Found 8 talks width keyword gamma rays bursts

Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions. Currently known black holes are fed by material stripped from a low-mass star or by the wind of a massive companion. Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars. However, among the ~80 Be X-ray binaries known in the Galaxy (~150 including the Magellanic Clouds), only pulsating neutron stars have been found as companions, which is known as the missing Be/black-hole X-ray binary problem. In this talk I present the first dynamical evidence for a 3.8-6.9 Msun black hole orbiting the Be star and gamma-ray candidate MWC 656 (=AGL J2241+4454). This discovery has been allowed by the detection of a HeII emission line from an accretion disc encircling the black hole. We find the black hole is X-ray quiescent with Lx<1.6 × 10−7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect by conventional X-ray surveys and may be more abundant than predicted by population synthesis models.

Gamma-Ray Bursts (GRBs) are the most powerful sources of electromagnetic radiation in the Universe. There are many open questions about their origin and their nature, and the answers should be searched in the large amount of data collected during these last years. We focused on the study of the their X-ray and optical afterglow properties, as observed by the Swift X-Ray Telescope (XRT) and ground-based optical telescopes. We investigated the observer and rest-frame properties of all GRBs observed by Swift between December 2004 and December 2010 with spectroscopic redshift through a comprehensive statistical analysis of the XRT light-curves of GRBs carried out in a model-independent way. We found out a three parameter correlation that is followed both by short and long GRBs. We also carried out a systematic analysis of the optical data available in literature for the same GRBs to investigate the GRB emission mechanisms and to study their environment properties. Our analysis shows that the gas-to-dust ratios of GRBs are larger than the values calculated for the Milky Way, the Large Magellanic Cloud, and the Small Magellanic Cloud. In this talk I will show the major results of the analysis of this large set of data.

Long Gamma-Ray Bursts are flashes of high-energy radiation and are linked to the death of massive stars. I will first summarize the main aspects of GRB astronomy, ranging from gamma to infrared frequencies, and secondly I will show how long GRBs pinpoint star-forming galaxies. Afterwards, I will present recent results which indicate as the GRB host population resembles all kind of star-forming galaxies, even the most dusty ones, almost invisible in optical-dedicated surveys.

Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually associated with supernovae (Woosley et al. 2006). They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium (Zhang et al. 2004). Here we report observations of the peculiar GRB 101225A (the "Christmas burst"). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to 1.6 Gpc by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a helium star-neutron star merger that underwent a common envelope phase expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which gets thermalized by interacting with the dense, previously ejected material and thus creating the observed black-body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy (Campana et al. 2011).

Long gamma-ray bursts are supposed to be connected to the death of very massive stars. Due to their brightness, we can detect them to much larger distances than supernovae. Using them as powerful lightsources, they allow us to study star-forming high redshift galaxies and their interstellar medium in great detail with medium and high resolution spectroscopy. Despite the large redshift ranged spanned by GRBs, there is surprisingly little evolution in the properties of their host galaxies which might indicate that GRBs can only occur under certain conditions. This can be investigated from a few bursts at very low redshifts where we can resolve their host galaxies e.g. with integral field spectroscopy. The immediate surroundings might allow us some conclusions on the progenitors of GRBs.