Re: Integral sees the Galactic centre playing hide and seek (Forwarded)

ESA News
http://www.esa.int

18 January 2007

Integral sees the Galactic centre playing hide and seek

ESA'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.

For more information

Erik Kuulkers, ESA Integral Science Operations Centre
Email: erik.kuulkers @ sciops.esa.int

Christoph Winkler, ESA Integral Project Scientist
Email: christoph.winkler @ esa.int

[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMGOVRMTWE_index_1.html ]

If you gather and separate X-Ray signals for 5 years, you will find
that
the core of the galaxy does not produce any X-Ray signals.

You can't just detemine what's there.

.

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