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Gamma-ray Bursts

Group members: Ruben Salvaterra, Sandro Mereghetti, Andrea De Luca, Andrea Tiengo, Paolo Esposito, Andrea Guliani

INTEGRAL/IBAS GRBs. Our institute involvement in the GRB field started with the realization of the INTEGRAL Burst Alert System (IBAS) and is now continuing with the study of afterglows with XMM-Newton and Swift. IBAS has been realized under the IASF-Milano responsibility as part of the Italian contribution to the INTEGRAL Science Data Center (ISDC). IBAS is a software system, running on ground at the ISDC, to detect and localize in near real time the GRBs detected by the INTEGRAL instruments. In particular, the GRBs in the IBIS field of view can be localized with an uncertainty of 2 arcmin and their positions distributed within a few seconds. IBAS started to operate immediately after the INTEGRAL launch at the end of 2002, providing the first rapid and accurate localizations at a rate of about one GRB/month. IASF scientists are currently responsible for the maintenance and operations of IBAS, in collaboration with the ISDC, and for the quick look interactive analysis performed after the automatic alerts delivery in order to validate the events, derive refined information and distribute the GCN notices. Besides studying the GRB prompt emission with INTEGRAL data, we have also been active in the observations of GRB afterglows, mainly exploiting our expertise with the XMM-Newton EPIC instrument, but also using Swift XRT data.

Statistical study of the GRB population. Complete samples are at the base of any population studies. We are now studing the rest-frame properties of a well-selected complete sub-sample of long GRBs detected by Swift. The sample is constructed by considering: i) burst with favorable observing conditions for ground-based follow-up otical/NIR observations and ii) GRBs bright in the 15-150 keV Swift/BAT band. This sample has a redshift completeness level of 90% (95% is reached by considering also available redshift constraints). For the first time, our complete sample allows us to study in an unbiased way the properties of long GRBs and their evolution with cosmic time. It also provides a critical test for population synthesis models aimed to make reliable prediction for future GRB mission. Special attention is paid to compute the rate of high redshift GRBs and to charaterize their properties and the envirorment in which they exploded.

Galactic dust studies with GRB afterglows. A line of activity consists in the study of X-ray scattering rings observed around GRBs. These are produced by dust layers in our Galaxy and provide a mean to derive extremely accurate distances for the dust clouds, and in some cases also to constrain the properties of the prompt GRB emission at soft X-ray energy. We have developed a sensitive method to optimize the detection and study of faint dust scattering expanding rings. An automatic pipeline has been implemented to semi-automatically search for dust rings in all the new bursts observed with the Swift/XRT and XMM-Newton/EPIC instruments.