Analytical techniques for polarimetric imaging of accretion flows in Schwarzschild metric. (arXiv:2109.04827v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Loktev_V/0/1/0/all/0/1">Vladislav Loktev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Veledina_A/0/1/0/all/0/1">Alexandra Veledina</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Poutanen_J/0/1/0/all/0/1">Juri Poutanen</a>
Emission from an accretion disc around compact objects, such as neutron stars
and black holes, is expected to be significantly polarized. The polarization
can be used to put constraints on geometrical and physical parameters of the
compact sources — their radii, masses and spins — as well as to determine the
orbital parameters. The radiation escaping from the innermost parts of the disc
is strongly affected by the gravitational field of the compact object and
relativistic velocities of the matter. The straightforward calculation of the
observed polarization signatures involves computationally expensive ray-tracing
technique. At the same time, having fast computational routines for direct data
fitting becomes increasingly important in light of the currently observed
images of the accretion flow around supermassive black hole in M87 by the Event
Horizon Telescope, infrared polarization signatures coming from Sgr A*, as well
as for the upcoming X-ray polarization measurements by the Imaging X-ray
Polarimetry Explorer and enhanced X-ray Timing and Polarimetry mission. In this
work, we obtain an exact analytical expression for the rotation angle of
polarization plane in Schwarzschild metric accounting for the effects of light
bending and relativistic aberration. We show that the calculation of the
observed flux, polarization degree and polarization angle as a function of
energy can be performed analytically with high accuracy using approximate
light-bending formula, lifting the need for the pre-computed tabular models in
fitting routines.
Emission from an accretion disc around compact objects, such as neutron stars
and black holes, is expected to be significantly polarized. The polarization
can be used to put constraints on geometrical and physical parameters of the
compact sources — their radii, masses and spins — as well as to determine the
orbital parameters. The radiation escaping from the innermost parts of the disc
is strongly affected by the gravitational field of the compact object and
relativistic velocities of the matter. The straightforward calculation of the
observed polarization signatures involves computationally expensive ray-tracing
technique. At the same time, having fast computational routines for direct data
fitting becomes increasingly important in light of the currently observed
images of the accretion flow around supermassive black hole in M87 by the Event
Horizon Telescope, infrared polarization signatures coming from Sgr A*, as well
as for the upcoming X-ray polarization measurements by the Imaging X-ray
Polarimetry Explorer and enhanced X-ray Timing and Polarimetry mission. In this
work, we obtain an exact analytical expression for the rotation angle of
polarization plane in Schwarzschild metric accounting for the effects of light
bending and relativistic aberration. We show that the calculation of the
observed flux, polarization degree and polarization angle as a function of
energy can be performed analytically with high accuracy using approximate
light-bending formula, lifting the need for the pre-computed tabular models in
fitting routines.
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