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 NASA’s
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/...blackhole.html ]