What Determines Unique Spectra of Super-Eddington Accretors?: Origin of Optically Thick and Low Temperature Coronae in Super-Eddington Accretion Flows. (arXiv:2012.05386v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Kawanaka_N/0/1/0/all/0/1">Norita Kawanaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mineshige_S/0/1/0/all/0/1">Shin Mineshige</a>
Existence of relatively cool ($k_B T lesssim 10~{rm keV}$) and optically
thick ($tau gtrsim 3$) coronae are inferred above super-Eddington accretion
flow such as ultraluminous X-ray sources (ULXs), GRS 1915+105, and narrow-line
Seyfert 1 galaxies (NLS1), which contrasts the cases in sub-Eddington accretion
flows, which are associated with coronae with $k_B T sim 100~{rm keV}$ and
$tau sim 1$. To understand their physical origin, we investigate the emission
properties of the corona which is formed by the gas blown off the
super-Eddington inner disk by radiation pressure. We assume that the corona is
heated by the reconnection of magnetic loops emerged from the underlying disk.
We show that this radiation pressure driven wind can act as an optically thick
corona which upscatters thermal soft photons from the underlying disk, and that
with a reasonable parameter set we can theoretically reproduce the coronal
optical depth and temperature which are inferred by spectral fittings of
observational data. By contrast, the coronal optical depth cannot be so high in
sub-Eddington cases, since the coronal material is supplied from the disk via
evaporation and there is a maximum limit on the evaporation rate. We support
that the low temperature, optically thick Comptonization should be a key
signature of super-Eddington accretion flow.
Existence of relatively cool ($k_B T lesssim 10~{rm keV}$) and optically
thick ($tau gtrsim 3$) coronae are inferred above super-Eddington accretion
flow such as ultraluminous X-ray sources (ULXs), GRS 1915+105, and narrow-line
Seyfert 1 galaxies (NLS1), which contrasts the cases in sub-Eddington accretion
flows, which are associated with coronae with $k_B T sim 100~{rm keV}$ and
$tau sim 1$. To understand their physical origin, we investigate the emission
properties of the corona which is formed by the gas blown off the
super-Eddington inner disk by radiation pressure. We assume that the corona is
heated by the reconnection of magnetic loops emerged from the underlying disk.
We show that this radiation pressure driven wind can act as an optically thick
corona which upscatters thermal soft photons from the underlying disk, and that
with a reasonable parameter set we can theoretically reproduce the coronal
optical depth and temperature which are inferred by spectral fittings of
observational data. By contrast, the coronal optical depth cannot be so high in
sub-Eddington cases, since the coronal material is supplied from the disk via
evaporation and there is a maximum limit on the evaporation rate. We support
that the low temperature, optically thick Comptonization should be a key
signature of super-Eddington accretion flow.
http://arxiv.org/icons/sfx.gif
