Was the “naked burst” GRB 050421 really naked?

Abstract

Context. A few long gamma-ray bursts such as GRB 050421 show no afterglow emission beyond the usual initial steep decay phase. It has been suggested that these events correspond to "naked" bursts that occur in a very low density environment. We reconsider this possibility in the context of various scenarios for the origin of the afterglow. Aims. In the standard model where the afterglow results from the forward shock as well as in the alternative model where the afterglow comes from the reverse shock, we aim to obtain constraints on the density of the environment, the microphysics parameters, or the Lorentz factor of the ejecta, which are imposed by the absence of a detected afterglow. Methods. For the two models we compute the afterglow evolution for different values of the external density (uniform or wind medium) and various burst parameters. We then compare our results to the Swift data of GRB 050421, which is the best example of a long burst without afterglow. Results. In the standard model we show that consistency with the data imposes that the external density does not exceed 10 −5 cm −3 or that the microphysics parameters are very small with e < ∼ 10 −2 and B < ∼ 10 −4 . If the afterglow is caused by the reverse shock, we find that its contribution can be strongly reduced if the central source has mainly emitted fast-moving material (with less than 10-30% of the kinetic energy at Γ < 100) and was located in a dense environment. Conclusions. The two considered scenarios therefore lead to opposite constraints on the circumburst medium. The high-density environment, favored by the reverse shock model, better corresponds to what is expected if the burst progenitor was a massive star.

Accounting for the XRT early steep decay in models of the prompt gamma-ray burst emission

“…The prompt emission is associated with internal shocks and the high latitude emission is in excellent agreement with XRT data (see Fig. 2 in Hascoët et al 2011).…”

“…As a last example, the "naked burst" GRB 050421 is a good case of a GRB with a variable light curve followed by an early steep decay that was well observed by the XRT because of its especially long duration (Godet et al 2006). This burst was modeled in detail by Hascoët et al (2011). The prompt emission is associated with internal shocks and the high latitude emission is in excellent agreement with XRT data (see Fig.…”

X-ray flares from dense shells formed in gamma-ray burst explosions

“…These features are not explained by the standard forward shock model (Meszaros & Rees 1997;Sari et al 1998), and it was proposed that the observed afterglow is produced by a long-lived reverse shock inside the GRB ejecta (Uhm & Beloborodov 2007;Genet et al 2007). The dynamics and emission of the reverse shock are particularly sensitive to the structure of the ejecta (the distribution of its Lorentz factor, density, and magnetic fields) which may explain the rich phenomenology of the early afterglow (Hascoët et al 2011(Hascoët et al , 2012Uhm et al 2012;Hascoët et al 2014b).…”

Relativistic simulations of long-lived reverse shocks in stratified ejecta: the origin of flares in GRB afterglows

“…This approach has been extended by Genet et al (2007); Hascoët et al (2011Hascoët et al ( , 2012Hascoët et al ( , 2014 to include the deceleration phase. This allows to follow the forward shock in the external medium and the reverse shock in the ejecta.…”