ESA’s gamma-ray observatory Integral is making excellent progress,
mapping the Galaxy at key gamma-ray wavelengths. It is now poised
to give astronomers their truest picture yet of recent changes in the
Milky Way’s chemical composition. At the same time, it has confirmed an
‘antimatter’ mystery at the centre of the Galaxy.

Since its formation from a cloud of hydrogen and helium gas,
around 12 000 million years ago, the Milky Way has gradually been
enriched with heavier chemical elements. This has allowed planets and,
indeed, life on Earth to form.

Today, one of those heavier elements – radioactive aluminium –
is spread throughout the Galaxy and, as it decays into magnesium,
gives out gamma rays with a wavelength known as the ‘1809 keV line.’
Integral has been mapping this emission with the aim of understanding
exactly what is producing all this aluminium.

In particular, Integral is looking at the aluminium ‘hot spots’ that
dot the Galaxy to determine whether these are caused by individual
celestial objects or the chance alignment of many objects.

Astronomers believe that the most likely sources of the aluminium are
supernovae (exploding high-mass stars) and, since the decay time of the
aluminium is around one million years, Integral’s map shows how many
stars
have died in recent celestial history. Other possible sources of the
aluminium include ‘red giant’ stars or hot blue stars that give out the
element naturally.

To decide between these options, Integral is also mapping radioactive
iron, which is only produced in supernovae. Theories suggest that,
during a supernova blast, aluminium and iron should be produced together
in the same region of the exploding star. Thus, if the iron’s
distribution coincides with that of the aluminium, it will prove that
the overwhelming majority of aluminium comes indeed from supernovae.

These measurements are difficult and have not been possible so far,
since the gamma-ray signature of radioactive iron is about six times
fainter than that of the aluminium. However, as ESA’s powerful Integral
observatory accumulates more data in the course of the next year,
it will finally be possible to reveal the signature of radioactive iron.
This test will tell astronomers whether their theories of how elements
form are correct.

In addition to these maps, Integral is also looking deeply into the
centre of the Galaxy, to make the most detailed map ever of
‘antimatter’ there.

Antimatter is like a mirror image to normal matter and is produced
during
extremely energetic atomic processes: for example, the radioactive
decay of
aluminium. Its signature is known as the ‘511 keV line.’ Even though
Integral’s observations are not yet complete, they show that there
is too much antimatter in the centre of the Galaxy to be coming from
aluminium decay alone. They also show clearly that there must be many
sources of antimatter because it is not concentrated around a single
point.

There are many possible sources for this antimatter. As well as
supernovae,
old red stars and hot blue stars, there are jets from neutron stars
and
black holes, stellar flares, gamma-ray bursts and interaction between
cosmic rays and the dusty gas clouds of interstellar space.

Chris Winkler, ESA’s Integral’s Project Scientist, says: “We have
collected excellent data in the first few months of activity but
we can and will do much more in the next year. Integral’s accuracy and
sensitivity have already exceeded our expectations and, in the month
t come, we could get the answers to some of astronomy’s most
intriguing questions.”

Note to Editors:

These and other preliminary results, plus a thorough description
of the Integral spacecraft and mission are published this month
in a dedicated issue of the journal Astronomy and Astrophysics.

At its 105th meeting on 6 October 2003, ESA’s Science Programme
Committee unanimouslydecided to extend the Integral mission until
December 2008.

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.

The SPI spectrometer on board Integral measures the energy of
incoming gamma rays with extraordinary accuracy. It is more sensitive
to faint radiation than any previous gamma ray instrument and allows
the
precise nature of gamma ray sources to be determined. SPI’s Principal
Investigators are: J.-P. Roques, (CESR Toulouse, France), V.
Schoenfelder
(MPE Garching, Germany).

The text of this release and images can be found at:
http://www.esa.int/sci_mediacentre/