An External Inverse Compton Emission Model of Gamma-Ray Burst High-Energy Lags

arXiv:0912.3277v1 [astro-ph.HE] 16 Dec 2009 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1 An External Inverse Compton Emission Model of Gamma-Ray Burst High-Energy Lags K. Toma1,2, X. F. Wu1,2,3, and P. M´esz´aros1,2,4 1Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA 2Center for Particle Astrophysics, Pennsylvania State University, University Park, PA 16802, USA 3Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China and 4Department of Physics, Pennsylvania State University, University Park, PA 16802, USA The Fermi satellite has been reporting the detailed temporal properties of gamma-ray bursts (GRBs) in an extremely broad spectral range, 8 keV - 300 GeV, in particular, the unexpected delays of the GeV emission onsets behind the MeV emission of some GRBs. We focus on GRB 080916C, one of the Fermi-LAT GRBs for which the data of the delayed high-energy emission are quite extensive, and we show that the behavior of the high-energy emission of this burst can be explained by a model in which the prompt emission consists of two components: one is the MeV component due to the synchrotron-self-Compton radiation of electrons accelerated in the internal shock of the jet and the other is the high-energy component due to inverse Compton scattering of the photospheric X-ray emission of the expanding cocoon offthe same electrons in the jet. Such an external inverse Compton effect could be important for other Fermi-LAT GRBs, including short GRBs as well. In this model, the delay timescale is directly linked to the physical properties of GRB progenitor. 1. Introduction Gamma-ray bursts (GRBs) were only sparsely ob- served in the > 100 MeV range, until the Fermi satel- lite was launched on June 11 2008 [1]. Now Fermi provides extremely broad energy coverage, 8 keV − 300 GeV, with high sensitivity for GRBs, and is ac- cumulating a wealth of new data which open a com- pletely new window on the physics of GRBs. The high-energy temporal and spectral data provided by Fermi can severely constrain the physical parameters of the GRB emission region and the circumburst en- vironment, which will lead to a deeper understanding of the central engine and the GRB progenitors, and will also constrain models of high-energy cosmic ray acceleration [2, 3]. GRB 080916C has the largest isotropic γ-ray energy release so far, Eγ,iso ≃8.8×1054 erg (with redshift z ≃ 4.35). Fermi LAT obtained its high-energy emission data quite extensively, showing several important new properties [4]: (i) The time-resolved spectra (with resolution ∼5− 50 s) are well fitted by a smoothly broken power- law function (the so-called Band function) from 8 keV up to a photon with energy ≈13.2 GeV. (ii) The ε > 100 MeV emission is not detected to- gether with the first ε <∼1 MeV pulse and the onset of the ε > 100 MeV emission coincides with the rise of the second pulse (≈5 s after the trigger). (iii) Most of the emission in the second pulse shifts towards later times as higher energies are con- sidered. (iv) The ε > 100 MeV emission lasts at least 1400 s, while photons with ε < 100 MeV are not de- tected past 200 s. Some other Fermi-LAT GRBs also display high- energy lags, similar to the properties (ii) and/or (iii) [4, 5], and then they should be very important to un- derstand the prompt emission mechanism of GRBs. We will call the ε <∼1 MeV emission and the ε > 100 MeV emission ”MeV emission” and ”high-energy emission”, respectively. A simple physical picture for the property (i) is that the prompt emission consists of a single emission com- ponent, such as synchrotron radiation of electrons ac- celerated in internal shocks of a relativistic jet. In this picture, the peak of the MeV pulse could be at- tributed to the cessation of the emission production (i.e., the shock crossing of the shell) and the subse- quent emission could come from the high latitude re- gions of the shell. Thus the observed high-energy lag for the second pulse (property (iii)) requires that the electron energy spectrum should be harder systemat- ically in the higher latitude region. This would imply that the particle acceleration process should definitely depend on the global parameters of the jet, e.g., the angle-dependent relative Lorentz factor of the collid- ing shells, but such a theory has not been formulated yet. The property (ii) could be just due to the fact that the two pulses originate in two internal shocks with different physical conditions for which the elec- tron energy spectrum of the second internal shock is harder than that of the first one [4]. Another picture is that the prompt emission con- sists of the MeV component and a delayed high-energy component. The latter component could be produced by hadronic effects (i.e., photo-pion process and pro- ton synchrotron emission) [6, 7], but they require ex- eConf C091122 2 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 tremely large tota

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