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Register a new userRadiation Magnetohydrodynamic Simulations of Sub-Eddington accretion Flows in AGN: Origin of Soft X-ray Excess and Rapid Time Variabilities
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We investigate the origin of the soft X-ray excess component in Seyfert galaxies observed when their luminosity exceeds 0.1% of the Eddington luminosity ($L_{mathrm{Edd}}$). The evolution of a dense blob in radiatively inefficient accretion flow (RIAF) is simulated by applying a radiation magnetohydrodynamic code, CANS+R. When the accretion rate onto a $10^7M_{odot}$ black hole exceeds 10% of the Eddington accretion rate ($dot M_{rm Edd}=L_{rm Edd}/c^2$, where $c$ is the speed of light)}, the dense blob shrinks vertically because of radiative cooling and forms a Thomson thick, relatively cool ($sim10^{7-8}$ K) region. The cool region coexists with the optically thin, hot ($Tsim10^{11}~mathrm{K}$) RIAF near the black hole. The cool disk is responsible for the soft X-ray emission, while hard X-rays are emitted from the hot inner accretion flow. The soft X-ray emitting region coexists with the optically thin, hot ($T sim 10^{11}~mathrm{K}$), radiatively inefficient accretion flow (RIAF) near the black hole. Such a hybrid structure of hot and cool accretion flows is consistent with the observations of both hard and soft X-ray emissions from `changing-look active galactic nuclei (CLAGN). Furthermore, we find that quasi-periodic oscillations (QPOs) are excited in the soft X-ray emitting region. These oscillations can be the origin of rapid X-ray time variabilities observed in CLAGN.
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(Abridged) Narrow Line Seyfert 1 (NLS1) galaxies have low mass black holes and mass accretion rates close to (or exceeding) Eddington, so a standard blackbody accretion disc should peak in the EUV. However, the lack of true absorption opacity in the disc means that the emission is better approximated by a colour temperature corrected blackbody, and this colour temperature correction is large enough ($sim 2.4$) that the bare disc emission from a zero spin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray excess seen in these objects must be intrinsic emission from the disc unless the vertical structure is very different to that predicted. However, the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by cool, optically thick material. We associate this with the disc itself, so it must ultimately be powered by mass accretion. We build an energetically self consistent model assuming that the emission thermalises at large radii, but that at smaller radii the gravitational energy is split between powering optically thick Comptonised disc emission (forming the soft X-ray excess) and an optically thin corona above the disc (forming the tail to higher energies). We show examples of this model fit to the extreme NLS1 REJ1034+396, and to the much lower Eddington fraction Broad Line Seyfert 1 PG1048+231. We use these to guide our fits and interpretations of three template spectra made from co-adding multiple sources to track out a sequence of AGN spectra as a function of $L/L_{Edd}$. The new model is publically available within the {sc xspec} spectral fitting package.
The X-ray spectra of many active galactic nuclei (AGN) 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 $sim$ 1 keV), optically thick ($tausimeq$ 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 AGN 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 broad-band 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 WA and a neutral reflection. The model gives a good fit (reduced $chi^2 <1.5$) 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 $sim$10-40. These values are consistent with a warm corona covering a large fraction of a quasi-passive accretion disc, 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.
We use global three dimensional radiation magneto-hydrodynamic simulations to study the properties of inner regions of accretion disks around a 5times 10^8 solar mass black hole with mass accretion rates reaching 7% and 20% of the Eddington value. This region of the disk is supported by magnetic pressure with surface density significantly smaller than the values predicted by the standard thin disk model but with a much larger disk scale height. The disks do not show any sign of thermal instability over many thermal time scales. More than half of the accretion is driven by radiation viscosity in the optically thin corona region for the lower accretion rate case, while accretion in the optically thick part of the disk is driven by the Maxwell and Reynolds stresses from MRI turbulence. Coronae with gas temperatures > 10^8 K are generated only in the inner approx 10 gravitational radii in both simulations, being more compact in the higher accretion rate case. In contrast to the thin disk model, surface density increases with increasing mass accretion rate, which causes less dissipation in the optically thin region and a relatively weaker corona. The simulation results may explain the formation of X-ray coronae in Active Galactic Nuclei (AGNs), the compact size of such coronae, and the observed trend of optical to X-ray luminosity with Eddington ratio for many AGNs.
X-ray reverberation is a powerful technique which maps out the structure of the inner regions of accretion disks around black holes using the echoes of the coronal emission reflected by the disk. While the theory of X-ray reverberation has been developed almost exclusively for standard thin disks, recently reverberation lags have been observed from likely super-Eddington accretion sources such as the jetted tidal disruption event Swift J1644+57. In this paper, we extend X-ray reverberation studies into the super-Eddington accretion regime, focusing on investigating the lags in the Fe K{alpha} line region. We find that the coronal photons are mostly reflected by the fast and optically thick winds launched from super-Eddington accretion flow, and this funnel-like reflection geometry produces lag-frequency and lag-energy spectra with unique characteristics. The lag-frequency spectra exhibits a step-function like decline near the first zero-crossing point. As a result, the shape of the lag-energy spectra remains almost independent of the choice of frequency bands and linearly scales with the black hole mass for a large range of parameter spaces. Not only can these morphological differences be used to distinguish super-Eddington accretion systems from sub-Eddington systems, they are also key for constraining the reflection geometry and extracting parameters from the observed lags. When explaining the X-ray reverberation lags of Swift J1644+57, we find that the super-Eddington disk geometry is preferred over the thin disk, for which we obtain a black hole mass of 5-6 million solar masses and a coronal height around 10 gravitational radii by fitting the lag spectra to our modeling.
We present for the first time the timing and spectral analyses for a narrow-line Seyfert 1 galaxy, SBS 1353+564, using it{XMM-Newton} and it{Swift} multi-band observations from 2007 to 2019. Our main results are as follows: 1) The temporal variability of SBS 1353+564 is random, while the hardness ratio is relatively constant over a time span of 13 years; 2) We find a prominent soft X-ray excess feature below 2 keV, which cannot be well described by a simple blackbody component; 3) After comparing the two most prevailing models for interpreting the origin of the soft X-ray excess, we find that the relativistically smeared reflection model is unable to fit the data above 5 keV well and the X-ray spectra do not show any reflection features, such as the Fe Kalpha emission line. However, the warm corona model can obtain a good fitting result. For the warm corona model, we try to use three different sets of spin values to fit the data and derive different best-fitting parameter sets; 4) We compare the UV/optical spectral data with the extrapolated values of the warm corona model to determine which spin value is more appropriate for this source, and we find that the warm corona model with non-spin can sufficiently account for the soft X-ray excess in SBS 1353+564.
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