GRB afterglow 090510 is (so far) the best-monitored afterglow in the optical, X-ray, and above 100 MeV, measurements covering 2-3 decades in time at each frequency. Owing to its power-law temporal decay and power-law spectrum, it seems very likely that the highest energy emission is from the forward-shock energizing the ambient medium (the standard blast-wave model for GRB afterglows), the GeV flux and its decay rate being consistent with that model's expectations. However, the synchrotron emission from a collimated outflow (the standard jet model) has difficulties in accounting for the lower-energy afterglow emission, where a simultaneous break occurs at 2 ks in the optical and X-ray light-curves, but with the optical flux decay (before and after the break) being much slower than in the X-rays (at same time). The measured X-ray and GeV fluxes are incompatible with the higher-energy afterglow emission being from same spectral component as the lower-energy afterglow emission, which suggests a synchrotron self-Compton model for this afterglow. Cessation of energy injection in the blast-wave and an ambient medium with a wind-like n ~ r^{-2} density can explain all features of the optical and X-ray light-curves of GRB afterglow 090510. Such an ambient medium radial structure is incompatible with this short-GRB originating from the merger of two compact stars.
The existence of the afterglow emission following Gamma-Ray Bursts (GRBs) has been predicted by Paczyński & Rhoads (1993) and Mészáros & Rees (1997a) and has been been monitored in the radio (catalog of Frail et al 2003), optical (catalog of Kann et al 2010, Swift-UVOT catalog of Oates et al 2009, Roming et al 2009), and X-ray (catalogs: BeppoSAX -De Pasquale et al 2006, Swift -O'Brien et al 2006, Willingale et al 2007) for hundreds of afterglows and, more recently, it has been detected above 100 MeV in a few cases.
The general afterglow picture is that the relativistic outflow that produced the prompt (burst) emission interacts with the ambient medium, driving two shocks: one into the circumburst medium (the “forward” shock), leading to a progressive deceleration of the blast-wave) and one into the GRB ejecta (the “reverse” shock). The shocks accelerate electrons to relativistic energies (up to at least 10 GeV, in the comoving frame) through the first-order Fermi process or by the plasma two-stream instability (Medvedev & Loeb 1999). Magnetic dissipation in a Poynting outflow has also been proposed as the origin of relativistic electrons in GRB outflows (Mészáros & Rees 1997b, Lyutikov 2006). Turbulence in the shocked fluid or the mentioned Weibel instability are believed to be the origin of the ∼ 1 Gauss magnetic field required to explain the X-ray afterglow emission with synchrotron emission from the forward-shock.
The radio, optical, and X-ray afterglow emission has been identified most often with synchrotron emission from the ambient medium energized by the forward-shock (e.g. Mészáros & Rees 1997a, Sari, Piran & Narayan 1997). The reasons for that is that, for an impulsive GRB ejecta release, most of the ejecta energy is quickly transferred to the forward-shock, and that the forward-shock emission flux is expected to decay as a power-law for long times (as observed in GRB afterglows) without any further assumptions/requirements: just the power-law deceleration of the blast-wave and the power-law spectrum of the shock-accelerated electrons suffice to yield a synchrotron flux with a power-law decay in time.
The emission from the GRB ejecta energized by the reverse-shock has also been proposed to be the afterglow source, more likely at optical frequencies (Mészáros & Rees 1997a, Panaitescu & Mészáros 1998), and was identified as the origin of the bright optical counterpart of GRB 990123 (Sari & Piran 1999). Due to the fast cooling of the ejecta electrons, the reverse-shock emission may account for the afterglow emission lasting for days only if new electrons are accelerated at that shock, i.e. only if there is a continuous influx of ejecta crossing the reverse-shock. In this case, the afterglow light-curve also depends on the rate at which the new ejecta cross the reverse-shock, a fact used by Uhm & Beloborodov (2007) and by Genet, Daigne & Mochkovitch (2007) to explain the plateaus displayed by the X-ray afterglow light-curves at 1-10 ks. As shown in those articles, the reverse-shock emission may also account for the various degree of coupling between the optical and X-ray afterglow light-curves, although the reason for chromatic X-ray light-curve breaks lies in the tracking of the reverse-shock electron distribution and not in some fundamental/simple afterglow process.
In this work, we model the broadband emission of afterglow 090510 by calculating the synchrotron emission from both shocks. Kumar & Barniol Duran (2010) and Corsi et al (2010) have identified the emission of this afterglow at all wavelengths with synchrotron from the forward-shock. Analytically and numerically, we find that, if that were true, then the X-ray emission after the 2 ks break would be too dim for the GeV flux measured by Fermi-LAT at 100 s. To solve this incompatibility, the optical and GeV emissions must be from different emission processes, and we propose that the latter is inverse-Compton scatterings. The fast decay of the X-ray flux after the 2 ks break indicates that the X-ray is also inverse-Compton. As shown by Panaitescu & Kumar (2000), inverse-Compton scatterings in the forward-shock could be the dominant emission process in the X-ray if the ambient medium is denser than about 100 protons/cm -3 but, so far, there is only one case (000926 - Harrison et al 2001, Panaitescu & Kumar 2002) where the X-ray afterglow light-curve and flux warranted its explanation with the synchrotron self-Compton model.
A successful model for the multiwavelength emission of GRB afterglow must account for the following properties (using the notation Fν ∝ t -α ν -β for the power-law afterglow flux):
(i) an achromatic light-curve break (at 2 ks after trigger), appearing at the same time in the optical and X-ray, with the optical flux slowly rising at a power-law index αo1 = -0.2 ± 0.1 followed αo2 = 1.1 ± 0.1 after the break, and the X-ray flux decaying with αx1 = 0.74 ± 0.03 and αx2 = 2.2 ± 0.1, (ii) a GeV light-curve with a power-law d
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