The X-ray spectra of many active galactic nuclei (AGNs) show a soft X-ray excess below 1–2 keV on top of the extrapolated high-energy power law. The origin of this component is uncertain. It could be ...a signature of relativistically blurred, ionized reflection or the high-energy tail of thermal Comptonization in a warm (kT ~ 1 keV), optically thick (τ ≃ 10–20) corona producing the optical/UV to soft X-ray emission. The purpose of the present paper is to test the warm corona model on a statistically significant sample of unabsorbed, radio-quiet AGNs with XMM-Newton archival data, providing simultaneous optical/UV and X-ray coverage. The sample has 22 objects and 100 observations. We use two thermal Comptonization components to fit the broadband spectra, one for the warm corona emission and one for the high-energy continuum. In the optical/UV, we also include the reddening, the small blue bump, and the Galactic extinction. In the X-rays, we include a warm absorber and a neutral reflection. The model gives a good fit (reduced χ2 < 1.5) to more than 90% of the sample. We find the temperature of the warm corona to be uniformly distributed in the 0.1–1 keV range, while the optical depth is in the range ~10–40. These values are consistent with a warm corona covering a large fraction of a quasi-passive accretion disk, i.e., that mostly reprocesses the warm corona emission. The disk intrinsic emission represents no more than 20% of the disk total emission. According to this interpretation, most of the accretion power would be released in the upper layers of the accretion flow.
A soft X-ray excess above the 2–10 keV power-law extrapolation is generally observed in the X-ray spectra of active galactic nuclei. The origin of this excess is still not well understood. Presently ...there are two competitive models: blurred ionized reflection and warm Comptonization. In the case of warm Comptonization, observations suggest a corona temperature in the range 0.1–2 keV and a corona optical depth of about 10–20. Moreover, radiative constraints from spectral fits with Comptonization models suggest that most of the accretion power should be released in the warm corona and the disk below is basically non-dissipative, radiating only the reprocessed emission from the corona. However, the true radiative properties of such a warm and optically thick plasma are not well known. For instance, the importance of the Comptonization process, the potential presence of strong absorption and/or emission features, and the spectral shape of the output spectrum have been studied only very recently. Here, we present simulations of warm and optically thick coronae using the
TITAN
radiative transfer code coupled with the
NOAR
Monte-Carlo code, the latter fully accounting for Compton scattering of continuum and lines. Illumination from above by hard X-ray emission and from below by an optically thick accretion disk are taken into account, as well as (uniform) internal heating. Our simulations show that for a large part of the parameter space, the warm corona with sufficient internal mechanical heating is dominated by Compton cooling and neither strong absorption nor emission lines are present in the outgoing spectra. In a smaller part of the parameter space, the calculated emission agrees with the spectral shape of the observed soft X-ray excess. Remarkably, this also corresponds to the conditions of radiative equilibrium of an extended warm corona covering a non-dissipative accretion disk almost entirely. These results confirm that warm Comptonization is a valuable model that can explain the origin of the soft X-ray excess.
Context. The primary X-ray emission in active galactic nuclei (AGNs) is widely believed to be due to Comptonisation of the thermal radiation from the accretion disc in a corona of hot electrons. The ...resulting spectra can, in first approximation, be modelled with a cut-off power law, the photon index and the high-energy roll-over encoding information on the physical properties of the X ray emitting region. The photon index and the high-energy curvature of AGNs (Γ, Ec) have been largely studied since the launch of X-ray satellites operating above 10 keV. However, high-precision measurements of these two observables have only been obtained in recent years thanks to the unprecedented sensitivity of NuSTAR up to 79 keV. Aims. We aim at deriving relations between Γ, Ec phenomenological parameters and the intrinsic properties of the X-ray-emitting region (the hot corona), namely the optical depth and temperature. Methods. We use MoCA (Monte Carlo code for Comptonisation in Astrophysics) to produce synthetic spectra for the case of an AGN with MBH = 1.5 × 108 M⊙ and ṁ = 0.1 and then compared them with the widely used power-law model with an exponential high-energy cutoff. Results. We provide phenomenological relations relating Γ and Ec with the opacity and temperature of the coronal electrons for the case of spherical and slab-like coronae. These relations give origin to a well defined parameter space which fully contains the observed values. Exploiting the increasing number of high-energy cut-offs quoted in the literature, we report on the comparison of physical quantities obtained using MoCA with those estimated using commonly adopted spectral Comptonisation models. Finally, we discuss the negligible impact of different black hole masses and accretion rates on the inferred relations.
