Science at ISOC


The research carried out by the ISOC scientists is mostly focused on the study of sources in our Galaxy, the Milky way. The Galactic Center region and any compact object in the Galactic Bulge, plane or halo can be of interest to any member of the team, composed by Guillaume Belanger, Peter Kretschmar, Erik Kuulkers, Celia Sanchez Fernandez.

More precisely, our main scientific goals are:

Accreting X-ray binaries

Low Mass X-ray binaries

In these systems, a companion star usually filling its Roche Lobe, transfers mass to a compact object, either a Black Hole (BH) or a Neutron star (NS). The accretion of material by the compact object, a process far more energetically efficient than nuclear fusion is the source of the enormous power output of these systems. Because the two stars orbit each other, the transferred material has angular momentum which this causes it to spiral into the compact object in an accretion disk, where the gravitational potential energy of the accreted matter is slowly released as electromagnetic radiation.

X-ray burst movie

X-ray bursters

Type I X-ray bursts are thermonuclear explosions on the surface of weakly magnetized accreting NS in LMXB systems. In an X-ray burst, Hydrogen orHellium–rich material, accreted from the companion star, and piled on the (hard) surface of the NS over hours, or days, is burnt in a few seconds as the result of a thermal instability on the surface of the compact object. Type-I X-ray bursts are detected in the system X-ray light curves, as is in this energy domain where most of their energy is released. 

Accreting X-ray pulsars

In accreting X-ray pulsars, the high magnetic field of the Neutron Star (typically B≈1012 Gauss), funnels the accreted matter onto hotspots at the magnetic poles. The gas that supplies the X-ray pulsar can reach the neutron star by a variety of ways that depend on the size and shape of the neutron star's orbital path and the nature of the companion star. Some companion stars of X-ray pulsars are very massive young stars, usually OB supergiants (see stellar classification), that emit a radiation driven stellar wind from their surface, while in other system, the neutron star is so close to its companion that mass transfer takes place via Roche-Lobe overflow.


Microquasars are stellar-size version of quasars. These accreting X-ray binary systems display strong and variable radio emission, often resolved as a pair of radio jets, which sometimes shows apparent superluminal motion. Microquasars are very important for the study of relativistic jet phenomena. The jets are formed close to the compact object, and timescales near the compact object are proportional to the mass of the compact object. Therefore, ordinary quasars take centuries to go through variations a microquasar experiences in one day.

Combining data from various high-energy missions (INTEGRAL, RXTE, XMM-Newton, Suzaku, Swift or Maxi) and ground based instruments (operating from radio to Optical/NIR), members of the team can study the many faces of these active systems. The combination of multiwavelength information is essential to understand the various physical mechanisms at work in X-ray binaries.

Galactic Bulge monitoring

The Galactic bulge extends from the Galactic centre to a radius of about 3 kpc, and so spans the central 40 degrees of our Galaxy, corresponding to a distance of about 10000 light years. The region at its very heart, near the nucleus of the Galaxy, is one of the most dynamic places in the Milky Way. Since its launch INTEGRAL has kept close watch on the ever-changing high-energy landscape of the bulge. From 2005 onwards three and a half hour 'snapshot' observations are done approximately every three days (once per orbit), whenever possible, as part of the Galactic bulge monitoring program. As a service to the scientific community the X-ray and gamma-ray light curves and images are made available soon after the observations have been performed. For more details on the program and on its scientific results see:


The Galactic Center

The Central Molecular Zone

The CMZ, at the heart of which lies dormant the four million supermassive black hole Sagittarius A*, spans only a few degrees when view from the Earth. Its spherical volume is one millionth that of the Galaxy's, and yet contains 10% of all the Milky Way's stores of molecular gas. This means that the density of gas there is one hundred thousand times greater than the Galactic average. Almost all of the gas is bound to specific orbits in which the material revolves around the dark core of the Galaxy, and at least half is in giant molecular clouds, among the largest and the densest in the Galaxy. The high stellar density, the intense radiation fields, the extreme gas densities, the vigorous orbital dynamics and the gravitational sheers, all combine in various complex ways to make this region very unique indeed. There is a wealth of phenomena to study at all wavelengths, but the most energetic X-rays and soft gamma-rays carry information from the deepest depths of the CMZ.

Vibrant Activity in Sagittarius

Looking up into the night sky from somewhere in the southern hemisphere, the Galactic centre lies just to right of the archer Sagittarius' head, and is flanked on the left side by the long Scorpius with Sco X-1, the brightest X-ray source, discovered in 1962 by Giacconi. The Galactic centre, however, was detected as a radio source as far back as 1932 by Karl Jansky: the birth of radio astronomy. In 1974, Balick and Brown discovered that within the radio bright Sagittarius A complex at the Galactic centre, was a very bright point source, unusual in its features, lying at the dynamical centre of the Milky Way. In fact, Sgr A* is so unusual that there is only one such object in the entire Galaxy. And even if it was suspected to be the signature of a supermassive black hole from the very early days, it is after several decades of observations that we have traced the orbits of several stars that make the case for a supermassive black hole very compelling.

Sgr A* was seen flaring in X-rays by Chandra and XMM-Newton several times between 1999 and 2010. The flares are bright with respect to the quiescent X-ray emission, but dim compared to even the smallest hiccups in active galactic nuclei. However, close to a decade of Integral observations combined with all XMM-Newton data revealed the incredibly lucky detection of fading hard X-rays form the giant molecular cloud Sgr B2, about 50 light years from Sgr A*. This is a clear signature of a very bright 10-year flare from the massive black hole around 150 years ago. But even just a few years from now, Sgr B2 will not be visible at all by Integral, and the X-rays producing the 6.4 keV fluorescence seen by XMM-Newton will have passed on. This is indeed a very special time for high-energy observations of the Galactic Centre.


More details are provided in the ISOC scientists' personal pages.