Dying Star Reveals More Evidence for New Kind of Black Hole (Forwarded)

Christopher Wanjek
Goddard Space Flight Center, Greenbelt, Md. January 5, 2006

Dying Star Reveals More Evidence for New Kind of Black Hole

Scientists using NASA's Rossi X-ray Timing Explorer have found a doomed
star orbiting what appears to be a medium-sized black hole -- a theorized
"in-between" category of black hole that has eluded confirmation and
frustrated scientists for more than a decade.

With the discovery of the star and its orbital period, scientists are now
one step away from measuring the mass of such a black hole, a step which
would help verify its existence. The star's period and location already
fit into the main theory of how these black holes could form.

A team led by Prof. Philip Kaaret of the University of Iowa, Iowa City,
announced these results today in Science Express. The results will also
appear in the Jan. 27 issue of Science.

"We caught this otherwise ordinary star in a unique stage in its
evolution, toward the end of its life when it has bloated into a red giant
phase," said Kaaret. "As a result, gas from the star is spilling into the
black hole, causing the whole region to light up. This is a well-studied
region of the sky, and we spotted the star with a little luck and a lot of
perseverance."

A black hole is an object so dense and with a gravitational force so
intense that nothing, not even light, can escape its pull once within its
boundary. A black hole region becomes visible when matter falls toward it
and heats to high temperatures. This light is emitted before the matter
crosses the border, called the event horizon.

Our galaxy is filled with millions of stellar-mass black holes, each with
the mass of a few suns. These form from the collapse of very massive
stars. Most galaxies possess at their core a supermassive black hole,
containing the mass of millions to billions of suns confined to a region
no larger than our solar system. Scientists do not know how these form,
but it likely entails the collapse of enormous quantities of primordial
gas.

"In the past decade, several satellites have found evidence of a new class
of black holes, which could be between 100 and 10,000 solar masses," said
Dr. Jean Swank, Rossi Explorer project scientist at NASA¹s Goddard Space
Flight Center, Greenbelt, Md. "There has been debate about the masses and
how these black holes would form. Rossi has provided major new insight."

These suspected mid-mass black holes are called ultra-luminous X-ray
objects because they are bright sources of X-rays. In fact, most of these
black hole mass estimates have been based solely on a calculation of how
strong a gravitational pull is needed to produce light of a given
intensity.

Kaaret's group at the University of Iowa, which includes Prof. Cornelia
Lang and Melanie Simet, an undergraduate, made a measurement that can be
used in the equation to directly calculate mass. Using straightforward
Newtonian physics, scientists can calculate an object's mass once they
know an orbital period and velocity of smaller objects rotating around it.

"We found a rise and fall in X-ray light every 62 days, likely caused by
the orbit of the companion star around the black hole," said Simet. "The
velocity will be hard to determine, however, because the star is located
in such a dust-obscured area. This makes it hard for optical and infrared
telescopes to observe the star and make velocity calculations. Yet for
now, knowing just the orbital period is very revealing."

The suspected mid-mass black hole, known as M82 X-1, is a well-studied
ultra-luminous X-ray object in a nearby star cluster containing about a
million stars packed into a region only about 100 light years across. A
leading theory proposes that a multitude of star collisions over a short
period in a crowded region will create a short-lived gigantic star that
collapses into a 1,000-solar-mass black hole. The cluster near M82 X-1 has
a high-enough density to form such a black hole. No normal companion could
provide enough fuel to make M82 X-1 shine so brightly. But the 62-day
orbital period implies that the companion must have a very low density.
This fits the scenario of a bloated super-giant star losing mass at a rate
high enough to fuel M82 X-1.

"With this discovery of the orbital period, we now have a consistent
picture of the whole evolution of a mid-mass black hole binary," said
Kaaret. "It was formed in a 'super' star cluster; the black hole then
captured a companion star; the companion star evolved to the giant stage;
and we now see it as an extremely luminous X-ray source because the
companion star has expanded and is feeding the black hole."

[NOTE: Images supporting this release are available at
http://www.nasa.gov/centers/goddard/...lackhole.html]