reprogram an hour of our observing time for
broadband imaging of the region containing
M83’s new ULX.
When we compared these Gemini data to the
April 2009 Magellan images, we discovered
that a faint blue star had now appeared at
the position of the X-ray source, with a visual
magnitude of ~ 24! But did this source correspond with one of the faint red stars seen in
the earlier HST data? Only new HST images
would tell us that.
Knowing that an optical counterpart had appeared, our team went to work and successfully proposed for follow-up director’s discretionary time with HST/WFC3, to pin down
the source’s exact location and measure its
properties. Our team obtained these data in
late July 2011, and, indeed, a blue star was
now clearly visible at the location of the X-ray
source. Furthermore, the new source did not
correspond exactly with any of the red stars
visible in the earlier HST data.
An upper limit from the HST photometry
implied that the mass-donating companion must be about 4 MSun or less. This was an
unexpected result because most ULXs are
thought to be young objects because their
visible counterparts look like short-lived,
massive blue stars. In this case, because we
have pre-outburst data, we know the blue
light arises from the reprocessing of X-rays
— either on the heated face of the companion star or in portions of the accretion disk
around the black hole — and does not indicate an OB star companion.
In another surprise, applying standard accretion models to this object points to a likely
black hole mass in excess of 40 MSun and perhaps as high as 100 MSun — quite high for a
stellar binary system. If the emission was
beamed rather than isotropic, a more typical
mass estimate like 15 MSun would be possible,
but the time variability characteristics of the
source, to the extent that we know them, do
not support a beamed model for this source.
While we expect to discover many new
surprises as we continue to analyze these
fabulous data sets, it will be hard to top the
combination of serendipity and excitement
generated by this result!
The paper reporting these results appeared in
the May 10, 2012, issue of The Astrophysical Journal, 750: 152, with Roberto Soria (Curtin University, Australia) as the first author. Other team
members on this study include Knox Long and
Brad Whitmore (STScI), Kip Kuntz (Johns Hopkins
University), and Paul Plucinsky (Harvard-Smithsonian Center for Astrophysics).
Bill Blair is a research professor in the Physics and
Astronomy Department at Johns Hopkins University in Baltimore, Maryland. His e-mail address is:
[email protected]
Frank Winkler is the Gamaliel Painter Bicentennial
Professor of Physics at Middlebury College in Vermont. His e-mail address is:
[email protected]
Since pre-outburst data are not available for
most ULXs, it raises the question whether a
significant number of them may also have
low-mass donor stars as well. In any event,
with this case, we have proof of the existence of an older population of ULXs, which
may be invisible most of the time, and only
brighten sporadically.
16
GeminiFocus
June2012