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Register a new userX-ray and infrared diagnostics of nearby active galactic nuclei with MAXI and AKARI
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Nearby active galactic nuclei were diagnosed in the X-ray and mid-to-far infrared wavelengths, with Monitor of All-sky X-ray Image (MAXI) and the Japanese infrared observatory AKARI, respectively. Among the X-ray sources listed in the second release of the MAXI all-sky X-ray source catalog, 100 ones are currently identified as a non-blazar-type active galactic nucleus. These include 95 Seyfert galaxies and 5 quasars, and they are composed of 73 type-1 and 27 type-2 objects. The AKARI all-sky survey point source catalog was searched for their mid- and far-infrared counterparts at 9, 18, and 90 $mu$m. As a result, 69 Seyfert galaxies in the MAXI catalog (48 type-1 and 21 type-2 ones) were found to be detected with AKARI. The X-ray (3-4 keV and 4-10 keV) and infrared luminosities of these objects were investigated, together with their color information. Adopting the canonical photon index, $Gamma = 1.9$, of the intrinsic X-ray spectrum of the Seyfert galaxies, the X-ray hardness ratio between the 3-4 and 4-10 keV ranges derived with MAXI was roughly converted into the absorption column density. After the X-ray luminosity was corrected for absorption from the estimated column density, the well-known X-ray-to-infrared luminosity correlation was confirmed at least in the Compton-thin regime. In contrast, NGC 1365, only one Compton-thick object in the MAXI catalog, was found to deviate from the correlation toward a significantly lower X-ray luminosity by nearly an order of magnitude. It was verified that the relation between the X-ray hardness below 10 keV and X-ray-to-infrared color acts as an effective tool to pick up Compton-thick objects. The difference in the infrared colors between the type-1 and type-2 Seyfert galaxies and its physical implication on the classification and unification of active galactic nuclei were briefly discussed.
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The highly energetic outflows from Active Galactic Nuclei detected in X-rays are one of the most powerful mechanisms by which the central supermassive black hole (SMBH) interacts with the host galaxy. The last two decades of high resolution X-ray spectroscopy with XMM and Chandra have improved our understanding of the nature of these outflowing ionized absorbers and we are now poised to take the next giant leap with higher spectral resolution and higher throughput observatories to understand the physics and impact of these outflows on the host galaxy gas. The future studies on X-ray outflows not only have the potential to unravel some of the currently outstanding puzzles in astronomy, such as the physical basis behind the MBH$-sigma$ relation, the cooling flow problem in intra-cluster medium (ICM), and the evolution of the quasar luminosity function across cosmic timescales, but also provide rare insights into the dynamics and nature of matter in the immediate vicinity of the SMBH. Higher spectral resolution ($le 0.5$ eV at $1$ keV) observations will be required to identify individual absorption lines and study the asymmetries and shifts in the line profiles revealing important information about outflow structures and their impact. Higher effective area ($ge 1000 rm ,cm^{2}$) will be required to study the outflows in distant quasars, particularly at the quasar peak era (redshift $1le zle 3$) when the AGN population was the brightest. Thus, it is imperative that we develop next generation X-ray telescopes with high spectral resolution and high throughput for unveiling the properties and impact of highly energetic X-ray outflows. A simultaneous high resolution UV + X-ray mission will encompass the crucial AGN ionizing continuum, and also characterize the simultaneous detections of UV and X-ray outflows, which map different spatial scales along the line of sight.
Recent time-resolved spectral studies of a few Active Galactic Nuclei in hard X-rays revealed occultations of the X-ray primary source probably by Broad Line Region (BLR) clouds. An important open question on the structure of the circumnuclear medium of AGN is whether this phenomenon is common, i.e. whether a significant fraction of the X-ray absorption in AGN is due to BLR clouds. Here we present the first attempt to perform this kind of analysis in a homogeneous way, on a statistically representative sample of AGN, consisting of the ~40 brightest sources with long XMM-Newton and/or Suzaku observations. We describe our method, based on a simple analysis of hardness-ratio light curves, and its validation through a complete spectroscopic analysis of a few cases. We find that X-ray eclipses, most probably due to clouds at the distance of the BLR, are common in sources where the expected occultation time is compatible with the observation time, while they are not found in sources with longer estimated occultation times. Overall, our results show that occultations by BLR clouds may be responsible for most of the observed X-ray spectral variability at energies higher than 2 keV, on time scales longer than a few ks.
