Stephen Justham and Jifeng Liu
Weighing the Black Hole in
M101 ULX-1
Astronomers have measured the mass of an ultra-luminous X-ray source,
producing a puzzle over how to explain the observed X-ray properties and
leaving a hole in the quest for intermediate-mass black holes.
M101 ULX-1 is a transient ultra-luminous Xray source with characteristics expected of
an accreting, intermediate-mass black hole
(IMBH). A series of Gemini spectra have detected a Wolf-Rayet star in the system and
revealed its orbital motion. This constrains
the mass of the black hole in M101 ULX-1;
the object is too massive to be a neutron
star but very unlikely to be an intermediatemass black hole. The data also show that
the black hole accretes from the wind of
the star, not the overflow of the donor star’s
Roche lobe, as illustrated in Figure 1.
“Ultra-luminous X-ray sources” (ULXs) sit
at the intersection of two fundamental
problems in astrophysics, since this class of systems contains objects which appear to be
more luminous than the Eddington limit allows for stellar-mass black holes. That definition
is somewhat imprecise because we don’t know the definitive upper mass limit for “stellarmass” black holes. Nonetheless, the questions raised by the existence of these systems are
clear: Is the Eddington limit somehow exceeded in ULXs? Or do ULXs contain black holes
with higher-than-expected masses, perhaps even intermediate-mass IMBHs?
January2014 2013 Year in Review
GeminiFocus
Figure 1.
Artist’s impression
of M101 ULX-1. In
the foreground is the
black hole, surrounded
by an accretion disk;
matter falling into
the black hole via
the disk produces the
X-ray luminosity of the
system. That matter
originates from the
wind of a Wolf-Rayet
star, shown in the
distance. In the far
background is one
of the spiral arms of
M101.
Gemini illustration by
Lynette Cook.
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