Non-Thermal emission from the photospheres of Gamma-Ray Burst outflows. I: High frequency tails

The standard model that has been developed over the years assumes that internal shocks are responsible for the dissipation (Rees & Mészáros 1994), while electrons gyrating in a shock-generated magnetic field produce the radiation via the synchrotron process (Mészáros et al. 1994;Piran 1999;Lloyd & Petrosian 2000). Both mechanisms are fraught with numerous problems.

Internal shocks assume that the outflow is released by the central engine with fluctuations in the Lorentz factor, so that different parts of the flow collide with each other, producing shocks that dissipate energy. Unfortunately, the internal shock mechanism has very little predictive power, since any behavior of the light curve can be explained by a suitable choice of the ejection history of the central engine. Since the physics of the emission from the engine is largely unconstrained, it is very hard to disprove internal shocks based on any observation. The only robust prediction of internal shocks as a dissipation mechanism is the low efficiency of the process (Kobayashi et al. 1997;Lazzati et al. 1999;Spada et al. 2000), due to the fact that internal shocks can dissipate only the energy associated with differential motions and not the energy associated to the bulk motion, which is much larger. Observations, instead, show that at least for a fraction of bursts, the efficiency of the prompt phase is 50 per cent, if not higher (Zhang et al. 2007). An alternative model that is becoming increasingly popular is magnetic dissipation in a Poynting dominated outflow (e.g., Thompson 1994;Spruit et al. 2001;Giannios & Spruit 2005). Even though magnetic dissipation could in principle provide very high efficiencies, it is, again, a very uncertain process and as such provides very little predictive power to allow for meaningful comparisons with observations. Synchrotron radiation, with possible self-Compton components, has long been considered the best candidate to explain the prompt emission of GRBs. While synchrotron radiation can easily explain the non-thermal nature of the observed spectrum, more thorough scrutiny reveals several important problems. First, optically thin synchrotron emission has a very well defined limiting slope α ≥ -1/3 (where F(ν) ∝ ν -α ) due to the synchrotron radiation spectrum of a single electron (the so-called line of death of synchrotron emission). However, at least several cases of GRBs with α < -1/3 have been detected (Crider et al. 1997;Preece et al. 1998;Ghirlanda et al. 2002Ghirlanda et al. , 2003)). Second, the typical slope of the lowfrequency part of the GRB spectrum is α = 0 (Kaneko et al. 2006), which is not a natural slope of synchrotron radiation from shock-accelerated electrons. Third, the high frequency spectrum has a highly variable slope β, not easily explained by synchrotron radiation from shock accelerated electrons that should produce a fairly standard spectrum with α ∼ 1 (as in the X-ray afterglows). Finally, for the prompt emission to be efficient, electrons are expected to cool fast (Ghisellini et al. 2000), but the typical slope of cooling electrons (α = 1/2) is not observed in GRB spectra (Kaneko et al. 2006).

These well known difficulties of synchrotron emission have favored the study of various alternatives, including quasithermal Comptonization (Ghisellini & Celotti 1999), jitter radiation (Medvedev & Loeb 1999;Medvedev 2000;Workman et al. 2007;Morsony et al. 2008), bulk Compton (Lazzati et al. 2000); and photospheric emission (Mészáros & Rees 2000;Mészáros et al. 2002;Rees & Mészáros 2005;Pe’er et al. 2005Pe’er et al. , 2006;;Giannios 2006;Giannios & Spruit 2007;Pe’er et al. 2007;Thompson et al. 2007;Ryde & Pe’er 2009, Lazzati et al. 2009). In this paper we expand the analysis of photospheric emission as a GRB prompt radiation mechanism, studying the conditions under which the photosphere can produce a high-frequency component with a non-thermal power-law shape up to tens of MeV. Photospheric radiation has two great advantages with respect to synchrotron emission: it does not require a dissipation mechanism and it naturally provides high efficiency. Photospheric radiation does not require a dissipation mechanism if the radiation is released before the full acceleration of the fireball and therefore at a stage when the fireball still contains a large fraction of internal energy. Numerical simulations have shown (Lazzati et al. 2009, hereafter LMB09) that this is indeed the ca

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