The spectral and timing properties of an oscillating hot thermal corona are investigated. This oscillation is assumed to be due to a magneto-acoustic wave propagating within the corona and triggered ...by an external, non-specified, excitation. A cylindrical geometry is adopted and, neglecting the rotation, the wave equation is solved for different boundary conditions. The resulting X-ray luminosity, through thermal Comptonization of embedded soft photons, is then computed analytically, assuming linear dependence between the local pressure disturbance and the radiative modulation. These calculations are also compared to Monte Carlo simulations. The main results of this study are as follows. (1) The corona plays the role of a low bandpass medium, its response to a white noise excitation being a flat-top noise power spectral density (PSD) at low frequencies and a red noise at high frequency. (2) Resonant peaks are present in the PSD. Their powers depend on the boundary conditions chosen and, more specifically, on the impedance adaptation with the external medium at the corona inner boundary. (3) The flat-top noise level and break as well as the resonant peak frequencies are inversely proportional to the external radius rj. (4) Computed rms and f-spectra exhibit an overall increase in the variability with energy. Comparison with observed variability features, especially in the hard-intermediate states of X-ray binaries, are discussed.
Supermassive black holes in the nuclei of active galaxies expel large amounts of matter through powerful winds of ionized gas. The archetypal active galaxy NGC 5548 has been studied for decades, and ...high-resolution x-ray and ultraviolet (UV) observations have previously shown a persistent ionized outflow. An observing campaign in 2013 with six space observatories shows the nucleus to be obscured by a long-lasting, clumpy stream of ionized gas not seen before. It blocks 90% of the soft x-ray emission and causes simultaneous deep, broad UV absorption troughs. The outflow velocities of this gas are up to five times faster than those in the persistent outflow, and, at a distance of only a few light days from the nucleus, it may likely originate from the accretion disk.
Context. We discuss the results of the hot corona parameters of active galactic nuclei (AGN) that have been recently measured with NuSTAR. The values taken from the literature of a sample of 19 ...bright Seyfert galaxies are analysed. Aims. The aim of this work is to look for correlations between coronal parameters, such as the photon index and cut-off energy (when a phenomenological model is adopted) or the optical depth and temperature (when a Comptonization model is used), and other parameters of the systems, such as the black hole mass or the Eddington ratio. Methods. We analysed the coronal parameters of the 19 unobscured, bright Seyfert galaxies that are present in the Swift/BAT 70-month catalogue and that have been observed by NuSTAR, alone or simultaneously with others X-ray observatories, such as Swift, Suzaku, or XMM-Newton. Results. We found an anti-correlation with a significance level >98% between the coronal optical depth and the coronal temperature of our sample. On the other hand, no correlation between the above parameters and the black hole mass, the accretion rate, and the intrinsic spectral slope of the sources is found.
Context. In past years, several observations of AGN and X-ray binaries suggested the existence of a warm (T ~ 0.5 − 1 keV) and optically thick (τcor ~ 10 − 20) corona covering the inner parts of the ...accretion disk. These properties are directly derived from spectral fitting in UV to soft-X-rays using Comptonization models. However, whether such a medium can be both in radiative and hydrostatic equilibrium with an accretion disk is still uncertain. Aims. We investigate the properties of such warm, optically thick coronae and put constraints on their existence. Methods. We solve the radiative transfer equation for grey atmosphere analytically in a pure scattering medium, including local dissipation as an additional heating term in the warm corona. The temperature profile of the warm corona is calculated assuming that it is cooled by Compton scattering, with the underlying dissipative disk providing photons to the corona. Results. Our analytic calculations show that a dissipative thick corona (τcor in the range 10–12) on top of a standard accretion disk can reach temperatures of the order of 0.5–1 keV in its upper layers provided that the disk is passive. However, in the absence of strong magnetic fields, the requirement of a Compton cooled corona in hydrostatic equilibrium in the vertical direction sets an upper limit on the Thomson optical depth τcor ≲ 5. We show that this value cannot be exceeded independently of the accretion disk parameters. However, magnetic pressure can extend this result to larger optical depths. Namely, a dissipative corona might have an optical depth up to ~20 when the magnetic pressure is 100 times higher than the gas pressure. Conclusions. The observation of warm coronae with Thomson depth larger than ≃5 puts tight constraints on the physics of the accretion disk/corona systems and requires either strong magnetic fields or vertical outflows to stabilize the system.
