The Very Bright and Nearby GRB 130427A: The Extra Hard Spectral Component and Implications for Very High-energy Gamma-ray Observations of Gamma-ray Bursts

The extended high-energy gamma-ray (>100 MeV) emission occurring after the prompt gamma-ray bursts (GRBs) is usually characterized by a single power-law spectrum, which has been explained as the afterglow synchrotron radiation. We report on the Fermi Large Area Telescope (LAT) observations of the >100 MeV emission from the very bright and nearby GRB 130427A, up to ~100 GeV. By performing time-resolved spectral fits of GRB 130427A, we found a strong evidence of an extra hard spectral component above a few GeV that exists in the extended high-energy emission of this GRB. This extra spectral component may represent the first clear evidence of the long sought-after afterglow inverse Compton emission. Prospects for observations at the very high-energy gamma-rays, i.e., above 100 GeV, are described.Comment: 6 pages, 2 figures, presented at the 4th High Energy Phenomena in Relativistic Outflows (HEPRO IV) meeting held in Heidelberg, July 23-26, 2013; to be published in IJMPC

Gamma-ray burst studies using the H.E.S.S. Cherenkov array

Gamma-ray bursts (GRBs) are the most intense and unpredictable gamma-ray events from the Universe. Without prior signal, an enormous amount of energy is released for seconds, energizing particles and generating the observed 10e5-10e6 eV gamma-ray photons. The emitting regions can produce Very-High-Energy (VHE) gamma-ray photons of energy about 10e11 eV during and after the burst. These VHE gamma-rays may be attenuated in the source or by the extragalactic background light (EBL). The H.E.S.S. array of four imaging atmospheric Cherenkov telescopes (IACT) is sensitive to VHE gamma-rays. H.E.S.S. observations of 34 GRBs were carried out during 2003-2008, with the shortest response time being six minutes. No evidence of VHE gamma-rays was found. Flux upper limits derived for GRB030329 and GRB060505 are compared and are found consistent with the synchrotron self-Compton flux calculated in the context of relativistic blast-wave model. Absorption by EBL was taken into consideration. Accidentally, GRB060602B was observed with H.E.S.S. throughout the GRB duration, during which no signal of VHE gamma-rays was found. The distance scale and origin of GRB060602B remain unclear and different possibilities and implications are presented. Future prospects of VHE gamma-ray observations of GRBs are discussed

Discovery of an extra hard spectral component in the high-energy afterglow emission of GRB 130427A

The extended high-energy gamma-ray (>100 MeV) emission occurred after the prompt gamma-ray bursts (GRBs) is usually characterized by a single power-law spectrum, which has been explained as the afterglow synchrotron radiation. The afterglow inverse-Compton emission has long been predicted to be able to produce a high-energy component as well, but previous observations have not revealed such a signature clearly, probably due to the small number of >10 GeV photons even for the brightest GRBs known so far. In this Letter, we report on the Fermi Large Area Telescope (LAT) observations of the >100 MeV emission from the very bright and nearby GRB 130427A. We characterize the time-resolved spectra of the GeV emission from the GRB onset to the afterglow phase. By performing time-resolved spectral fits of GRB 130427A, we found a strong evidence of an extra hard spectral component that exists in the extended high-energy emission of this GRB. We argue that this hard component may arise from the afterglow inverse Compton emission.Comment: 5 pages, 2 figures, 2 tables, ApJL, in pres

Evidence of a spectral break in the gamma-ray emission of the disk component of Large Magellanic Cloud: a hadronic origin?

It has been suggested that high-energy gamma-ray emission ($>100{\rm MeV}$) of nearby star-forming galaxies may be produced predominantly by cosmic rays colliding with the interstellar medium through neutral pion decay. The pion-decay mechanism predicts a unique spectral signature in the gamma-ray spectrum, characterized by a fast rising spectrum and a spectral break below a few hundreds of MeV. We here report the evidence of a spectral break around 500 MeV in the disk emission of Large Magellanic Cloud (LMC), which is found in the analysis of the gamma-ray data extending down to 60 MeV observed by {\it Fermi}-Large Area Telescope. The break is well consistent with the pion-decay model for the gamma-ray emission, although leptonic models, such as the electron bremsstrahlung emission, cannot be ruled out completely.Comment: 11 pages, 4 figures, Accepted by Ap