Figure 3.
Before and after
outburst images in
X-rays and in optical
light (as viewed with
HST/WFC3). The
colors in the X-ray
images are the same
as Figure 2. The
HST data show red
(I band), green (V
band), and blue (B
band). The source
corresponding to the
ULX is obvious in the
right hand panels.
Note the scale
changes from top to
bottom panels.
second. Like most compact X-ray sources,
we believe ULXs derive their energy from
the accretion of material from a more-orless normal star onto a highly collapsed
one (a neutron star or black hole).
What leads to classification as a ULX is
a luminosity so high that its central engine must be a black hole with a mass
larger than that of the most massive normal stars. (A simple relationship exists,
known as the Eddington limit, which
sets the maximum luminosity that can
be sustained through normal accretion
onto an object of a given mass; at a higher luminosity, the radiation pressure outward would exceed the gravitational attraction and choke off the accretion process.)
While we knew X-ray sources could vary in
brightness from one observation to another,
we didn’t expect to see such a strong source
present where none had appeared before.
Again, this one easily shined as the brightest
single source (out of over 250) in the entire
galaxy! Calculating an upper limit from the
earlier Chandra observation in 2000-2001, we
determined that the source had brightened
by at least a factor of 3000. Many previous
X-ray missions (back to the 1970s) have targeted M83, but our searches of all the earlier
data showed no evidence for any previous appearance of this source.
Chandra obtained the new data periodically
over the entire calendar year of 2011, and we
were also able to obtain Swift X-ray Telescope
monitoring between some of the Chandra
observations. From these we found that the
new source varied somewhat in brightness
and hardness ratio, but remained bright at
least through the end of 2011.
The new ULX in M83 ranks among the closest such sources known — close enough, we
thought, that it might be possible to see optical emission from the donor star providing
the material that keeps the source glowing.
15
GeminiFocus
This would give us some real physical insight
into what’s happening in the system. Through
great fortune, HST had observed the field
containing the source just 16 months earlier! However, pulling up those data revealed
nothing at all unusual at the position of the Xray source — just a few exceedingly faint red,
low-mass stars of about 27th magnitude.
This discovery meant two things: 1) that the
X-ray source had likely “turned on” sometime
between August 2009 and December 2010,
and; 2) that the normal, mass-donating star
was an old, red star of relatively low mass. For
most ULXs, if their galaxies are close enough
for optical counterparts to be seen at all, the
counterparts are very blue and thought to be
much more massive (O or B) stellar companions to the black holes.
A Gemini Discovery
Another stroke of luck hit in April 2011, when
we were scheduled to observe on the 8-m
Gemini South telescope in Chile