Flares In Long And Short Gamma Ray Bursts

📝 Abstract

The many similarities between the prompt emission pulses in gamma ray bursts (GRBs) and X-ray flares during the fast decay and afterglow phases of GRBs suggest a common origin. In the cannonball (CB) model of GRBs, this common origin is mass accretion episodes of fall-back matter on a newly born compact object. The prompt emission pulses are produced by a bipolar jet of highly relativistic plasmoids (CBs) ejected in the early, major episodes of mass accretion. As the accretion material is consumed, one may expect the engine’s activity to weaken. X-ray flares ending the prompt emission and during the afterglow phase are produced in such delayed episodes of mass accretion. The common engine, environment and radiation mechanisms (inverse Compton scattering and synchrotron radiation) produce their observed similarities. Flares in both long GRBs and short hard gamma ray bursts (SHBs) can also be produced by bipolar ejections of CBs following a phase transition in compact objects due to loss of angular momentum and/or cooling. Optical flares, however, are mostly produced in collisions of CBs with massive stellar winds/ejecta or with density bumps along their path. In this paper we show that the master formulae of the CB model of GRBs and SHBs, which reproduce very well their prompt emission pulses and their smooth afterglows, seem to reproduce also very well the lightcurves and spectral evolution of the prominent X-ray and optical flares that are well sampled.

💡 Analysis

The many similarities between the prompt emission pulses in gamma ray bursts (GRBs) and X-ray flares during the fast decay and afterglow phases of GRBs suggest a common origin. In the cannonball (CB) model of GRBs, this common origin is mass accretion episodes of fall-back matter on a newly born compact object. The prompt emission pulses are produced by a bipolar jet of highly relativistic plasmoids (CBs) ejected in the early, major episodes of mass accretion. As the accretion material is consumed, one may expect the engine’s activity to weaken. X-ray flares ending the prompt emission and during the afterglow phase are produced in such delayed episodes of mass accretion. The common engine, environment and radiation mechanisms (inverse Compton scattering and synchrotron radiation) produce their observed similarities. Flares in both long GRBs and short hard gamma ray bursts (SHBs) can also be produced by bipolar ejections of CBs following a phase transition in compact objects due to loss of angular momentum and/or cooling. Optical flares, however, are mostly produced in collisions of CBs with massive stellar winds/ejecta or with density bumps along their path. In this paper we show that the master formulae of the CB model of GRBs and SHBs, which reproduce very well their prompt emission pulses and their smooth afterglows, seem to reproduce also very well the lightcurves and spectral evolution of the prominent X-ray and optical flares that are well sampled.

📄 Content

arXiv:0908.0650v2 [astro-ph.HE] 26 Jan 2010 Flares In Long And Short Gamma Ray Bursts Shlomo Dado1 and Arnon Dar2 ABSTRACT The many similarities between the prompt emission pulses in gamma ray bursts (GRBs) and X-ray flares during the fast decay and afterglow phases of GRBs suggest a common origin. In the cannonball (CB) model of GRBs, this common origin is mass accretion episodes of fall-back matter on a newly born compact object. The prompt emission pulses are produced by a bipolar jet of highly relativistic plasmoids (CBs) ejected in the early, major episodes of mass accretion. As the accretion material is consumed, one may expect the engine’s activity to weaken. X-ray flares ending the prompt emission and during the afterglow phase are produced in such delayed episodes of mass accretion. The common engine, environment and radiation mechanisms (inverse Compton scat- tering and synchrotron radiation) produce their observed similarities. Flares in both long GRBs and short hard gamma ray bursts (SHBs) can also be produced by bipolar ejections of CBs following a phase transition in compact objects due to loss of angular momentum and/or cooling. Optical flares, however, are mostly produced in collisions of CBs with massive stellar winds/ejecta or with density bumps along their path. In this paper we show that the master formulae of the CB model of GRBs and SHBs, which reproduce very well their prompt emis- sion pulses and their smooth afterglows, seem to reproduce also very well the lightcurves and spectral evolution of the prominent X-ray and optical flares that are well sampled. Subject headings: gamma rays: bursts 1. Introduction A flaring activity during the afterglow (AG) phase of a gamma ray burst (GRB) was first observed in the late-time AG of GRB 970508 with the Narrow Field Instrument (NFI) 1dado@phep3.technion.ac.il Physics Department, Technion, Haifa 32000, Israel 2arnon@physics.technion.ac.il Physics Department, Technion, Haifa 32000, Israel – 2 – aboard the BeppoSAX satellite in the X-ray band (Piro et al. 1998), and with ground based telescopes in the optical band (Pian et al. 1998; Galama et al. 1998a). It was interpreted in the framework of the fireball (FB) model of GRBs as a delayed burst from the central GRB engine (Piro et al. 1998). Alternatively, in the cannonball (CB) model of GRBs it was interpreted as a synchrotron radiation (SR) flare from an encounter of the highly relativis- tic jetted ejecta from an underlying supernova (SN) explosion with a density jump in the interstellar medium (Dado et al. 2002, 2004). Late-time flares were later discovered in the broadband AG of several other GRBs that were localized by the BeppoSAX satellite, most notably in GRB 000301C at t ∼4 days after burst (Berger et al. 2000; Sagar et al. 2000) where the flare was attributed to gravitational microlensing (Garnavich et al. 2000) and in GRB 030329 (Lipkin et al. 2004) where the flare was interpreted in the framework of the FB model as due to ‘refreshed shocks’ generated by a late activity of the central engine (Granot et al. 2003). In the CB model, however, these flares were well reproduced by the emission of SR from encounters of the jetted ejecta from an underlying SN explosion in a star formation region with density jumps within or at the border of a super bubble created by the star formation region (Dado et al. 2002, 2004). Early-time X-ray flares ending the prompt emission phase were also detected by the wide field camera (WFC) aboard BeppoSAX in a few GRBs such as GRB 011121. In the FB model they were attributed to the onset of the external shock in the circumburst material. In the CB model they were interpreted as being due to the last episodes of bipolar CB ejections from a shutting offcentral engine. Shortly after the launch of the Swift satellite in November 2004, data collected with its X-ray telescope (XRT) showed that X-ray flares are quite common in all the phases of the emission from GRBs. In more than 50% of the GRBs observed with the Swift X-ray telescope (XRT), flares were observed at the end of the prompt emission and/or the early AG phase (Burrows et al. 2005, 2007; Falcone et al. 2007). In some cases X-ray flares were observed also at very late times, of the order of several days after the prompt emission. Although the information on flares is much more sketchy compared to that on the prompt gamma ray pulses, their spectral and temporal behaviours show clearly that the X-ray flares during the prompt γ-ray emission follow the pattern of the γ-ray pulses, suggesting they are the low energy part of these pulses. The X-ray flares ending the prompt emission and those superimposed on the early-time afterglow have a fast spectral evolution. Their peak intensities decrease with time, and their spectral and temporal behaviours are similar to those of the prompt X/γ-ray pulses, except that they are progressively softer and last longer. In most cases their γ-ray emission probably is below the detection sensitivit

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