ESA's Integral Solves Thirty-Year Old Gamma-Ray Mystery
|Artist's view of Integral|
Credits: ESA. Illustration by D. Ducros
17 March 2004
(ESA Press release)
ESA's Integral gamma-ray observatory has resolved the diffuse glow of
gamma rays in the center of our Galaxy and has shown that most of it is
produced by a hundred individual sources.
Integral's high sensitivity and pointing precision have allowed it to
detect these celestial objects where all other telescopes, for more than
thirty years, had seen nothing but a mysterious, blurry fog of gamma
During the spring and autumn of 2003, Integral observed the central
regions of our Galaxy, collecting some of the perpetual glow of diffuse
low-energy gamma rays that bathe the entire Galaxy.
|The central regions of our galaxy, the
Milky Way, as seen by Integral in gamma rays. With its superior ability
to see faint details, Integral correctly reveals the individual sources
that comprised the foggy, gamma-ray background seen by previous
observatories. The brightest 91 objects seen in this image were
classified by Integral as individual sources, while the others appear
too faint to be properly characterized at this stage.|
Credits: ESA, F. Lebrun (CEA-Saclay)
These gamma rays were first discovered in the mid-1970s by high-flying
balloon-borne experiments. Astronomers refer to them as the 'soft'
Galactic gamma-ray background, with energies similar to those used in
medical X-ray equipment.
Initially, astronomers believed that the glow was caused by interactions
involving the atoms of the gas that pervades the Galaxy. While this
theory could explain the diffuse nature of the emission, since the gas
is ubiquitous, it failed to match the observed power of the gamma rays.
The gamma rays produced by the proposed mechanisms would have to be much weaker
than those observed. The mystery has remained unanswered for
Now Integral's gamma-ray telescope IBIS, built for ESA by an
international consortium led by Principal Investigator Pietro Ubertini
(IAS/CNR, Rome, Italy), has seen clearly that, instead of a fog produced
by the interstellar medium, most of the gamma-rays are coming from
individual celestial objects. In the view of previous
instruments, these objects appeared to merge together.
In a paper published in Nature, Francois Lebrun (CEA Saclay, Gif sur
Yfette, France) and his collaborators report the discovery of 91
gamma-ray sources toward the direction of the Galactic center. Much
to the team's surprise, almost half of these sources do not fall in
any class of known gamma-ray objects. They probably represent a new
population of gamma-ray emitters.
The first clues about a new class of gamma-ray objects came last
October, when Integral discovered an intriguing gamma-ray source, known
as IGRJ16318-4848. The data from Integral and ESA's other high-energy
observatory XMM-Newton suggested that this object is a binary system,
probably including a black hole or neutron star, embedded in a thick
cocoon of cold gas and dust. When gas from the companion star is
accelerated and swallowed by the black hole, energy is released at all
wavelengths, mostly in the gamma rays.
However, Lebrun is cautious to draw premature conclusions about the
sources detected in the Galactic center. Other interpretations are also
possible that do not involve black holes. For instance, these objects
could be the remains of exploded stars that are being energized by
rapidly rotating celestial 'powerhouses', known as pulsars.
Observations with another Integral instrument (SPI, the Spectrometer on
Integral) could provide Lebrun and his team with more information on the
nature of these sources. SPI measures the energy of incoming gamma rays
with extraordinary accuracy and allows scientist to gain a better
understanding of the physical mechanisms that generate them.
However, regardless of the precise nature of these gamma-ray sources,
Integral's observations have convincingly shown that the energy output
from these new objects accounts for almost ninety per cent of the soft
gamma-ray background coming from the center of the Galaxy. This result
raises the tantalizing possibility that objects of this type hide
everywhere in the Galaxy, not just in its center.
Again, Lebrun is cautious, saying, "It is tempting to think that we can
simply extrapolate our results to the entire Galaxy. However, we have
only looked toward its center and that is a peculiar place compared to
Next on Integral's list of things to do is to extend this work to the
rest of the Galaxy. Christoph Winkler, ESA's Integral Project Scientist,
says, "We now have to work on the whole disk region of the Galaxy. This
will be a tough and long job for Integral. But at the end, the reward
will be an exhaustive inventory of the most energetic celestial objects
in the Galaxy."