107 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
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
Investigating the X-ray polarization of lamp-post coronae in BHXRB
High-sensitivity X-ray polarimetric observations of black hole X-ray
binaries, which will soon become available with the launches of space-borne
X-ray observatories with sensitive X-ray polarimeters, will be able to put
independent constraints on the black hole as well as the accretion flow, and
possibly break degeneracies that cannot be resolved by spectral/timing
observations alone. In this work we perform a series of general relativistic
Monte-Carlo radiative transfer simulations to study the expected polarization
properties of X-ray radiation emerging from lamp-post coronae in black hole
X-ray binaries. We find that the polarization degree of the coronal emission of
black hole X-ray binaries is sensitive to the spin of the black hole, the
height of the corona, and the dynamics of the corona.Comment: Accepted for publication in 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
Spectra of Puffy Accretion Discs: the kynbb Fit
Puffy disc is a numerical model, expected to capture the properties of the
accretion flow in X-ray black hole binaries in the luminous, mildly
sub-Eddington state. We fit the kerrbb and kynbb spectral models in XSPEC to
synthetic spectra of puffy accretion discs, obtained in general relativistic
radiative magnetohydrodynamic simulations, to see if they correctly recover the
black hole spin and mass accretion rate assumed in the numerical simulation. We
conclude that neither of the two models is capable of correctly interpreting
the puffy disc parameters, which highlights a necessity to develop new, more
accurate, spectral models for the luminous regime of accretion in X-ray black
hole binaries. We propose that such spectral models should be based on the
results of numerical simulations of accretion.Comment: 6 pages, 4 figures, submitted to the AN as proceedings of XMM-Newton
2022 Science Worksho
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