New technique for 'weighing' black holes (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 16 May 2007 17:30:02 GMT
ESA News
http://www.esa.int
16 May 2007
New technique for 'weighing' black holes
ESA's XMM-Newton has helped to find evidence for the existence of
controversial Intermediate Mass Black Holes. Scientists used a new,
recently proven method for determining the mass of black holes.
Nikolai Shaposhnikov and Lev Titarchuk, at NASA's Goddard Space Flight
Center (GSFC), have used the technique to determine the mass of the black
hole, Cygnus X-1, located in the constellation Cygnus (the Swan)
approximately 10 000 light years away in our Galaxy, the Milky Way.
The elegant technique, first suggested by Titarchuk in 1998, shows that
Cygnus X-1, part of a binary system, contains 8.7 solar masses, with a
margin of error of only 0.8 solar masses. Cygnus X-1 was one of the first
compelling black hole candidates to emerge in the early 1970s. The system
consists of a blue supergiant and a massive but invisible companion.
Alternative techniques have previously suggested that the invisible object
was a black hole of about 10 solar masses. "This agreement gives us a lot
of confidence that our method works," says Shaposhnikov. It can help
determine a black hole's mass when alternative techniques fail," adds
Titarchuk.
Working independently from Shaposhnikov and Titarchuk, Tod Strohmayer and
Richard Mushotzky, also from GSFC, and four colleagues, used Titarchuk's
technique on XMM data and stumbled upon an Intermediate Mass Black Hole
(IMBH) -- the existence of which is in theory controversial.
They estimated that an ultraluminous X-ray source in the nearby galaxy,
NGC 5408, harbours a black hole with a mass of about 2 000 Suns."This is
one of the best indications to date for an IMBH," says Strohmayer.
The existence of IMBHs is controversial because there is no widely
accepted mechanism for how they could form. But they would fill in a huge
gap between black holes such as Cygnus X-1 -- which form from collapsing
massive stars and contain perhaps 5 to 20 solar masses -- and the
'monsters' (up to thousand million solar masses) that lurk in the cores of
large galaxies.
Titarchuk's method takes advantage of a relationship between a black hole
and its surrounding accretion disk. Gas orbiting in these disks eventually
spirals 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 scales directly with the mass of the black hole. The
more massive the black hole, the farther this congestion occurs 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 close relationship between the frequency of QPOs
and the spectrum, telling astronomers how efficiently the black hole is
producing X-rays.
Using RXTE, Shaposhnikov and Titarchuk have applied this method to three
stellar-mass black holes in the Milky Way and shown that the derived
masses from the QPOs concur with mass measurements from other techniques.
Using ESA's XMM-Newton X-ray observatory, Strohmayer, Mushotzky, and their
colleagues detected two QPOs in NGC 5408 X-1.
NGC 5408 X-1 is the brightest X-ray source in the small, irregular 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, imply that it is powered by an IMBH.
"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 IMBH, but the preponderance of evidence suggests
that it is."
One of the study's coauthors, Roberto Soria of the Harvard-Smithsonian
Center for Astrophysics, thinks the black hole's mass is closer to one
hundred Suns.
Notes for editors
The findings described in this article will appear in two papers to be
published in the Astrophysical journals.
The first, "Quasi-Periodic Variability in NGC 5408 X-1", is by
T.Strohmayer, R.Mushotzky, L. Winter, R. Soria, P. Uttley, M. Cropper.
The second paper, "Determination of Black Hole Mass in Cyg X-1 by Scaling
of Spectral Index-QPO Frequency Correlation", is by N. Shaposhnikov and
L.Titarchuk.
For more information
Tod Strohmayer, NASA Goddard Space Flight Center, USA
Email: stroh @ milkyway.gsfc.nasa.gov
Nikolai Shaposhnikov, NASA Goddard Space Flight Center, USA
Email: nikolai @ milkyway.gsfc.nasa.gov
Norbert Schartel, ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMDMAV681F_index_1.html ]
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