ESA’s Integral discovers hidden black holes
Integral, ESA’s powerful gamma-ray space telescope, has discovered what seems
to be a new class of astronomical objects. These are binary systems, probably
including a black hole or a neutron star, embedded in a thick cocoon of cold
gas. They have remained invisible so far to all other telescopes. Integral
was launched exactly one year ago today to study the most energetic phenomena
in the universe.
Integral detected the first of these objects, called IGRJ16318-4848, on 29
January 2003. Although astronomers did not know its distance, they were sure
it was in our Galaxy. Also, after some analysis, researchers concluded that
the new object could be a binary system comprising a compact object, such
as a neutron star or a black hole, and a very massive companion star.
When gas from the companion star is accelerated and swallowed by the more
compact object, energy is released at all wavelengths, from the gamma rays
through to visible and infrared light. About 300 binary systems like those
are known to exist in our galactic neighbourhood and IGRJ16318-4848 could
simply have been one more. But something did not fit: why this particular
object had not been discovered so far?
Astronomers, who have been observing the object regularly, guess that it had
remained invisible because there must be a very thick shell of obscuring
material surrounding it. If that was the case, only the most energetic
radiation from the object could get through the shell; less-energetic
radiation would be blocked. That could explain why space telescopes that
are sensitive only to low-energy radiation had overlooked the object, while
Integral, specialised in detecting very energetic emissions, did see it.
To test their theory, astronomers turned to ESA’s XMM-Newton space
observatory, which observes the sky in the X-ray wavelengths. As well
as being sensitive to high-energy radiation, XMM-Newton is also able to
check for the presence of obscuring material. Indeed, XMM-Newton detected
this object last February, as well as the existence of a dense ‘cocoon’
of cold gas with a diameter of similar size to that of the Earth’s orbit
around the Sun.
This obscuring material forming the cocoon is probably ‘stellar wind’,
namely gas ejected by the supermassive companion star. Astronomers think
that this gas may be accreted by the compact black hole, forming a dense
shell around it. This obscuring cloud traps most of the energy produced
inside it.
The main author of these results, Roland Walter of the Integral Science
Data Centre, Switzerland, explained: “Only photons with the highest
energies [above 10 keV] could escape from that cocoon. IGR J16318-4848 has
therefore not been detected by surveys performed at lower energies, nor
by previous gamma-ray missions that were much less sensitive than Integral.”
The question now is to find out how many of these objects lurk in the Galaxy.
XMM-Newton and Integral together are the perfect tools to do the job.
They have already discovered two more new sources embedded in obscuring
material. Future observations are planned.
Christoph Winkler, ESA Project Scientist for Integral, said: “These early
examples of using two complementary ESA high-energy missions, Integral and
XMM-Newton, shows the potential for future discoveries in high-energy
astrophysics.”
Notes to Editors:
The paper explaining these results will be published in November in a special
issue of Astronomy and Astrophysics dedicated to Integral, on the
occasion of its first anniversary.
Integral
The International Gamma Ray Astrophysics Laboratory (Integral) is the first
space observatory that can simultaneously observe celestial objects in
gamma rays, X-rays and visible light. Integral was launched on a Russian
Proton rocket on 17 October 2002 into a highly elliptical orbit around Earth.
Its principal targets include regions of the galaxy where chemical elements
are being produced and compact objects, such as black holes.
XMM-Newton
XMM-Newton can detect more X-ray sources than any previous satellite and is
helping to solve many cosmic mysteries of the violent Universe, from black
holes to the formation of galaxies. It was launched on 10 December 1999,
using an Ariane-5 rocket from French Guiana. It is expected to return data
for a decade. XMM-Newton’s high-tech design uses over 170 wafer-thin
cylindrical mirrors spread over three telescopes. Its orbit takes it almost
a third of the way to the Moon, so that astronomers can enjoy long,
uninterrupted views of celestial objects.
The text of this release and images can be found at:
http://www.esa.int/sci_mediacentre/
Contacts:
Christoph Winkler
ESA/ESTEC, The Netherlands
Tel: + 31 71 565 3591
E-mail: Christoph.Winkler@rssd.esa.int
Roland Walter
Integral Science Data Centre, Switzerland
Tel: + 41 22 950 91 28
E-mail: Roland.Walter@obs.unige.ch
For further information, please contact:
ESA Media Relations Service
Tel: +33 (0)1.53.69.71.55
Fax: +33 (0)1.53.69.76.90