Context
. It has been suggested that the cycles of activity of X-ray binaries (XRB) are triggered by a switch in the dominant disk torque responsible for accretion. As the disk accretion rate ...increases, the disk innermost regions therefore change from a jet-emitting disk (JED) to a standard accretion disk (SAD).
Aims
. While JEDs have been proven to successfully reproduce X-ray binary hard states, the existence of an outer cold SAD introduces an extra nonlocal cooling term. We investigate the thermal structure and associated spectra of such a hybrid disk configuration.
Methods
. We use a two-temperature plasma code, allowing for outside-in computation of the disk local thermal equilibrium with self-consistent advection and optically thin-to-thick transitions in both radiation and gas supported regimes. The nonlocal inverse Compton cooling introduced by the external soft photons is computed by the BELM code.
Results
. This additional cooling term has a profound influence on JED solutions, allowing a smooth temperature transition from the outer SAD to the inner JED. We explore the full parameter space in disk accretion rate and transition radius, and show that the whole domain in X-ray luminosities and hardness ratios covered by standard XRB cycles is well reproduced by such hybrid disk configurations. Precisely, a reasonable combination of these parameters allows us to reproduce the 3–200 keV spectra of each of five canonical XRB states. Along with these X-ray signatures, JED-SAD configurations also naturally account for the radio emission whenever it is observed.
Conclusions
. By varying only the radial transition radius and the accretion rate, hybrid disk configurations combining an inner JED and an outer SAD are able to simultaneously reproduce the X-ray spectral states and radio emission of X-ray binaries during their outburst. Adjusting these two parameters, it is then possible to reproduce a full cycle. This will be shown in a forthcoming paper.
In 2016 we carried out a Swift monitoring programme to track the X-ray hardness variability of eight type-I AGN over a year. The purpose of this monitoring was to find intense obscuration events in ...AGN, and thereby study them by triggering joint XMM-Newton, NuSTAR, and HST observations. We successfully accomplished this for NGC 3783 in December 2016. We found heavy X-ray absorption produced by an obscuring outflow in this AGN. As a result of this obscuration, interesting absorption features appear in the UV and X-ray spectra, which are not present in the previous epochs. Namely, the obscuration produces broad and blue-shifted UV absorption lines of Lyα, C iv, and N v, together with a new high-ionisation component producing Fe xxv and Fe xxvi absorption lines. In soft X-rays, only narrow emission lines stand out above the diminished continuum as they are not absorbed by the obscurer. Our analysis shows that the obscurer partially covers the central source with a column density of few 1023 cm-2, outflowing with a velocity of few thousand km s-1. The obscuration in NGC 3783 is variable and lasts for about a month. Unlike the commonly seen warm-absorber winds at pc-scale distances from the black hole, the eclipsing wind in NGC 3783 is located at about 10 light days. Our results suggest that the obscuration is produced by an inhomogeneous and clumpy medium, consistent with clouds in the base of a radiatively driven disk wind at the outer broad-line region of the AGN.
We present results from a joint XMM–Newton/NuSTAR monitoring of the Seyfert 1 NGC 4593, consisting of 5 × 20 ks simultaneous observations spaced by 2 d, performed in 2015 January. The source is ...variable, both in flux and spectral shape, on time-scales down to a few ks and with a clear softer-when-brighter behaviour. In agreement with past observations, we find the presence of a warm absorber well described by a two-phase ionized outflow. The source exhibits a cold, narrow and constant Fe Kα line at 6.4 keV, and a broad component is also detected. The broad-band (0.3–79 keV) spectrum is well described by a primary power law with Γ ≃ 1.6–1.8 and an exponential cut-off varying from
$90^{+ 40}_{- 20}$
to >700 keV, two distinct reflection components, and a variable soft excess correlated with the primary power law. This campaign shows that probing the variability of Seyfert 1 galaxies on different time-scales is of prime importance to investigate the high-energy emission of active galactic nuclei.
PDF
