The INTEGRAL view of Gamma-Ray Bursts

After more than six and half years in orbit, the ESA space observatory INTEGRAL has provided new, exciting results in the soft gamma-ray energy range (from a few keV to a few MeV). With the discovery of about 700 hard X-Ray sources, it has changed our previous view of a sky composed of peculiar and “monster” sources. The new high energy sky is in fact full of a large variety of normal, very energetic emitters, characterized by new accretion and acceleration processes (see also IBIS cat4 (Bird et al., 2010). At the same time, about one GRB/month is detected and imaged by the two main gamma-ray instruments on board: IBIS and SPI. In this paper, we review the major achievements of the INTEGRAL observatory in the field of Gamma-Ray Bursts. We summarize the global properties of Gamma-Ray Bursts detected by INTEGRAL, with respect to their duration, spectral index, and peak flux distributions. We recall INTEGRAL results on the spectral lag analysis, showing how long-lag GRBs appear to form a separate population at low peak fluxes. We review the outcome of polarisation studies performed by using INTEGRAL data. Finally, concerning single GRB studies, we highlight the properties of particularly interesting Gamma-Ray Bursts in the INTEGRAL sample.     

QSO number counts to B < 24.5 from hubble space telescope WFPC2 images

We have conducted a survey for faint quasars to B < 24.5, z < 2.1 based on archival high-latitude Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images covering 0.04 deg² of the sky in three filters spanning 2300–9600 Å. Taking advantage of the superb 0.1″ resolution of WFPC2, we have developed an approach to select quasar candidates based on their morphology and position in multicolor space. We derive cumulative surface densities and compare them with our previous survey in the Groth–Westphal Strip and other faint quasar surveys that utilize very different selection approaches. In most cases the agreement with these surveys is good, however, we find approximately twice as many quasar candidates to B = 24.5 as the COMBO-17 survey. Although we are confident that our approach eliminates most contaminating stars and galaxies, spectroscopic follow-up will be required to determine the level of residual contamination and confirm or refute this excess.