Subphotospheric heating in GRBs: analysis and modeling of GRB090902B as observed by Fermi

2011 Fermi Symposium, Roma., May. 9-12 1 Subphotospheric heating in GRBs: analysis and modeling of GRB090902B as observed by Fermi T. Nymark, M. Axelsson, C. Lundman, E. Moretti, F. Ryde Department of physics, KTH, Stockholm, Sweden A. Pe’er Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA We analyze the spectral evolution of GRB 090902B and show that subphotospheric dissipation can explain both the spectra and the spectral evolution. The emission from a GRB photosphere can give rise to a variety of spectral shapes. The spectrum can have a shape close to that of a Planck function (as is observed during the first half of GRB090902B) or be broadened, resembling a typical Band function (as is observed during the second half of GRB090902B). The shape mainly depends on the strength and location of the dissipation in the jet, the ratio of the energy densities of thermal photons and of the electrons at the dissipation site, as well as on the strength of the magnetic field. We further discuss numerical models of the dissipation and relate these to the observed spectra. I. INTRODUCTION After more than 40 years of observations the prompt emission in gamma ray bursts is still not fully un- derstood. The most widely accepted model for the emission is the fireball model [1], [2], in which the emission is caused by dissipation in a relativistically expanding fireball. This model predicts the presence of a thermal component arising from the photosphere, where the flow which is initially opaque becomes opti- cally thin. A reasonable, first assumption is that this component has a shape resembling a Planck function, although this is expected to be slightly modified by relativistic and geometric effects (e.g. [3], [4], [5], [6], [7]). However, the majority of GRB spectra are well fit with a Band function and do not show clear signs of black body emission. On the other hand, Planck- like spectra have been observed in a few bursts [8, 9] and, in addition, in several cases there is evidence of a subdominant thermal component accompanying the non-thermal emission, e.g. in GRB110724B [10]. This has led to a renewed interest in the existence of pho- tospheric emission in GRB spectra. The presence of dissipation in the GRB outflow is unquestionable, since the spectrum is non-thermal, containing a high energy tail. The nature of the dissipation process is, however, not known, but pos- sible scenarios include magnetic reconnection in a Poynting-flux dominated outflow [11],[12],[2], internal shocks in the flow [13], [1] and collisional heating [6]. The location of the dissipation is also unknown, but in the internal shock scenario shocks due to colliding shells with different Lorentz factors are expected to occur mainly in the optically thin region of the flow, while oblique shocks can lead to dissipation below the photosphere. In addition, a change in the Lorentz fac- tor of the flow can lead to a shift in the location of the photosphere, so that the dissipation shifts from being located above the photosphere to occurring below the photosphere, with corresponding changes in the ob- served spectrum. This sub-photospheric dissipation is the subject of the present study. Non-thermal Component Photosphere Dissipation, e.g., shocks FIG. 1: Typically the dissipation due to internal shocks oc- curs above the photosphere, leading to strong non-thermal emission. However, a change in the Lorentz factor of the flow can shift the position of the photosphere, leading to the dissipation site being located below the photosphere. II. NUMERICAL MODELING We consider a highly relativistic jet. There is strong thermal emission from the base of the outflow, which is advected outward with the flow and released at the photosphere, where the optical depth drops to unity. Initially the thermal emission takes the form of a Planck function in the comoving frame, and if there is no dissipation below the photosphere the emerging spectrum will also have this shape. Although transfor- mation to the observer frame in addition to geomet- ric effects alters the spectrum somewhat, the observed spectrum should still have a largely Planck-like shape. If, however, part of the kinetic energy is dissipated below the photosphere, the spectrum will be modified and can take on a quite a complex shape [14]. We assume that the photosphere is located above the saturation radius where the flow ceases to acceler- ate, and that the dissipation occurs close to the pho- tosphere, at optical depths of a few. Whatever the nature of the dissipation process the result is that a population of energetic electrons is created. These eConf C110509 arXiv:1111.0308v1 [astro-ph.HE] 1 Nov 2011 2 2011 Fermi Symposium, Roma., May. 9-12 cool via synchrotron emission and Compton scattering of low energy photons. The energetic photons which are created through these processes may in turn un- dergo inverse Compton scattering. In addition pair production and annihilation modifies the particle and photon popu

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