Scientists pioneer technique for 'weighing' black holes

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Scientists pioneer technique for 'weighing' black holes

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Scientists pioneer technique for 'weighing' black holes
The nature of ultraluminous X-ray sources remains controversial, but
many astronomers think that some or most of these objects consist of
an intermediate-mass black hole that accretes matter from a companion
star. Recent evidence suggests that a ULX in the galaxy NGC 5408 has a
black hole with about 2,000 solar masses. Credit: NASA

Two astrophysicists at NASA's Goddard Space Flight Center in
Greenbelt, Md., Nikolai Shaposhnikov and Lev Titarchuk, have
successfully tested a new method for determining the masses of black

This elegant technique, which Titarchuk first suggested in 1998, shows
that the black hole in a binary system known as Cygnus X-1 contains
8.7 times the mass of our sun, with a margin of error of only 0.8
solar mass.

Cygnus X-1 was the first compelling black hole candidate to emerge in
the early 1970s. The system consists of a blue supergiant star and a
massive but invisible companion. Optical observations of the star's
wobble have suggested that the invisible object is a black hole
containing about 10 solar masses. "This agreement gives us a lot of
confidence that our method works," says Shaposhnikov.

"Our method can determine a black hole's mass when alternative
techniques fail," adds Titarchuk, who is also a research professor at
George Mason University, Arlington, Va., also works at the Naval
Research Laboratory, Washington. Shaposhnikov works for the
Universities Space Research Association, a part of the Center for
Research and Exploration in Space Science and Technology within NASA

Working independently, Tod Strohmayer and Richard Mushotzky of Goddard
and four colleagues used Titarchuk's technique to estimate that an
ultra-luminous X-ray source in the small, nearby galaxy NGC 5408
harbors a black hole with a mass of about 2,000 suns.

"This is one of the best indications to date for an intermediate-mass
black hole," says Strohmayer. This type of black hole fills in a huge
gap between black holes such as Cygnus X-1, which come from collapsing
massive stars and contain perhaps 5 to 20 solar masses, and monster
black holes containing millions or even billions of solar masses,
which lurk in the cores of large galaxies.

Titarchuk's method takes advantage of a relationship between a black
hole and the surrounding disk of matter spiraling into it, called an
accretion disk. Gas orbiting in these disks eventually falls into the
black hole. When a black hole's accretion rate increases to a high
level, material piles up near the black hole in a hot region that
Titarchuk likens to a traffic jam. Titarchuk has shown that the
distance from the black hole where this congestion occurs is on a
direct scale with the mass of the black hole. The more massive the
black hole, the farther this congestion occurs from the black hole,
and the longer the orbital period.

In his model, hot gas piling up in the congestion region is linked to
observations of X-ray intensity variations that repeat on a nearly but
not perfectly periodic basis. These quasi-periodic oscillations (QPOs)
are observed in many black hole systems. The QPOs are accompanied by
simple, predictable changes in the system's spectrum as the
surrounding gas heats and cools in response to the changing accretion
rate. Precise timing observations from NASA's Rossi X-ray Timing
Explorer (RXTE) satellite have shown a tight relationship between the
frequency of QPOs and the spectrum, telling astronomers how
efficiently the black hole is producing X-ray radiation.

Using RXTE, Shaposhnikov and Titarchuk have applied this method to
three stellar-mass black holes in our Milky Way Galaxy, and showed
that the derived masses from the QPOs concur with mass measurements
from other techniques. The paper outlining their results is scheduled
to appear in the July 1 issue of Astrophysical Journal.

Using the European Space Agency's XMM-Newton X-ray observatory,
Strohmayer, Mushotzky, and their colleagues detected two QPOs in NGC
5408 X-1. This object is the brightest X-ray source in theirregular
galaxy NGC 5408, 16 million light-years from Earth in the
constellation Centaurus. The QPO frequencies, as well as the
luminosity and spectral characteristics of the source, implies that it
is powered by an intermediate-mass black hole.

"We had two other ways of estimating the mass of the black hole, and
all three methods agree within a factor of two," says Mushotzky. "We
don't have proof this is an intermediate-mass black hole, but the
preponderance of evidence suggests that it is."

The existence of IMBHs remains controversial because there is no
widely accepted mechanism for how they could form. One of the study's
co-authors, Roberto Soria of the Harvard-Smithsonian Center for
Astrophysics, Cambridge, Mass. thinks the black hole's mass could be
closer to 100 suns.

Source: Goddard Space Flight Center


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