Figure 2.
ly-understood process of accretion, and the
conditions which prevail in the strong gravity
close to a black hole.
Measuring the masses of the components in
ULXs is hard, partly because the great luminosity of the accretion disk overwhelms the
light from the star. However, rare systems
are transient, sometimes entering a state in
which they are sufficiently luminous to qualify as ULXs, sometimes returning to a quiescent state in which it might be possible to
directly detect the motion of the star.
M101 ULX-1 is one such system. It was detected as the brightest X-ray source in the
galaxy M101 but since then has regularly
been observed in lower-luminosity states.
Moreover, M101 ULX-1 is one of the ULXs
from which the X-ray spectral energy distribution contains no hint of a Comptonising
corona, which reduces the chance that the
presently-proposed super-Eddington accretion mechanisms are helping to explain
the high luminosity. Furthermore, the X-ray
spectrum is easily fitted by a standard thermal accretion disk with a super-soft temperature of only 100 or 200 electron volts, which
implies that the inner disk temperature is
exceptionally cool (see Figure 2). This combination of spectral characteristics, combined
with the high outburst luminosity, is exactly
the set of properties which one would expect an IMBH to display.
Quantities derived from
fits to the X-ray spectra
of a variety of X-ray
sources indicate that the
accretion disk properties
divide naturally into
two groups. M101
ULX-1 (shown in red)
is a member of the
class which apparently
maintains very cool
inner disk temperatures
whilst attaining a high
luminosity, as would
be expected for IMBHs.
Galactic black-hole X-ray
binaries (labelled as
GBHXRB) and two other
known Wolf-Rayet blackhole X-ray binaries lie in a
distinctly different region
of the parameter space.
The dotted lines describe
the expected variation
in disk luminosity with
a fixed inner disk radius
(which, naively, would
be correlated with black
hole mass). For further
details, please see Liu
et al., 2013.
Gemini Observations of a Black
Hole Donor Star
Based on these arguments, a Gemini proposal was approved to try to detect the motion of the donor star in M101 ULX-1. This
resulted in 10 spectra with exposures ranging between 3200 and 9600 seconds, and a
combined integration time of 15.6 hours.
The first discovery from these observations
was the nature of the companion star. Clear
helium emission lines indicate that it is a
Wolf-Rayet star (an evolved and helium rich
January2014 2013 Year in Review
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