Time evolution of the spectral break in the high-energy extra component of GRB 090926A

被引:19
作者
Yassine, M. [1 ]
Piron, F. [1 ]
Mochkovitch, R. [2 ]
Daigne, F. [2 ]
机构
[1] Univ Montpellier, CNRS, IN2P3, Lab Univers & Particules Montpellier, F-34095 Montpellier, France
[2] UPMC, CNRS, UMR 7095, Inst Astrophys Paris, F-75014 Paris, France
关键词
gamma-ray burst: individual: GRB 090926A; radiation mechanisms: non-thermal; GAMMA-RAY-BURSTS; FERMI OBSERVATIONS; GEV EMISSION; INTERNAL SHOCKS; PEAK ENERGY; MODEL; AFTERGLOW; FIREBALL; PROMPT; PHOTOSPHERES;
D O I
10.1051/0004-6361/201630353
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
Aims. The prompt light curve of the long GRB 090926A reveals a short pulse similar to 10 s after the beginning of the burst emission, which has been observed by the Fermi observatory from the keV to the GeV energy domain. During this bright spike, the high-energy emission from GRB 090926A underwent a sudden hardening above 10 MeV in the form of an additional power-law component exhibiting a spectral attenuation at a few hundreds of MeV. This high-energy break has been previously interpreted in terms of gamma-ray opacity to pair creation and has been used to estimate the bulk Lorentz factor of the outflow. In this article, we report on a new time-resolved analysis of the GRB 090926A broadband spectrum during its prompt phase and on its interpretation in the framework of prompt emission models. Methods. We characterized the emission from GRB 090926A at the highest energies with Pass 8 data from the Fermi Large Area Telescope (LAT), which off er a greater sensitivity than any data set used in previous studies of this burst, particularly in the 30 100 MeV energy band. Then, we combined the LAT data with the Fermi Gamma-ray Burst Monitor (GBM) in joint spectral fits to characterize the time evolution of the broadband spectrum from keV to GeV energies. We paid careful attention to the systematic effects that arise from the uncertainties on the LAT response. Finally, we performed a temporal analysis of the light curves and we computed the variability timescales from keV to GeV energies during and after the bright spike. Results. Our analysis confirms and better constrains the spectral break, which has been previously reported during the bright spike. Furthermore, it reveals that the spectral attenuation persists at later times with an increase of the break characteristic energy up to the GeV domain until the end of the prompt phase. We discuss these results in terms of keV MeV synchroton radiation of electrons accelerated during the dissipation of the jet energy and inverse Compton emission at higher energies. We interpret the high-energy spectral break as caused by photon opacity to pair creation. Requiring that all emissions are produced above the photosphere of GRB 090926A, we compute the bulk Lorentz factor of the outflow,. The latter decreases from 230 during the spike to 100 at the end of the prompt emission. Assuming, instead, that the spectral break reflects the natural curvature of the inverse Compton spectrum, lower limits corresponding to larger values of are also derived. Combined with the extreme temporal variability of GRB 090926A, these Lorentz factors lead to emission radii R similar to 10(14) cm, which are consistent with an internal origin of both the keV MeV and GeV prompt emissions.
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页数:17
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