X-ray surveys have revealed a new class of active galactic nuclei (AGN) with a very low observed fraction of scattered soft X-rays, f_scat < 0.5%. Based on X-ray modeling these X-ray new-type, or low observed X-ray scattering (hereafter:low-scattering) sources have been interpreted as deeply-buried AGN with a high covering factor of gas. In this paper we address the questions whether the host galaxies of low-scattering AGN may contribute to the observed X-ray properties, and whether we can find any direct evidence for high covering factors from the infrared (IR) emission. We find that X-ray low-scattering AGN are preferentially hosted by highly-inclined galaxies or merger systems as compared to other Seyfert galaxies, increasing the likelihood that the line-of-sight toward the AGN intersects with high columns of host-galactic gas and dust. Moreover, while a detailed analysis of the IR emission of low-scattering AGN ESO 103-G35 remains inconclusive, we do not find any indication of systematically higher dust covering factors in a sample of low-scattering AGN based on their IR emission. For ESO 103-G35, we constrained the temperature, mass and location of the IR emitting dust which is consistent with expectations for the dusty torus. However, a deep silicate absorption feature probably from much cooler dust suggests an additional screen absorber on larger scales within the host galaxy. Taking these findings together, we propose that the low f_scat observed in low-scattering AGN is not necessarily the result of circumnuclear dust but could originate from interference of host-galactic gas with a column density of the order of 10^22 cm^-2 with the line-of-sight. We discuss implications of this hypothesis for X-ray models, high-ionization emission lines, and observed star-formation activity in these objects.
We present results on a systematic study of flux variability on hourly time-scales in a large sample of active galactic nuclei (AGN) in the 3-79 keV band using data from Nuclear Spectroscopic Telescope Array. Our sample consists of 4 BL Lac objects (BL Lacs), 3 flat spectrum radio quasars (FSRQs) 24 Seyfert 1, 42 Seyfert 2 and 8 narrow line Seyfert 1 (NLSy1) galaxies. We find that in the 3-79 keV band, about 65% of the sources in our sample show significant variations on hourly time scales. Using Mann-Whitney U-test and Kolmogorov-Smirnov test, we find no difference in the variability behaviour between Seyfert 1 and 2 galaxies. The blazar sources (FSRQs and BL Lacs) in our sample, are more variable than Seyfert galaxies that include Seyfert 1 and Seyfert 2 in the soft (3-10 keV), hard (10-79 keV) and total (3-79 keV) bands. NLSy1 galaxies show the highest duty cycle of variability (87%), followed by BL Lacs (82%), Seyfert galaxies (56%) and FSRQs (23%). We obtained flux doubling/halving time in the hard X-ray band less than 10 min in 11 sources. The flux variations between the hard and soft bands in all the sources in our sample are consistent with zero lag.
We report the results of a Sloan Digital Sky Survey-IV eBOSS program to target X-ray sources and mid-infrared-selected WISE AGN candidates in a 36.8 deg$^2$ region of Stripe 82. About half this survey (15.6 deg$^2$) covers the largest contiguous portion of the Stripe 82 X-ray survey. This program represents the largest spectroscopic survey of AGN candidates selected solely by their WISE colors. We combine this sample with X-ray and WISE AGN in the field identified via other sources of spectroscopy, producing a catalog of 4847 sources that is 82% complete to $rsim22$. Based on X-ray luminosities or WISE colors, 4730 of these sources are AGN, with a median sample redshift of $zsim1$. About 30% of the AGN are optically obscured (i.e., lack broad lines in their optical spectra). BPT analysis, however, indicates that 50% of the WISE AGN at $z<0.5$ have emission line ratios consistent with star-forming galaxies, so whether they are buried AGN or star-forming galaxy contaminants is currently unclear. We find that 61% of X-ray AGN are not selected as MIR AGN, with 22% of X-ray AGN undetected by WISE. Most of these latter AGN have high X-ray luminosities ($L_{rm x} > 10^{44}$ erg s$^{-1}$), indicating that MIR selection misses a sizable fraction of the highest luminosity AGN, as well as lower luminosity sources where AGN heated dust is not dominating the MIR emission. Conversely, $sim$58% of WISE AGN are undetected by X-rays, though we do not find that they are preferentially redder than the X-ray detected WISE AGN.
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