated with fields turned back on themselves. The findings are consistent with the hypothesis that high-beta plasma sheets, both at and away from sector boundaries, are the heliospheric counterparts of the small coronal transients observed at the tips of helmet streamers, in which case the proposed mechanism for their release, interchange reconnection, could be responsible for the field inversions

X-ray constraints on the fraction of obscured AGN at high accretion luminosities

The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured AGN at high accretion luminosities, $L_X (\rm 2-10 \, keV) > 10^{44} \, erg \,s ^{-1}$, and out to redshift $z\approx1.5$. The sample covers an area of about $\rm14\,deg^2$ and provides constraints on the space density of powerful AGN over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, $\rm N_H\approx10^{24}\,cm^{-2}$. The fraction of obscured Compton-thin ($\rm N_H=10^{22}-10^{24}\,cm^{-2}$) AGN is estimated to be $\approx0.35$ for luminosities $L_X(\rm 2-10\,keV)>10^{44}\,erg\,s^{-1}$ independent of redshift. For less luminous sources the fraction of obscured Compton-thin AGN increases from $0.45\pm0.10$ at $z=0.25$ to $0.75\pm0.05$ at $z=1.25$. Studies that select AGN in the infrared via template fits to the observed Spectral Energy Distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGN (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGN as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGN is primarily a function of redshift. This fraction increases from about 20-30% at z=0.25 to about 50-70% at $z=1.5$.Comment: MNRAS accepte

Photometric redshifts for X-ray-selected active galactic nuclei in the eROSITA era

With the launch of eROSITA (extended Roentgen Survey with an Imaging Telescope Array), successfully occurred on 2019 July 13, we are facing the challenge of computing reliable photometric redshifts for 3 million of active galactic nuclei (AGNs) over the entire sky, having available only patchy and inhomogeneous ancillary data. While we have a good understanding of the photo-z quality obtainable for AGN using spectral energy distribution (SED)-fitting technique, we tested the capability of machine learning (ML), usually reliable in computing photo-z for QSO in wide and shallow areas with rich spectroscopic samples. Using MLPQNA as example of ML, we computed photo-z for the X-ray-selected sources in Stripe 82X, using the publicly available photometric and spectroscopic catalogues. Stripe 82X is at least as deep as eROSITA will be and wide enough to include also rare and bright AGNs. In addition, the availability of ancillary data mimics what can be available in the whole sky. We found that when optical, and near- and mid-infrared data are available, ML and SED fitting perform comparably well in terms of overall accuracy, realistic redshift probability density functions, and fraction of outliers, although they are not the same for the two methods. The results could further improve if the photometry available is accurate and including morphological information. Assuming that we can gather sufficient spectroscopy to build a representative training sample, with the current photometry coverage we can obtain reliable photo-z for a large fraction of sources in the Southern hemisphere well before the spectroscopic follow-up, thus timely enabling the eROSITA science return. The photo-z catalogue is released here

Finding Rare AGN: X-ray Number Counts of Chandra Sources in Stripe 82

We present the first results of a wide area X-ray survey within the Sloan Digital Sky Survey (SDSS) Stripe 82, a 300 deg$^2$ region of the sky with a substantial investment in multi-wavelength coverage. We analyzed archival {\it Chandra} observations that cover 7.5 deg$^2$ within Stripe 82 ("Stripe 82 ACX"), reaching 4.5$\sigma$ flux limits of 7.9$\times10^{-16}$, 3.4$\times10^{-15}$ and 1.8$\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the soft (0.5-2 keV), hard (2-7 keV) and full (0.5-7 keV) bands, to find 774, 239 and 1118 X-ray sources, respectively. Three hundred twenty-one sources are detected only in the full band and 9 sources are detected solely in the soft band. Utilizing data products from the {\it Chandra} Source Catalog, we construct independent Log$N$-Log$S$ relationships, detailing the number density of X-ray sources as a function of flux, which show general agreement with previous {\it Chandra} surveys. We compare the luminosity distribution of Stripe 82 ACX with the smaller, deeper CDF-S + E-CDFS surveys and with {\it Chandra}-COSMOS, illustrating the benefit of wide-area surveys in locating high luminosity AGN. We also investigate the differences and similarities of X-ray and optical selection to uncover obscured AGN in the local Universe. Finally, we estimate the population of AGN we expect to find with increased coverage of 100 deg$^2$ or 300 deg$^2$, which will provide unprecedented insight into the high redshift, high luminosity regime of black hole growth currently under-represented in X-ray surveys.Comment: Accepted for publication in MNRAS, 15 pages, 6 Figures, 2 Table

The Accretion History of AGNs. I. Supermassive Black Hole Population Synthesis Model

As matter accretes onto the central supermassive black holes in active galactic nuclei (AGNs), X-rays are emitted. We present a population synthesis model that accounts for the summed X-ray emission from growing black holes; modulo the efficiency of converting mass to X-rays, this is effectively a record of the accreted mass. We need this population synthesis model to reproduce observed constraints from X-ray surveys: the X-ray number counts, the observed fraction of Compton-thick AGNs [log (BH /cm-2 )>24], and the spectrum of the cosmic X-ray background (CXB), after accounting for selection biases. Over the past decade, X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT have provided greatly improved observational constraints. We find that no existing X-ray luminosity function (XLF) consistently reproduces all these observations. We take the uncertainty in AGN spectra into account and use a neural network to compute an XLF that fits all observed constraints, including observed Compton-thick number counts and fractions. This new population synthesis model suggests that, intrinsically, 50% +- 9% (56% +- 7%) of all AGNs within z ≃ 0.1 (1.0) are Compton-thick