 From February 2005, ESA's Integral gamma-ray observatory began to
regularly monitor the centre of our Galaxy, and its immediate
environment, known as the Galactic bulge. (Credits: ESA/ISDC)
*
The European Space Agency's gamma ray observatory Integral has caught
the centre of our galaxy in a moment of rare quiet. A handful of the
most energetic high-energy sources surrounding the black hole at the
centre of the Galaxy had all faded into a temporary silence when
Integral looked...
*
This unusual event is allowing astronomers to probe for even fainter
objects and may give them a glimpse of matter disappearing into the
massive black hole at the centre of our galaxy.
The Galactic
centre is one of the most dynamic places in our Galaxy. It is thought
to be home to a gigantic black hole, called Sagittarius A* (pronounced
'A star'). Since the beginning of the Integral mission, ESA's gamma ray
observatory has allowed astronomers to keep watch on this ever-changing
environment.
Integral has discovered many new sources of
high-energy radiation near the galactic centre. From February 2005,
Integral began to regularly monitor the centre of the Galaxy, and its
immediate environment, known as the Galactic bulge.
Erik Kuulkers
of ESA's Integral Science Operations Centre, ESAC, Spain, leads the
Galactic bulge monitoring programme. Integral now keeps its high-tech
eyes on about 80 high-energy sources in the galactic bulge. "Most of
these are X-ray binaries," says Kuulkers.
X-ray binaries are
made up of two stars in orbit around one another. One star is a
relatively normal star; the other is a collapsed star, such as a white
dwarf, neutron star or even a black hole. If the stars are close enough
together, the strong gravity of the collapsed star can pull off gaseous
material from the normal star. As this gas spirals down around the
collapsed star, it is heated to over a million degrees centigrade and
this causes it to emit high energy X-rays and gamma rays. The amount of
gas falling from one star to the other determines the brightness of the
X-ray and gamma-ray emission.
According to the Integral
observations in April 2006, the high-energy rays from about ten sources
closest to the galactic centre all faded temporarily. Kuulkers excludes
the possibility that a mysterious external force is acting on all the
objects to drive them into quiescence. "All the sources are variable
and it was just by accident or sheer luck that they had turned off
during that observation," he says with a smile.
The fortuitous
dimming allows astronomers to set new limits on how faint these X-ray
binaries can become. It also allows a number of new investigations to
be undertaken with the data.
"When these normally bright
sources are faint, we can look for even fainter sources," says
Kuulkers. These could be other X-ray binaries or the high-energy
radiation from giant molecular clouds interacting with past supernovae.
There is also the possibility of detecting the faint high-energy
radiation from the massive black hole in our Galaxy's centre.
Integral's
Galactic bulge monitoring programme will continue throughout this year.
The data is made available, within a day or two of being collected, to
the scientific community via the Internet from a dedicated webpage at
the Integral Science Data Centre (IDSC), Geneva, Switzerland. This way,
anyone interested in specific sources can watch for interesting changes
and trigger follow up observations with other telescopes in good time.
*Notes for editors
The
findings are accepted for publication in the Astronomy &
Astrophysics magazine, in the article titled: "The INTEGRAL Galactic
bulge monitoring program: the first 1.5 years", by E.Kuulkers et al.
Note: This story has been adapted from a news release issued by European Space Agency.
http://www.sciencedaily.com/releases/2007/01/070118094001.htm
See also:
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