A hard X-ray cut-off in the spectra of ultraluminous X-ray sources
Ultraluminous X-ray sources (ULXs) are point-like X-ray sources
found in nearby galaxies, with luminosities of
1039-1041 erg/s. Their
nature remains unknown. Two scenarios are being actively discussed:
subcritical (at a rate below the Eddington limit) accretion onto a black
hole of intermediate (hundreds to thousands of solar masses) mass and
supercritical accretion onto a black hole of stellar mass. Both scenarios
are of great interest and might in principle co-exist in nature.
Until recently, X-ray observations of ULXs were carried out
predominantly at energies below 10 keV. Hard X-ray observations could
provide additional valuable information, but were impossible because
even the brightest ULXs have X-ray fluxes of less than 1 mCrab and they
are usually located in actively star-forming galaxies with high number
density of X-ray sources. With INTEGRAL it has become possible for the
first time to overcome these difficulties. Since late 2009, INTEGRAL has
been performing a series of observations of the M81 group of galaxies,
which contains two bright ULXs: Holmberg IX X-1 and M82 X-1. As of
early 2013, nearly 6 Msec of observations were completed. As part of the
same project, the XMM-Newton observatory performed a series of X-ray
observations of Ho IX X-1 and M82 X-1 in 2011. The goal of the
coordinated observations by INTEGRAL and XMM-Newton was to study the
spectra of the ULXs in a broad energy range from below 1 keV to several
tens of keV.
The deep INTEGRAL observations resulted in just a weak detection of M82
X-1 and non-detection of Ho IX X-1 at energies above 20 keV (see top
panel). However, comparison of these observations with the data obtained
at lower energies by XMM-Newton reveals a clear cutoff at energies above
10 keV in the spectra of both ULXs (bottom panels). The broad-band spectra
can be well described by a model of Comptonization of radiation in a cloud
of gas of moderate temperature (2-3 keV) and large optical depth (15-25).
Such conditions are quite unusual for normal X-ray binaries, but can possibly
be fulfilled during supercritical accretion onto a stellar-mass black hole.
The presence of a cut-off in ULX spectra has now been confirmed by the
NuSTAR observatory, whose high sensitivity and angular resolution have
enabled yet more detailed studies of ULXs at energies above 10 keV.
Reference:
"Cutoff in the hard X-ray spectra of the ultraluminous X-ray sources
HoIX X-1 and M82 X-1"
Sazonov et al., 2014, Astronomy Letters, Volume 40, pp. 65-74
(ADS link:
http://adsabs.harvard.edu/abs/2014AstL...40...65S )
