We used a sample of GRBs detected by Fermi and Swift to reanalyze the correlation discovered by Amati et al. (2002) between Epi, the peak energy of the prompt GRB emission, and Eiso, the energy released by the GRB assuming isotropic emission. This co
rrelation has been disputed by various authors, and our aim is to assess whether it is an intrinsic GRB property or the consequence of selection effects. We constructed a sample of Fermi GRBs with homogeneous selection criteria, and we studied their distribution in the Epi-Eiso plane. Our sample is made of 43 GRBs with a redshift and 243 GRBs without a redshift. We show that GRBs with a redshift follow a broad Epi-Eiso relation, while GRBs without a redshift show several outliers. We use these samples to discuss the impact of selection effects associated with GRB detection and with redshift measurement. We find that the Epi-Eiso relation is partly due to intrinsic GRB properties and partly due to selection effects. The lower right boundary of the Epi-Eiso relation stems from a true lack of luminous GRBs with low Epi. In contrast, the upper left boundary is attributed to selection effects acting against the detection GRBs with low Eiso and large Epi that appear to have a lower signal-to-noise ratio. In addition, we demonstrate that GRBs with and without a redshift follow different distributions in the Epi-Eiso plane. GRBs with a redshift are concentrated near the lower right boundary of the Epi-Eiso relation. This suggests that it is easier to measure the redshift of GRBs close to the lower Epi-Eiso boundary. In this context, we attribute the controversy about the reality of the Amati relation to the complex nature of this relation resulting from the combination of a true physical boundary and biases favoring the detection and the measurement of the redshift of GRBs located close to this boundary.
Gamma-ray bursts are associated with catastrophic cosmic events. They appear when a new black hole, created after the explosion of a massive star or the merger of two compact stars, quickly accretes the matter around it and ejects a transient relativ
istic jet in our direction. This review discusses the various types of gamma-ray bursts, their progenitors, their beaming and their rate in the local universe. We emphasize the broad astrophysical interest of GRB studies, and the crucial role of high-energy satellites as exclusive suppliers of GRB alerts and initial locations.
Gamma-ray bursts (GRBs) were first detected thanks to their prompt emission, which was the only information available for decades. In 2010, while the high-energy prompt emission remains the main tool for the detection and the first localization of GR
B sources, our understanding of this crucial phase of GRBs has made great progress. We discuss some recent advances in this field, like the occasional detection of the prompt emission at all wavelengths, from optical to GeV; the existence of sub-luminous GRBs; the attempts to standardize GRBs; and the possible detection of polarization in two very bright GRBs. Despite these advances, tantalizing observational and theoretical challenges still exist, concerning the detection of the faintest GRBs, the panchromatic observation of GRBs from their very beginning, the origin of the prompt emission, or the understanding of the physics at work during this phase. Significant progress on this last topic is expected with SVOM thanks to the observation of dozens of GRBs from optical to MeV during the burst itself, and the measure of the redshift for the majority of them. SVOM will also change our view of the prompt GRB phase in another way. Within a few years, the sensitivity of sky surveys at optical and radio frequencies, and outside the electromagnetic domain in gravitational waves or neutrinos, will allow them to detect several new types of transient signals, and SVOM will be uniquely suited to identify which of these transients are associated with GRBs. This radically novel look at GRBs may elucidate the complex physics producing these bright flashes.
