366 research outputs found
MoCA: A Monte Carlo code for Comptonisation in Astrophysics. I. Description of the code and first results
We present a new Monte Carlo code for Comptonisation in Astrophysics (MoCA).
To our knowledge MoCA is the first code that uses a single photon approach in a
full special relativity scenario, and including also Klein-Nishina effects as
well as polarisation. In this paper we describe in detail how the code works,
and show first results from the case of extended coronae in accreting sources
Comptonising the accretion disc thermal emission. We explored both a slab and a
spherical geometry, to make comparison with public analytical codes more easy.
Our spectra are in good agreement with those from analytical codes for
low/moderate optical depths, but differ significantly, as expected, for optical
depths larger than a few. Klein-Nishina effects become relevant above 100 keV
depending on the optical thickness and thermal energy of the corona. We also
calculated the polarisation properties for the two geometries, which show that
X-ray polarimetry is a very useful tool to discriminate between them.Comment: 16 pages, 20 figure
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
On the Prospect of Constraining Black-Hole Spin Through X-ray Spectroscopy of Hotspots
Future X-ray instrumentation is expected to allow us to significantly improve
the constraints derivedfrom the Fe K lines in AGN, such as the black-hole
angular momentum (spin) and the inclination angle of the putative accretion
disk. We consider the possibility that measurements of the persistent,
time-averaged Fe K line emission from the disk could be supplemented by the
observation of a localized flare, or "hotspot", orbiting close to the black
hole. Although observationally challenging, such measurements would recover
some of the information loss that is inherent to the radially-integrated line
profiles. We present calculations for this scenario to assess the extent to
which, in principle, black-hole spin may be measured. We quantify the
feasibility of this approach using realistic assumptions about likely
measurement uncertainties.Comment: 7 pages, 7 figures. Accepted for publication in Ap
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
NGC 2992 in an X-ray high state observed by XMM: Response of the Relativistic Fe K Line to the Continuum
We present the analysis of an XMM observation of the Seyfert galaxy NGC 2992.
The source was found in its highest level of X-ray activity yet detected, a
factor higher in 2--10 keV flux than the historical minimum. NGC
2992 is known to exhibit X-ray flaring activity on timescales of days to weeks,
and the XMM data provide at least factor of better spectral resolution
in the Fe K band than any previously measured flaring X-ray state. We find that
there is a broad feature in the \sim 5-7 keV band which could be interpreted as
a relativistic Fe K emission line. Its flux appears to have increased
in tandem with the 2--10 keV continuum when compared to a previous Suzaku
observation when the continuum was a factor of lower than that during
the XMM observation. The XMM data are consistent with the general picture that
increased X-ray activity and corresponding changes in the Fe K line
emission occur in the innermost regions of the putative accretion disk. This
behavior contrasts with the behavior of other AGN in which the Fe K
line does not respond to variability in the X-ray.Comment: 30 pages, 6 figures, Accepted to Ap
X-ray polarization properties of partially ionized equatorial obscurers around accreting compact objects
We present the expected X-ray polarization signal resulting from distant
reprocessing material around black holes. Using a central isotropic power-law
emission at the center of the simulated model, we add distant equatorial and
axially symmetric media that are covering the central accreting sources. We
include partial ionization and partial transparency effects, and the impact of
various polarization and steepness of the primary radiation spectrum. The
results are obtained with the Monte Carlo code STOKES that considers both line
and continuum processes and computes the effects of scattering and absorption
inside static homogenous wedge-shaped and elliptical toroidal structures,
varying in relative size, composition and distance to the source. We provide
first order estimates for parsec-scale reprocessing in Compton-thin and
Compton-thick active galactic nuclei, as well as winds around accreting
stellar-mass compact objects, for observer's inclinations above and below the
grazing angle. The resulting reprocessed polarization can reach tens of % with
either parallel or perpendicular orientation with respect to the axis of
symmetry, depending on subtle details of the geometry, density and ionization
structure. We also show how principal parameters constrained from X-ray
spectroscopy or polarimetry in other wavelengths can lift the shown
degeneracies in X-ray polarization. We provide an application example of the
broad modelling discussion by revisiting the recent IXPE 2-8 keV X-ray
polarimetric observation of the accreting stellar-mass black hole in Cygnus X-3
from the perspective of partial transparency and ionization of the obscuring
outflows.Comment: 22 pages, 21 figures. Submitted to MNRA
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
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