204 research outputs found
A search for X-ray reprocessing echoes in the power spectral density functions of AGN
We present the results of a detailed study of the X-ray power spectra density
(PSD) functions of twelve X-ray bright AGN, using almost all the archival
XMM-Newton data. The total net exposure of the EPIC-pn light curves is larger
than 350 ks in all cases (and exceeds 1 Ms in the case of 1H 0707-497). In a
physical scenario in which X-ray reflection occurs in the inner part of the
accretion disc of AGN, the X-ray reflection component should be a filtered echo
of the X-ray continuum signal and should be equal to the convolution of the
primary emission with the response function of the disc. Our primary objective
is to search for these reflection features in the 5-7 keV (iron line) and 0.5-1
keV (soft) bands, where the X-ray reflection fraction is expected to be
dominant. We fit to the observed periodograms two models: a simple bending
power law model (BPL) and a BPL model convolved with the transfer function of
the accretion disc assuming the lamp-post geometry and X-ray reflection from a
homogeneous disc. We do not find any significant features in the best-fitting
BPL model residuals either in individual PSDs in the iron band, soft and full
band (0.3-10 keV) or in the average PSD residuals of the brightest and more
variable sources (with similar black hole mass estimates). The typical
amplitude of the soft and full-band residuals is around 3-5 per cent. It is
possible that the expected general relativistic effects are not detected
because they are intrinsically lower than the uncertainty of the current PSDs,
even in the strong relativistic case in which X-ray reflection occurs on a disc
around a fast rotating black hole having an X-ray source very close above it.
However, we could place strong constrains to the X-ray reflection geometry with
the current data sets if we knew in advance the intrinsic shape of the X-ray
PSDs, particularly its high frequency slope.Comment: Accepted for publication in MNRAS. The paper is 15 pages long and
contains 7 figures and 6 table
X-ray polarization from parsec-scale components of active galactic nuclei: observational prospects
We present a broad analysis of X-ray polarimetric observational prospects for
radio-quiet active galactic nuclei (AGN), focusing on the role of parsec-scale
components. We provide a revision of self-consistent type-1 and type-2 generic
AGN radiative transfer models that were obtained with a Monte Carlo code
STOKES, evaluating the effects of absorption and scattering. Our model consists
of a central disc-corona emission obtained with the KYNSTOKES code in the
lamp-post geometry, an equatorial wedge-shaped dusty torus and two symmetric
conical polar outflows. We argue that the information on the mutual
orientation, shape, relative size and composition of such components, usually
obtained from spectroscopy or polarimetry in other wavelengths, is essential
for the X-ray polarization analysis of the obscured type-2 AGNs. We provide
general detectability prospects for AGNs with 2-8 keV polarimeters on board of
the currently flying IXPE satellite and the forthcoming eXTP mission. Finally,
we assess the role of contemporary X-ray polarimetry in our understandings of
the unified AGN model after the first year and a half of IXPE operation.Comment: 20 pages, 31 figures, accepted for publication in MNRA
Constraints on a strong X-ray flare in the Seyfert galaxy MCG-6-30-15
We discuss implications of a strong flare event observed in the Seyfert
galaxy MCG-6-30-15 assuming that the emission is due to localized magnetic
reconnection. We conduct detailed radiative transfer modeling of the
reprocessed radiation for a primary source that is elevated above the disk. The
model includes relativistic effects and Keplerian motion around the black hole.
We show that for such a model setup the observed time-modulation must be
intrinsic to the primary source. Using a simple analytical model we then
investigate time delays between hard and soft X-rays during the flare. The
model considers an intrinsic delay between primary and reprocessed radiation,
which measures the geometrical distance of the flare source to the reprocessing
sites. The observed time delays are well reproduced if one assumes that the
reprocessing happens in magnetically confined, cold clouds.Comment: 4 pages, 2 figures, proceedings of a talk given at the symposium 238
at the IAU General Assembly 200
Thermal disc emission from a rotating black hole: X-ray polarization signatures
Thermal emission from the accretion disc around a black hole can be
polarized, due to Thomson scattering in a disc atmosphere. In Newtonian space,
the polarization angle must be either parallel or perpendicular to the
projection of the disc axis on the sky. As first pointed out by Stark and
Connors in 1977, General Relativity effects strongly modify the polarization
properties of the thermal radiation as observed at infinity. Among these
effects, the rotation of the polarization angle with energy is particularly
useful as a diagnostic tool.
In this paper, we extend the Stark and Connors calculations by including the
spectral hardening factor, several values of the optical depth of the
scattering atmosphere and rendering the results to the expected performances of
planned X-ray polarimeters. In particular, to assess the perspectives for the
next generation of X-ray polarimeters, we consider the expected sensitivity of
the detectors onboard the planned POLARIX and IXO missions. We assume the two
cases of a Schwarzschild and an extreme Kerr black hole with a standard thin
disc and a scattering atmosphere. We compute the expected polarization degree
and the angle as functions of the energy as they could be measured for
different inclinations of the observer, optical thickness of the atmosphere and
different values of the black hole spin. We assume the thermal emission
dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105
in the thermal state, we calculate the observed polarization.Comment: 8 pages, 7 figures, accepted by MNRA
A theoretical study of the time-lags due to Comptonization and the constraints on the X-ray corona in AGN
We study the Fourier time-lags due to the Comptonization of disc-emitted
photons in a spherical, uniform, and stationary X-ray corona, which located on
the rotational axis of the black hole. We use Monk, a general relativistic
Monte-Carlo radiative transfer code, to calculate Compton scattering of photons
emitted by a thin disc with a Novikov-Thorne temperature profile. We find that
the model time-lags due to Comptonization remain constant up to a
characteristic frequency and then rapidly decrease to zero at higher
frequencies. We provide equations which can be used to determine the time-lags
and cross spectra for a wide range of values for the corona radius,
temperature, optical depth, height, and for various accretion rates and black
hole masses. We also provide an equation for the X-ray luminosity of a single
corona, as a function of the its characteristics and location above the disc.
