Integral expands our view of the gamma-ray sky

The third Integral/IBIS survey catalog More Information ROLLOVER Credits: IBIS survey team

"The gamma-ray sky is notoriously variable and extremely unpredictable," says Anthony Dean, University of Southampton, UK, one of the original proposers of the Integral mission. Hence, the need for Integral's constant vigilance and an accurate catalog of all gamma-ray sources. With this, astronomers can target individual gamma-ray objects for more detailed, study.

For the past three and a half years, Integral has been collecting survey data. At the end of every year, the data has been turned into a catalog of sources.

During the first year, it concentrated on the regions close to the center of our galaxy and found more than 120 sources. During the following year, Integral broadened its reach and found almost 100 more sources.

Now Integral has observed over 70 percent of the sky, with a total exposure time of 40 million seconds. A European team of astronomers led by Antony Bird, University of Southampton, UK, have turned all three years' worth of data into the third Integral catalog of gamma-ray sources. It contains a total of 421 gamma-ray objects. Most have been identified as either binary stars in our Galaxy containing exotic objects such as black holes and neutron stars, or active galaxies, far away in space. But a puzzling quarter of sources remain unidentified so far.

Artist's impression of Integral More Information ROLLOVER Credits: ESA (Medialab)

One surprise has been the efficiency with which Integral has detected just one minor subclass of cataclysmic variable stars (CVs), the so-called intermediate polars. Initially astronomers were not sure that CVs would emit gamma rays. Indeed, Integral has already shown that only about one percent of them do. "At the moment, the reason why this should be is totally mysterious," says Dean.

As its surveys points further from our Galaxy, so Integral increasingly sees the active galaxies. These represent about a tenth of all galaxies and each one has some kind of extraordinary activity taking place in its core. It is widely accepted that this activity is driven by a gigantic black hole sucking matter out of existence.

Roman Krivonos, Max-Planck-Institute für Astrophysik, Germany, and colleagues have used the Integral survey to show that AGN are concentrated in the same places that ordinary galaxies are found. Whilst this is not an unexpected result, it is the first time such an AGN distribution has been seen at high-energies.

"Integral represents a milestone in gamma-ray astronomy," says Dean. Thirty years ago, NASA’s Einstein observatory produced a catalog of X-ray sources that became the standard reference document for all X-ray observatories – including ESA's XMM-Newton. "Integral is doing the same for gamma-ray astronomy," says Dean.

"We are in a golden age of gamma-ray astronomy," agrees Bird. And ESA's Integral is at the forefront of this brave new universe.

Source / Credit: ESA