Synchrotron Emissions in GRB Prompt Phase Using a Semi Leptonic and Hadronic Model

In this communication devoted to the prompt emission of GRBs, we claim that some important parameters associated to the magnetic field, such as its index profile, the index of its turbulence spectrum and its level of irregularities, will be measurable with GLAST. In particular the law relating the peak energy Epeak with the total energy E (like Amati's law) constrains the turbulence spectrum index and, among all existing theories of MHD turbulence, is compatible with the Kolmogorov scaling only. Thus, these data will allow a much better determination of the performances of GRBs as particle accelerators. This opens the possibility to characterize both electron and proton acceleration more seriously. We discuss the possible generation of UHECRs and of its signature through GeV-TeV synchrotron emission.Comment: 30th International Cosmic Ray Conference (ICRC2007) - Proceeding #107

Photospheric Emission in the Joint GBM and Konus Prompt Spectra of GRB 120323A

GRB 120323A is a very intense short Gamma Ray Burst (GRB) detected simultaneously during its prompt gamma-ray emission phase with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope and the Konus experiment on board the Wind satellite. GBM and Konus operate in the keV--MeV regime, however, the GBM range is broader both toward the low and the high parts of the gamma-ray spectrum. Analysis of such bright events provide a unique opportunity to check the consistency of the data analysis as well as cross-calibrate the two instruments. We performed time-integrated and coarse time-resolved spectral analysis of GRB 120323A prompt emission. We conclude that the analyses of GBM and Konus data are only consistent when using a double-hump spectral shape for both data sets; in contrast, the single-hump of the empirical Band function, traditionally used to fit GRB prompt emission spectra, leads to significant discrepancies between GBM and Konus analysis results. Our two-hump model is a combination of a thermal-like and a non-thermal component. We interpret the first component as a natural manifestation of the jet photospheric emission.Comment: 7 pages of article (3 figures and 1 table) + 3 pages of Appendix (3 figures). Submitted to ApJ on 2017 March 2

An overview of the current understanding of Gamma-ray Bursts in the Fermi era

Gamma-ray bursts are the most luminous explosions in the Universe, and their origin as well as mechanism are the focus of intense research and debate. More than three decades since their serendipitous discovery, followed by several breakthroughs from space-borne and ground-based observations, they remain one of the most interesting astrophysical phenomena yet to be completely understood. Since the launch of Fermi with its unprecedented energy band width spanning seven decades, the study of gamma-ray burst research has entered a new phase. Here we review the current theoretical understanding and observational highlights of gamma-ray burst astronomy and point out some of the potential promises of multi-wavelength observations in view of the upcoming ground based observational facilities.Comment: 45 pages, 18 figures; Special issue of the Bulletin of the Astronomical Society of India on Transients from Radio to Gamma rays, December, 2011, eds. D.J. Saikia and D.A. Gree

A Universal Scaling for the Energetics of Relativistic Jets From Black Hole Systems

Black holes generate collimated, relativistic jets which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies (active galactic nuclei; AGN). How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGNs is still unknown. Here we show that jets produced by AGNs and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGNs and GRBs lying at the low and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.Comment: Published in Science, 338, 1445 (2012), DOI: 10.1126/science.1227416. This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. Corrected typo in equation 4 of the supplementary materia

GRB 090227B: the missing link between the genuine short and long GRBs

The time-resolved spectral analysis of GRB090227B, made possible by the Fermi-GBM data, allows to identify in this source the missing link between the genuine short and long GRBs. Within the Fireshell model [...] we predict genuine short GRBs: bursts with the same inner engine of the long bursts but endowed with a severely low value of the Baryon load, B<~5x10^{-5}. A first energetically predominant emission occurs at the transparency of the e+e- plasma, the Proper-GRB (P-GRB), followed by a softer emission, the extended afterglow. The typical separation between the two emissions is expected to be [...] 10^{-3}-10^{-2}s. We identify the P-GRB [...] in the first 96ms of emission, where a thermal component with [...] kT=(517+/-28)keV and a flux comparable with the non thermal part of the spectrum is observed. This non thermal component as well as the subsequent emission, where there is no evidence for a thermal spectrum, is identified with the extended afterglow. We deduce a theoretical cosmological redshift z=1.61+/-0.14. We then derive the total energy E^{tot}_{e+e-}=(2.83+/-0.15)x10^{53}erg, [...] B=(4.13+/-0.05)x10^{-5}, the Lorentz factor at transparency \Gamma_tr=(1.44+/-0.01)x10^4, and the intrinsic duration \Delta t'~0.35s. We also determine the average density of the CircumBurst Medium (CBM), =(1.90+/-0.20)x10^{-5} #/cm^3. There is no evidence of beaming in the system. In view of the energetics and of the Baryon load of the source, as well as of the low interstellar medium and of the intrinsic time scale of the signal, we identify the GRB progenitor as a binary neutron star. From the recent progress in the theory of neutron stars, we obtain masses of the stars m_1=m_2=1.34M_Sun and their corresponding radii R_1=R_2=12.24km and thickness of their crusts ~0.47km, consistent with the above values of the Baryon load, of the energetics and of the time duration of the event.Comment: 14 pages, 14 figures, new version with some updated references, matching the one actually appeared on Ap

The Fermi view of gamma-ray bursts

Since its successful launch in June 2008, the {\it Fermi} Gamma-ray Space Telescope has made important breakthroughs in the understanding of the Gamma-Ray Burst (GRB) phenomemon. The combination of the GBM and the LAT instruments onboard the {\it Fermi} observatory has provided a wealth of information from its observations of GRBs over seven decades in energy. We present brief descriptions of the {\it Fermi} instruments and their capabilities for GRB science, and report highlights from {\it Fermi} observations of high-energy prompt and extended GRB emission. The main physical implications of these results are discussed, along with open questions regarding GRB modelling. We emphasize future synergies with ground-based \v{C}erenkov telescopes at the time of the SVOM mission.Comment: 10 pages, 10 figures -- Published in a special issue of Comptes Rendus Physique "GRB studies in the SVOM era", Eds. F. Daigne, G. Dubu

Searching for Needles in Haystacks - Using the Fermi/GBM to find GRB gamma-rays with the Fermi/LAT Detector

From the launch of the Fermi Gamma-ray Space Telescope to July 9, 2010, the Gamma-ray Burst Monitor (GBM) has detected 497 probable GRB events. Twenty-two of these satisfy the simultaneous requirements of an estimated burst direction within 52^\circ of the Fermi Large Area Telescope (LAT) boresight and a low energy fluence exceeding 5 $\mu$erg/cm^2. Using matched filter techniques, the spatially correlated Fermi/LAT photon data above 100 MeV have been examined for evidence of bursts that have so far evaded detection at these energies. High energy emission is detected with great confidence for one event, GRB 090228A. Since the LAT has significantly better angular resolution than the GBM, real-time application of these methods could open the door to optical identification and richer characterization of a larger fraction of the relatively rare GRBs that include high energy emission.Comment: 4 pages, 3 figures, 2 tables, accepted for publication in ApJ Letters, minor revision