Remarkably, the observed X-ray time-lags of nearby, bright active galaxies can
be successfully reproduced by inverse Comptonization process of multiple
dynamic coronae.Comment: 16 pages, 17 figures; accepted for publication in MNRA
Steep X-ray reflection emissivity profiles in AGN as the result of radially structured disc ionization
The flare model for X-ray variability of NGC 4258
We study the variability mechanism of active galactic nuclei (AGN) within the
framework of the flare model. To this end we examine the case of Seyfert/LINER
galaxy NGC 4258, which is observed at high inclination angle and exhibits rapid
fluctuations of the X-ray light curve. We construct a model light curve based
on the assumption of magnetic flares localized in the equatorial plane and
orbiting with Keplerian speed at each given radius. We calculate the level of
variability as a function of the inclination of an observer, taking into
account all effects of general relativity near a rotating supermassive black
hole. The variability level is a monotonic function of the source inclination.
It rises more rapidly for larger values of the black hole spin (Kerr parameter)
and for steeper emissivity (index beta of the radial profile). We compare the
expected level of variability for the viewing angle 81.6 deg, as inferred for
NGC 4258, with the case of moderate viewing angles about 30 deg, typical for
Seyfert type-1 galaxies. Highly inclined sources such as this one are
particularly suitable to test the flare model because the effects of orbital
motion, Doppler boosting and light bending are all expected to have maximum
when the accretion disk is seen almost edge-on. The model is consistent with
the NGC 4258 variability, where the obscuring material is thought to be
localized mainly towards the equatorial plane rather than forming a
geometrically thick torus. Once the intrinsic time-scales of the flare duration
are determined to better precision, this kind of highly inclined objects with a
precisely known mass of the black hole can be used to set independent
constraints on the spin parameter.Comment: 7 pages, 3 figures; Astronomy & Astrophysics (this version includes
minor language corrections
Revisiting UV/optical continuum time lags in AGN
In this paper, we present an updated version of our model (KYNXiltr) which
considers thermal reverberation of a standard Novikov-Thorne accretion disc
illuminated by an X-ray point-like source. Previously, the model considered
only two cases of black hole spins, and assumed a colour correction factor
. Now, we extend the model to any spin value and colour
correction. In addition, we consider two scenarios of powering the X-ray
corona, either via accretion, or external to the accretion disc. We use
KYNXiltr to fit the observed time lags obtained from intense monitoring of four
local Seyfert galaxies (NGC 5548, NGC 4395, Mrk 817, and Fairall 9). We
consider various combinations of black hole spin, colour correction, corona
height, and fraction of accretion power transferred to the corona. The model
fits well the overall time-lags spectrum in these sources (for a large
parameter space). For NGC 4593 only, we detect a significant excess of delays
in the U-band. The contribution of the diffuse BLR emission in the time-lags
spectrum of this source is significant. It is possible to reduce the large
best-fitting parameter space by combining the results with additional
information, such as the observed Eddington ratio and average X-ray luminosity.
We also provide an update to the analytic expression provided by Kammoun et
al., for an X-ray source that is not powered by the accretion process, which
can be used for any value of colour correction, and for two values of the black
hole spin (0 and 0.998).Comment: Accepted for publication in MNRA
The model constraints from the observed trends for the quasi-periodic oscillation in RE J1034+396
We analyze the time variability of the X-ray emission of RE J1034+396 -- an
active galactic nucleus with the first firm detection of a quasi-periodic
oscillations (QPO). Based on the results of a wavelet analysis, we find a drift
in the QPO central frequency. The change in the QPO frequency correlates with
the change in the X-ray flux with a short time delay. The data specifically
suggest a linear dependence between the QPO period and the flux, and this gives
important constraints on the QPO models. In particular, it excludes explanation
in terms of the orbiting hot spot model close to a black hole. Linear
structures such as shocks, spiral waves, or very distant flares are favored.Comment: Astronomy & Astrophysics, in pres
Signatures of X-ray reverberation in the power spectra of AGN
Aims: we study the effects of X-ray reprocessing in the power spectra (PSDs) of active galactic nuclei (AGNs).Methods: we compute fully relativistic disc response functions in the case of lamp-post geometry using the full observed reflection spectrum for various X-ray source heights, disc inclination, and spin values of the central black hole. Since the observed PSD is equal to the product of the intrinsic power spectrum with the transfer function (i.e. the Fourier transform of the disc response function), we are able to predict the observed PSDs in the case of X-ray illumination of the inner disc.Results: the observed PSD should show a prominent dip at high frequencies and an oscillatory behaviour with a decreasing amplitude at higher frequencies. The reverberation echo features should be more prominent in energy bands where the reflection component is more pronounced. The frequency of the dip is independent of energy, and it is mainly determined by the black hole mass and the X-ray source height. The amplitude of the dip increases with increasing black hole spin and inclination angle, as long as the height of the lamp is smaller than ~10 gravitational radii.Conclusions: the detection of the X-ray reverberation signals in the PSDs can provide further evidence for X-ray illumination of the inner disc in AGN. Our results are largely independent of the assumed geometry of the disc-corona system, as long as it does not change with time, and the disc response function is characterized by a sharp rise, a plateau, and a decline at longer times. Irrespective of the geometry, the frequency of the main dip should decrease with increasing mean time of the response function, and the amplitude of the dip should increase with increasing reflection fractio
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