Abstract
Using archival data from Suzaku, XMM-Newton, and NuSTAR, nine representative
Ultra-Luminous X-ray sources (ULXs) in nearby galaxies were studied. Their
X-ray spectra were all reproduced with a multi-color disk emission model plus
its Comptonization. However, the spectral shapes of individual sources changed
systematically depending on the luminosity, and defined three typical spectral
states. These states differ either in the ratio between the Comptonizing
electron temperature and the innermost disk temperature, or in the product of
Compton y-parameter and fraction of the Comptonized disk photons. The
luminosity range at which a particular state emerges was found to scatter by a
factor of up to 16 among the eight ULXs. By further assuming that the spectral
state is uniquely determined by the Eddington ratio, the sample ULXs are
inferred to exhibit a similar scatter in their masses. This gives a
model-independent support to the interpretation of ULXs in terms of relatively
massive black holes. None of the spectra showed noticeable local structures.
Especially, no Fe K-shell absorption/emission lines were detected, with upper
limits of $30-40$ eV in equivalent width from the brightest three among the
sample; NGC 1313 X-1, Holmberg IX X-1, and IC 342 X-1. These properties
disfavor ordinary mass accretion from a massive companion star, and suggest
direct Bondi-Hoyle accretion from dense parts of the interstellar medium.
Description
A New Possible Accretion Scenario for Ultra-Luminous X-ray Sources
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