Scientists Find High Energy Systems Hidden in 'Gas Cocoon' (Forwarded)

Rob Gutro
Goddard Space Flight Center, Greenbelt, Md. February 6, 2007
(Phone: 301/286-4044)

Scientists Find High Energy Systems Hidden in 'Gas Cocoon'

Astronomers have found a new class of objects in space: a neutron star
orbiting inside a cocoon of cold gas and/or dust that hides a bloated
supergiant star. In a strange twist of fate, these objects may be
tremendously luminous, but the enshrouding cocoon absorbs almost all their
emission, making them nearly invisible to telescopes on Earth until now.

These findings were presented on Feb. 5, 2007 by Dr. Sylvain Chaty of the
University Paris 7 and Service d'Astrophysique, CEA, France, at the first
Gamma Ray Large-Area Space Telescope (GLAST) scientific Symposium meeting
in Palo Alto, Calif.

This result, obtained through multi-wavelength observations, is of special
interest because it revolutionizes the view scientists have on the
formation, evolution, and fate of massive stars in close binary (twin
star) systems. Furthermore, this clearly demonstrates that the GLAST
satellite will likely discover more of these new and unexpected celestial
objects.

This new class of objects was discovered using the European "INTErnational
Gamma-Ray Astrophysics Laboratory" (INTEGRAL) satellite. Twenty of these
binary systems were found, with estimated distances lying between 7,000
and 25,000 light years from Earth, putting them all inside of our own
Milky Way Galaxy. The nature of these sources was revealed through
multi-wavelength observations, mainly from optical to mid-infrared (MIR)
wavelengths, using European Southern Observatory (ESO) facilities.

Scientists have found that most of these sources are made up of a compact
object orbiting a supergiant star, an enormous star with 30 times the
Sun's mass and 20 times its diameter. Stars like this eject a huge amount
of cold gas and/or dust at a rate equivalent to emitting the mass of our
Sun in just 100,000 years. This type of object is called a High Mass X-ray
Binary System (HMXB) and in most cases the compact object is a neutron
star, an object of about 1.4 solar masses concentrated in a radius of only
10 kilometers (6.2 miles). Normally, an object like this would be an
intense source of X-rays as the tremendous gravity and magnetic fields of
the neutron star interact with the dense gas and dust emitted from the
more massive supergiant star. However, for this new class of objects the
cocoon of cold gas and/or dust is so dense it absorbs most, but not quite
all, of the high energy X-rays. Only multi-wavelength observations, from
X-rays to infrared, were able to reveal the nature of such objects.

These systems seem to divide into two classes, likely depending on the
size and eccentricity (ellipticity) of the orbit of the neutron star
around its companion. In the first class of objects, such as IGR
J16318-4848, the neutron star orbits around the supergiant star along a
roughly circular orbit, like the Earth does around the Sun. However, in
this case, the orbit is far smaller: the distance from the neutron star to
the supergiant is less than the distance of Mercury from the Sun -- even
though the supergiant star's radius is 20 times bigger than that of the
Sun.

Since the cocoon of cold gas/dust totally blankets the whole system, the
neutron star stays permanently inside this dense cocoon, so there is a
persistent source of X-rays. But in the second class, such as IGR
J17544-2619, the orbit is more eccentric, and the neutron star crosses
only periodically into this dense cocoon of cold gas/dust covering the
supergiant star, causing intermittent emission of X-rays during that time.

As Dr. Chaty, Associate Professor of Astrophysics at University Paris 7
and Service d'Astrophysique, CEA, France, points out, these findings are
important because " until now only a few high energy systems hosting
supergiant stars were known, and none were exhibiting such a high
absorption. We now have an example where we know the extent of the
material causing this absorption, and also where it is coming from."

"This finding will help us to understand the formation, evolution and fate
of such massive stars in high energy binary systems. This result
demonstrates that astronomers will likely discover previously unknown
objects with the GLAST satellite, and that a multi-wavelength approach can
be decisive to unveil their nature."

Further work, both observational and theoretical, is therefore needed to
study the environment of supergiant stars in binary star systems. GLAST,
with its sensitivity to very high-energy gamma rays, will provide an ideal
platform for follow-up observations. GLAST is slated for launch in the
fall of 2007. It is a collaborative effort between NASA, the U.S.
Department of Energy and institutions in France, Germany, Japan, Italy and
Sweden. General Dynamics has been chosen to build the spacecraft.

[NOTE: Images and weblinks supporting this release are available at
http://www.nasa.gov/centers/goddard/...as_cocoon.html ]