Does the disk in the hard state of XTE J1752-223 extend to the innermost stable circular orbit?. (arXiv:2006.12829v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Zdziarski_A/0/1/0/all/0/1">Andrzej Zdziarski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marco_B/0/1/0/all/0/1">Barbara De Marco</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Szanecki_M/0/1/0/all/0/1">Michal Szanecki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Niedzwiecki_A/0/1/0/all/0/1">Andrzej Niedzwiecki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Markowitz_A/0/1/0/all/0/1">Alex Markowitz</a>
The accreting black-hole binary XTE J1752–223 was observed in a stable hard
state for 25 d by RXTE, yielding a 3–140 keV spectrum of unprecedented
statistical quality. Its published model required a single Comptonization
spectrum reflecting from a disk close to the innermost stable circular orbit.
We studied that model as well as a number of other single-Comptonization models
(yielding similarly low inner radii), but found they violate a number of basic
physical constraints, e.g., their compactness is much above the maximum allowed
by pair equilibrium. We also studied the contemporaneous 0.55–6 keV spectrum
from the Swift/XRT and found it well fitted by an absorbed power law and a disk
blackbody with the innermost temperature of 0.1 keV. The normalization of the
disk blackbody corresponds to an inner radius of $gtrsim$20 gravitational
radii and its temperature, to irradiation of the truncated disk by a hot inner
flow. We have also developed a Comptonization/reflection model including the
disk irradiation and intrinsic dissipation, but found that it does not yield
any satisfactory fits. On the other hand, we found that the $leq$10 keV band
from RXTE is much better fitted by a reflection from a disk with the inner
radius $gtrsim$100 gravitational radii, which model then underpredicts the
spectrum at $>$10 keV by $<$10%. We argue that the most plausible explanation
of the above results is inhomogeneity of the source, with the local spectra
hardening with the decreasing radius. Our results support the presence of a
complex Comptonization region and a large disk truncation radius in this
source.
The accreting black-hole binary XTE J1752–223 was observed in a stable hard
state for 25 d by RXTE, yielding a 3–140 keV spectrum of unprecedented
statistical quality. Its published model required a single Comptonization
spectrum reflecting from a disk close to the innermost stable circular orbit.
We studied that model as well as a number of other single-Comptonization models
(yielding similarly low inner radii), but found they violate a number of basic
physical constraints, e.g., their compactness is much above the maximum allowed
by pair equilibrium. We also studied the contemporaneous 0.55–6 keV spectrum
from the Swift/XRT and found it well fitted by an absorbed power law and a disk
blackbody with the innermost temperature of 0.1 keV. The normalization of the
disk blackbody corresponds to an inner radius of $gtrsim$20 gravitational
radii and its temperature, to irradiation of the truncated disk by a hot inner
flow. We have also developed a Comptonization/reflection model including the
disk irradiation and intrinsic dissipation, but found that it does not yield
any satisfactory fits. On the other hand, we found that the $leq$10 keV band
from RXTE is much better fitted by a reflection from a disk with the inner
radius $gtrsim$100 gravitational radii, which model then underpredicts the
spectrum at $>$10 keV by $<$10%. We argue that the most plausible explanation
of the above results is inhomogeneity of the source, with the local spectra
hardening with the decreasing radius. Our results support the presence of a
complex Comptonization region and a large disk truncation radius in this
source.
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