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Register a new userThe physics and astrophysics of X-ray outflows from Active Galactic Nuclei
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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.
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Outflows from active galactic nuclei (AGN) are one of the fundamental mechanisms by which the central supermassive black hole interacts with its host galaxy. Detected in $ge 50%$ of nearby AGN, these outflows have been found to carry kinetic energy that is a significant fraction of AGN power, and thereby give negative feedback to their host galaxies. To understand the physical processes that regulate them, it is important to have a robust estimate of their physical and dynamical parameters. In this review we summarize our current understanding on the physics of the ionized outflows detected in absorption in the UV and X-ray wavelength bands. We discuss the most relevant observations and our current knowledge and uncertainties in the measurements of the outflow parameters. We also discuss their origin and acceleration mechanisms. The commissioning and concept studies of large telescope missions with high resolution spectrographs in UV/optical and X-rays along with rapid advancements in simulations offer great promise for discoveries in this field over the next decade.
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.
The central engines of Active Galactic Nuclei (AGNs) are powered by accreting supermassive black holes, and while AGNs are known to play an important role in galaxy evolution, the key physical processes occur on scales that are too small to be resolved spatially (aside from a few exceptional cases). Reverberation mapping is a powerful technique that overcomes this limitation by using echoes of light to determine the geometry and kinematics of the central regions. Variable ionizing radiation from close to the black hole drives correlated variability in surrounding gas/dust, but with a time delay due to the light travel time between the regions, allowing reverberation mapping to effectively replace spatial resolution with time resolution. Reverberation mapping is used to measure black hole masses and to probe the innermost X-ray emitting region, the UV/optical accretion disk, the broad emission line region and the dusty torus. In this article we provide an overview of the technique and its varied applications.
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.
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