Abstract
The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous active galactic nuclei. The continuum ...has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electron–positron pair production, as explored earlier (Paper I). Here, we examine hybrid plasmas in which a mixture of thermal and non-thermal particles is present. Pair production from the non-thermal component reduces the temperature leading to a wider temperature range more consistent with observations.
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.
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. 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.
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. High resolution X-ray spectra of black hole X-ray binaries (BHBs) show blueshifted absorption lines suggesting the presence of outflowing winds. Furthermore, observations show that the disk ...winds are equatorial and they occur in the Softer (disk dominated) states of the outburst and are less prominent or absent in the Harder (power-law dominated) states. Aims. We want to test whether the self-similar magneto-hydrodynamic (MHD) accretion-ejection models can explain the observational results for accretion disk winds in BHBs. In our models, the density at the base of the outflow from the accretion disk is not a free parameter. This mass loading is determined by solving the full set of dynamical MHD equations without neglecting any physical term. Thus, the physical properties of the outflow depend on and are controlled by the global structure of the disk. Methods. We studied different MHD solutions characterized by different values of the disk aspect ratio (ε) and the ejection efficiency (p). We also generate two kinds of MHD solutions depending on the absence (cold solution) or presence (warm solution) of heating at the disk surface. Such heating could be either from dissipation of energy due to MHD turbulence in the disk or from illumination of the disk surface. Warm solutions can have large (>0.1) values of p, which would imply larger wind mass loading at the base of the outflow. We use each of these MHD solutions to predict the physical parameters (distance, density, velocity, magnetic field, etc.) of an outflow. Motivated by observational results, we have put limits on the ionization parameter (ξ), column density, and timescales. Further constraints were derived for the allowed values of ξ from thermodynamic instability considerations, particularly for the Hard SED. These physical constraints were imposed on each of these outflows to select regions within it, which are consistent with the observed winds. Results. The cold MHD solutions are found to be inadequate and cannot account for winds because of their low ejection efficiency. On the contrary, warm solutions can have sufficiently high values of p( ≳ 0.1), which are required to explain the observed physical quantities in the wind. From our thermodynamic equilibrium curve analysis for the outflowing gas, we find that in the Hard state a range of ξ is unstable. This constraint makes it impossible to have any wind at all in the Hard state. Conclusions. Using the MHD outflow models we are able to explain the observed trends, i.e. that the winds are equatorial and that they are observable in the Soft states (and not expected in the Hard state) of the BHB outbursts.
ABSTRACT
Although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. Recent work suggests that Type I X-ray bursts ...from accreting neutron stars provide an opportunity to probe the characteristics of coronae. Several studies have observed hard X-ray shortages from the accretion disc during an X-ray burst implying strong coronal cooling by burst photons. Here, we use the plasma emission code eqpair to study the impact of X-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. Assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of ≳ 10 and cause a reduction of emission in the 30–50 keV band to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. This hard X-ray drop is intensified when the coronal optical depth and aspect ratio is increased. In contrast, depending on the properties of the burst and corona, the emission in the 8–24 keV band can either increase, by a factor of ≳ 20, or decrease, down to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. An increasing accretion rate during the X-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $\gtrsim 60{{\ \rm per\ cent}}$. These results indicate that changes of the hard X-ray flux during an X-ray burst probe the geometrical properties of the corona.
It is widely believed that the primary X-ray emission of AGN is due to the Comptonization of optical-UV photons from a hot electron corona, while the origin of the ‘soft-excess’ is still uncertain ...and matter of debate. A second Comptonization component, called warm corona, was therefore proposed to account for the soft-excess, and found in agreement with the optical-UV to X-ray emission of a sample of Seyfert galaxies. In this context, we exploit the broad-band XMM–Newton and NuSTAR simultaneous observations of the Seyfert galaxy NGC 4593 to further test the so-called ‘two corona model’. The NGC 4593 spectra are well reproduced by the model, from the optical/UV to the hard X-rays. Moreover, the data reveal a significant correlation between the hot and the warm corona parameters during our monitoring campaign.
The narrow-line Seyfert 1 galaxy IRAS 13224−3809 has been observed with XMM-Newton for 500 ks. The source is rapidly variable on time-scales down to a few 100 s. The spectrum shows strong broad Fe − ...K
and L emission features which are interpreted as arising from reflection from the inner parts of an accretion disc around a rapidly spinning black hole. Assuming a power law emissivity for the reflected flux and that the innermost radius corresponds to the innermost stable circular orbit, the black hole spin is measured to be 0.989 with a statistical precision better than 1 per cent. Systematic uncertainties are discussed. A soft X-ray lag of 100 s confirms this scenario. The bulk of the power-law continuum source is located at a radius of 2-3 gravitational radii.
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