The tiny black hole resides in a Milky Way
Galaxy binary system known as XTE J1650-500,
named for its sky coordinates in the southern
constellation Ara. NASA's Rossi X-ray Timing
Explorer (RXTE) satellite discovered the system
in 2001. Astronomers realized soon after J1650's
discovery that it harbors a normal star and a
relatively lightweight black hole. But the black
holeâ€™s mass had never been measured to high
Shaposhnikov and his Goddard colleague Lev
Titarchuk presented their results on Monday,
March 31, at the American Astronomical Society
High-Energy Astrophysics Division meeting in Los
Angeles, Calif. Titarchuk also works at George
Mason University in Fairfax, Va., and the US
Naval Research Laboratory in Washington, DC.
The method used by Shaposhnikov and Titarchuk
has been described in several papers in the
Astrophysical Journal. It uses a relationship
between black holes and the inner part of their
surrounding disks, where gas spirals inward
before making the fatal plunge. When the feeding
frenzy reaches a moderate rate, hot gas piles up
near the black hole and radiates a torrent of
X-rays. The X-ray intensity varies in a pattern
that repeats itself over a nearly regular
interval. This signal is called a quasi-periodic
oscillation, or QPO.
Astronomers have long suspected that a QPO's
frequency depends on the black hole's mass. In
1998, Titarchuk realized that the congestion
zone lies close in for small black holes, so the
QPO clock ticks quickly. As black holes increase
in mass, the congestion zone is pushed farther
out, so the QPO clock ticks slower and slower.
To measure the black hole masses, Shaposhnikov
and Titarchuk use archival data from RXTE, which
has made exquisitely precise measurements of QPO
frequencies in at least 15 black holes.
Last year, Shaposhnikov and Titarchuk applied
their QPO method to three black holes whose
masses had been measured by other techniques. In
their new paper, they extend their result to
seven other black holes, three of which have
well-determined masses. "In every case, our
measurement agrees with the other methods," says
Titarchuk. "We know our technique works because
it has passed every test with flying colors."
When Shaposhnikov and Titarchuk applied their
method to XTE J1650-500, they calculated a mass
of 3.8 Suns, with a margin of uncertainty of
only half a Sun. This value is well below the
previous black hole record holder with a
reliable mass measurement, GRO 1655-40, which
tips the scales at about 6.3 Suns.
Below some unknown critical threshold, a dying
star should produce a neutron star instead of a
black hole. Astronomers think the boundary
between black holes and neutron stars lies
somewhere between 1.7 and 2.7 solar masses.
Knowing this dividing line is important for
fundamental physics, because it will tell
scientists about the behavior of matter when it
is scrunched into conditions of extraordinarily
Despite the diminutive size of this new record
holder, future space travelers had better
beware. Smaller black holes like the one in
J1650 exert stronger tidal forces than the much
larger black holes found in the centers of
galaxies, which make the little guys more
dangerous to approach. "If you ventured too
close to J1650's black hole, its gravity would
tidally stretch your body into a strand of
spaghetti," says Shaposhnikov.
Shaposhnikov adds that RXTE is the only
instrument that can make the high-precision
timing observations necessary for this line of
research. "RXTE is absolutely crucial for these
black hole mass measurements," he says.
Adapted from materials provided by NASA/Goddard
Space Flight Center.