Report on 5th Integral Workshop
The workshop started with a session devoted to nucleosynthesis studies. The radioactive isotope 26Al is a key tracer of this process, and the emission of 1809 keV photons during its radioactive decay provides powerful means to study nucleosynthesis in our Galaxy. The Cygnus region is one of the most active nearby star forming regions in our Galaxy. Emission from 26Al has been previously detected in the Cygnus region by the COMPTEL observatory. INTEGRAL results confirm this detection and make it possible to measure with a much better precision the flux and spectral profile of the 1809 keV line. The line profile gives us information about the kinematics of the emitting region and the new INTEGRAL results show us that galactic rotation alone cannot explain the observations (Knoedelseder et al.). In fact there are hints of large scale motions within this active star forming region, possibly due to an expanding "bubble" of matter blown away by some newly born stars.
Another key result on gamma-ray line spectroscopy is the first map of the Galactic central region in the light of the 511 keV line. This emission is produced by the annihilation of electrons and their anti-particles, the positrons. A first map obtained by SPI shows a symmetric (about 10deg diameter) diffuse emission from the Centre of our Galaxy (Jean et al.). The emission can not be explained by a single source. Also no emission from the Galactic plane or from higher latitudes, as reported earlier, is visible. Furthermore, the data imposes strong constraints on the production rate of positrons.
The next topic to be discussed in the workshop was that of X-ray binary stars, which, together with Gamma-ray bursts, are by far the brightest sources observed by INTEGRAL. Not surprisingly therefore the "X-ray binary" session had the greatest number of contributions of this workshop.
One very exiting INTEGRAL result from its Galactic Plane Scan (GPS) program is the discovery of a new type of highly obscured X-ray binary sources which had so far escaped previous detection with other instruments (Walter et al.). Thanks to its good sensitivity in the hard X-ray band INTEGRAL can see them for the first time. These new sources are located within one of the spiral arms of our galaxy (the "Norma" arm) and are enshrouded in a so-called "Compton thick" environment. The obscuring gas resides in the likely binary system. It is most probably related to the accretion flow or the wind from the high mass stellar companion within the binary system. More INTEGRAL observations are necessary before we can understand the true nature of these "hidden" X-ray sources.
INTEGRAL was also pointed at the centre of our Galaxy. This region is known to harbor a super-massive black hole and it is particularly fascinating and puzzling because of it's relatively faint emission. This is difficult to understand for such a massive black hole. However, Goldwurm et al. reported on the discovery of a source, IGR J17456-2901, coincident with the Galactic Nucleus Sgr A* to within 0.9'. It is visible up to about 100 keV with a luminosity at 8 kiloparsecs of about 3x1035 ergs-1 in the 20-100 keV band. The new INTEGRAL source cannot unequivocally be associated to the Galactic Nucleus. But this is the first report of significant hard X-ray emission from within the inner 10' of the Galaxy and a contribution from the galactic super-massive black hole itself cannot be excluded.
The archetypal wind-accreting X-ray binary Vela X-1 has been observed extensively by INTEGRAL during the Vela Region Core Program observations, covering a wide sample of binary orbital phase intervals and different source activity levels. The long, 72 hour orbit of INTEGRAL made it possible to obtain for the first time extended and uninterrupted light curves, which reveal bright and short flares not detectable by other observatories (Staubert et al.). Even though Vela X-1 is known to be quite variable, the flares observed by INTEGRAL are of exceptional nature.
The galactic micro-quasar SS433 was observed simultaneously by INTEGRAL and large optical, ground-based telescopes (Cherepashchuk et al.). Exceptionally good data were collected during this combined observation campaign and the phase of eclipse of this binary system was very well covered. These data in particular can be used to derive the basic physical properties of the SS433 binary system.
Active Galactic Nuclei (AGN) are, by their great distance from us, relatively faint in the X-ray and Gamma-ray ranges compared to some bright X-ray emitting sources within our own Galaxy. They are, therefore, a more difficult type of source for INTEGRAL to observe. NGC 4388 is a so-called "Seyfert 2" galaxy. This type of AGN is often heavily obscured by matter close to the central massive black hole. The UV and soft X-ray light from the nuclear region is blocked and only very high energy X-ray radiation can escape through the absorbing matter, making Seyfert 2 galaxies particularly interesting sources for INTEGRAL. In this context NGC 4388 is the first Seyfert 2 seen by INTEGRAL (Beckmann et al.). Combined observations with the XMM-Newton observatory of ESA show that the X-ray spectrum of NGC 4388 is emitted by at least two, physically distinct, components.
Because of the strong absorption by the gas inside our Galaxy, it is difficult to observe extra-galactic sources located in the direction of our galactic plane. Despite this, a few AGN have been detected during INTEGRAL's Galactic Plane Scan (GPS) and Galactic Centre Deep Exposure (GCDE). However, their numbers are slightly lower than we would expect. This difference is not yet understood (Bassani et al.).
After one year in orbit a comprehensive and consistent analysis of the INTEGRAL/IBIS Core Program survey data has been made (Bird et al.), incorporating both the Galactic Plane Scans (GPS) and the Galactic Centre Deep Exposure (GCDE). This covers the first 100 revolutions of INTEGRAL operations. Using strict statistical criteria 124 sources have been detected, of which 40% are Low Mass X-ray Binaries and 31% are of undefined nature. 14 sources are completely new.
Last but not least the workshop included a session devoted to Gamma-ray bursts (GRBs). It was shown that the Integral Burst Alert System (IBAS, Mereghetti et al.) currently gives the best GRB localisations, in terms of speed and accuracy! The IBAS has the capability of handling not only GRBs, but also flaring events from Soft Gamma-ray Repeaters and known transient sources. At the time of the workshop 8 GRBs had been detected within the INTEGRAL field of view. These are all so-called "long bursts" (Kulkarni et al.). During the next years INTEGRAL will collect more GRB events and bring better statistics to this perplexing field of study.
The final morning of the workshop was enlivened by an urgent request for a "Target of Opportunity" observation submitted by a French astronomer to the INTEGRAL project scientist C Winkler, right then and there. It took no more than 55 minutes and two phone calls with the mission planner at ISOC to reschedule the pre-planned observing programme of INTEGRAL so it could observe the flaring X-ray binary star GX 339-4 on coming weekend! (This observation was successfully made and during this TOO, GRB #9 was detected)
The next INTEGRAL Workshop, the 6th of its kind, will be held in St Petersburg (Russia) in June 2006. By then INTEGRAL will surely deliver many more impressive discoveries.