181 research outputs found
X-ray polarization fluctuations induced by cloud eclipses in active galactic nuclei
Context: A fraction of active galactic nuclei (AGN) show dramatic X-ray
spectral changes on the day-to-week time scales associated with variation in
the line of sight of the cold absorber. Aims: We intend to model the
polarization fluctuations arising from an obscuration event, thereby offering a
method of determining whether flux variations are due to occultation or extreme
intrinsic emission variability. Methods: Undertaking 1 - 100 keV polarimetric
simulations with the Monte Carlo code STOKES, we simulated the journey of a
variety of cold gas clouds in front of an extended primary source. We varied
the hydrogen column density nH and size of the absorber, as well as the initial
polarization state of the emitting source, to cover a wide range of scenarios.
Results: For unpolarized primary fluxes, large (about 50deg) variations of the
polarization position angle psi are expected before and after an occultation
event, which is associated with very low residual polarization degrees (P lower
than 1 per cent). In the case of an emitting disk with intrinsic,
position-independent polarization, and for a given range of parameters, X-ray
eclipses significantly alter the observed polarization spectra, with most of
the variations seen in psi. Finally, non-uniformly polarized emitting regions
produce very distinctive polarization variations due to the successive covering
and uncovering of different portions of the disk. Plotted against time,
variations in P and psi form detectable P~Cygni type profiles that are
distinctive signatures of non-axisymmetric emission. Conclusions: Polarization
measurements are thus particularly adapted to distinguish between intrinsic
intensity fluctuations and external eclipsing events, constrain the geometry of
the covering medium, and test the hypothesis of non-uniformly emitting disks
predicted by general relativity.Comment: 8 pages, 5 figures. Accepted for publication in A&
General relativistic modelling of the negative reverberation X-ray time delays in AGN
We present the first systematic physical modelling of the time-lag spectra
between the soft (0.3-1 keV) and the hard (1.5-4 keV) X-ray energy bands, as a
function of Fourier frequency, in a sample of 12 active galactic nuclei which
have been observed by XMM-Newton. We concentrate particularly on the negative
X-ray time-lags (typically seen above Hz) i.e. soft band variations
lag the hard band variations, and we assume that they are produced by
reprocessing and reflection by the accretion disc within a lamp-post X-ray
source geometry. We also assume that the response of the accretion disc, in the
soft X-ray bands, is adequately described by the response in the neutral iron
line (Fe k) at 6.4 keV for which we use fully general relativistic
ray-tracing simulations to determine its time evolution. These response
functions, and thus the corresponding time-lag spectra, yield much more
realistic results than the commonly-used, but erroneous, top-hat models.
Additionally we parametrize the positive part of the time-lag spectra
(typically seen below Hz) by a power-law. We find that the
best-fitting BH masses, M, agree quite well with those derived by other
methods, thus providing us with a new tool for BH mass determination. We find
no evidence for any correlation between M and the BH spin parameter, ,
the viewing angle, , or the height of the X-ray source above the disc,
. Also on average, the X-ray source lies only around 3.7 gravitational radii
above the accretion disc and the viewing angles are distributed uniformly
between 20 and 60 degrees. Finally, there is a tentative indication that the
distribution of spin parameters may be bimodal above and below 0.62.Comment: Accepted for publication in MNRAS. The paper is 22 pages long and
contains 19 figures and 2 table
Light bending scenario for accreting black holes in X-ray polarimetry
We discuss a model of an X-ray illuminating source above an accretion disk of
a rotating black hole. Within the so called lamp-post scheme we compute the
expected (observed) polarization properties of the radiation reaching an
observer. We explore the dependencies on model parameters, employing Monte
Carlo radiation transfer computations of the X-ray reflection on the accretion
disk and taking general relativity effects into account. In particular, we
discuss the role of the black hole spin, of the observer viewing angle, and of
the primary X-ray source distance from the black hole. We give several examples
of the resulting polarization degree for two types of exemplary objects -
active galactic nuclei and Galactic black holes. In order to assess potential
observability of the polarization features, we assume the sensitivity of the
proposed New Hard X-ray Mission (NHXM).
