Three satellites needed to bring out 'shy star' (Forwarded)

ESA News
http://www.esa.int

13 July 2005

Three satellites needed to bring out 'shy star'

An international team of scientists has uncovered a rare type of neutron
star so elusive that it took three satellites to identify it.

The findings, made with ESA's Integral satellite and two NASA
satellites, reveals new insights about star birth and death in our
Galaxy. We report this discovery, highlighting the complementary nature
of European and US spacecraft, on the day in which ESA's Integral
celebrates 1000 days in orbit.

The neutron star, called IGR J16283-4838, is an ultra-dense 'ember' of
an exploded star and was first seen by Integral on 7 April 2005. This
neutron star is about 20,000 light years away, in a 'double hiding
place'. This means it is deep inside the spiral arm Norma of our Milky
Way galaxy, obscured by dust, and then buried in a two-star system
enshrouded by dense gas.

"We are always hunting for new sources," said Simona Soldi, the
scientist at the Integral Science Data Centre in Geneva, Switzerland,
who first saw the neutron star. "It is exciting to find something so
elusive. How many more sources like this are out there?"

Neutron stars are the core remains of 'supernovae', exploded stars once
about ten times as massive as our Sun. They contain about a Sun's worth
of mass compacted into a sphere about 20 kilometres across.

"Our Galaxy's spiral arms are loaded with neutron stars, black holes and
other exotic objects, but the problem is that the spiral arms are too
dusty to see through," said Dr Volker Beckmann at NASA Goddard
Spaceflight Centre, lead author of the combined results.

"The right combination of X-ray and gamma-ray telescopes could reveal
what is hiding there, and provide new clues about the true star
formation rate in our Galaxy," he added.

Because the Integral scientists could not immediately decipher the
nature of the object, they enlisted the help of NASA's Rossi X-ray
Timing Explorer and the newly launched Swift satellite to observe it in
different wavelengths.

Because gamma rays are hard to focus into sharp images, the science team
then used the X-ray telescope on Swift to determine a precise location.
In mid April 2005, Swift confirmed that the light was 'highly absorbed',
which means the binary system was filled with dense gas from the stellar
wind of the companion star.

Later the scientists used the Rossi Explorer to observe the source as it
faded away. This observation revealed a familiar light signature,
clinching the case for a fading high-mass X-ray binary with a neutron star.

IGR J16283-4838 is the seventh so-called 'highly absorbed', or hidden
neutron star to be identified. Neutron stars, created from fast-burning
massive stars, are intrinsically tied to star formation rates. They are
also energetic 'beacons' in regions too dusty to study in detail
otherwise. As more and more are discovered, new insights about what is
happening in the Galaxy's spiral arms begin to emerge.

IGR J16283-4838 revealed itself with an 'outburst' on or near its
surface. Neutron stars such as IGR J16283-4838 are often part of binary
systems, orbiting a normal star. Occasionally, gas from the normal star,
lured by gravity, crashes onto the surface of the neutron star and
releases a great amount of energy. These outbursts can last for weeks
before the system returns to dormancy for months or years.

Integral, the Rossi Explorer and Swift all detect X-rays and gamma rays,
which are far more energetic than the visible light that our eyes
detect. Yet each satellite has different capabilities. Integral has a
large field of view, enabling it to scan our Milky Way galaxy for
neutron stars and black hole activity.

Swift contains a high-resolution X-ray telescope, which allowed
scientists to zoom in on IGR J16283-4838. The Rossi Explorer has a
timing spectrometer, a device used to uncover properties of the light
source, such as speed and rapid variations in the order of milliseconds.

For more information:

Volker Beckmann, lead author of the results, NASA GSFC
E-mail: beckmann @ milkyway.gsfc.nasa.gov

Chris Winkler, ESA Integral Project Scientist
E-mail: christoph.winkler @ esa.int

These findings will appear in the 1 October issue of the Astrophysical
Journal (Vol 631, 2nd issue, http://arxiv.org/abs/astro-ph/0506170).

More about...

* ESA's gamma-ray astronomy mission
http://www.esa.int/SPECIALS/Integral/index.html
* Integral factsheet
http://www.esa.int/esaSC/SEMD9G1A6BD_index_0.html

Related articles

* Observations: Seeing in the gamma-ray wavelengths
http://www.esa.int/esaSC/SEM3A2T1VED_index_0.html
* Observations: Seeing in X-ray wavelengths
http://www.esa.int/esaSC/SEMTA2T1VED_index_0.html
* Observations: Seeing in microwave wavelengths
http://www.esa.int/esaSC/SEM4B2T1VED_index_0.html
* Black holes
http://www.esa.int/esaSC/SEMDJ71P4HD_index_0.html
* A gamma-ray burst bonanza
http://www.esa.int/esaCP/SEMIVX8YFDD_FeatureWeek_0.html
* Integral's first look at the gamma-ray Universe
http://www.esa.int/esaCP/ESADW18708D_index_0.html
* Integral -- tracking extreme radiation across the Universe
http://www.esa.int/esaCP/ESAI0BTHN6D_index_0.html

Related links

* Related NASA release
http://lheawww.gsfc.nasa.gov/users/b.../shy_star.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMSOI6DIAE_index_1.html]
This artist's impression illustrates neutron star IGR J16283-4838
flaring. This is due to the matter accreted from its companion star.

Credits: NASA/Dana Berry

[Image 2:
http://www.esa.int/esaCP/SEMSOI6DIAE...html#subhead2]
Artist's impression of neutron star IGR J16283-4838 orbiting its
companion star. Matter flowing from the companion to the neutron star,
attracted by strong gravity, occasionally flares up in X-ray and
gamma-ray light. Such flares last only for a few days or weeks but
reveal the location of an otherwise dim system.

Credits: NASA/Dana Berry