We examine the energy range from several keV to ~50 keV, so the iron-line
complex and the Compton hump are included in our model spectra. We find the
resultant polarization degree to increase at the higher end of the studied
energy band, i.e. at >~20 keV. Thus, the best results for polarimetry of
reflection spectra should be achieved at the Compton hump energy region. We
also obtain higher polarization degree for large spin values of the black hole,
small heights of the primary source, and low inclination angles of the
observer.Comment: 17 pages, 14 figures, accepted in Ap
On the driver of relativistic effects strength in Seyfert galaxies
Spectroscopy of X-ray emission lines emitted in accretion discs around
supermassive black holes is one of the most powerful probes of the accretion
flow physics and geometry, while also providing in principle observational
constraints on the black hole spin.[...] We aim at determining the ultimate
physical driver of the strength of this relativistic reprocessing feature. We
first extend the hard X-ray flux-limited sample of Seyfert galaxies studied so
far (FERO, de la Calle Perez et al. 2010) to obscured objects up to a column
density N_H=6x10^23 atoms/cm/cm. We verify that none of the line properties
depends on the AGN optical classification, as expected from the Seyfert
unification scenarios. There is also no correlation between the accretion disc
inclination, as derived from formal fits of the line profiles, and the optical
type or host galaxy aspect angle, suggesting that the innermost regions of the
accretion disc and the host galaxy plane are not aligned. [...]. Data are not
sensitive enough to the detailed ionisation state of the line-emitting disc.
However, the lack of dependency of the line EW on either the luminosity or the
rest-frame centroid energy rules out that disc ionisation plays an important
role on the EW dynamical range in Seyferts. The dynamical range of the
relativistically broadened K-alpha iron line EW in nearby Seyferts appears to
be mainly determined by the properties of the innermost accretion flow. We
discuss several mechanisms (disc ionisation, disc truncation, aberration due to
a mildly relativistic outflowing corona) which can explain this. [...]
Observational data are still not in contradiction with scenarios invoking
different mechanisms for the spectral complexity around the iron line, most
notably the "partial covering" absorption scenario. (abridged).Comment: Accepted for publication on Astronomy & Astrophysics. 14 pages, 9
figure
Modeling the X-ray fractional variability spectrum of Active Galactic Nuclei using multiple flares
Using Monte-Carlo simulations of X-ray flare distributions across the
accretion disk of active galactic nuclei (AGN), we obtain modeling results for
the energy-dependent fractional variability amplitude. Referring to previous
results of this model, we illustrate the relation between the shape of the
point-to-point fractional variability spectrum, F_pp, and the time-integrated
spectral energy distribution, F_E. The results confirm that the spectral shape
and variability of the iron Kalpha line are dominated by the flares closest to
the disk center.Comment: 2 pages, 1 figure, conference proceedings of the AGN meeting held in
October 2006 in Xi'an, China. To appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
Constraining global parameters of accreting black holes by modeling magnetic flares
We present modeling results for the reprocessed radiation expected from
magnetic flares above AGN accretion disks. Relativistic corrections for the
orbital motion of the flare and for the curved space-time in the vicinity of
the black hole are taken into account. We investigate the local emission
spectra, as seen in a frame co-orbiting with the disk, and the observed spectra
at infinity. We investigate long-term flares at different orbital phases and
short-term flares for various global parameters of the accreting black hole.
Particular emphasis is put on the relation between the iron Kalpha line and the
Compton hump as these two features can be simultaneously observed by the Suzaku
satellite and later by Simbol-X.Comment: 4 pages, 1 figure, 1 table, proceedings for a poster at the
international conference "The Extreme Universe in the Suzaku Era" held in
Kyoto, Japan, December 4-8, 200
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