Early afterglows in wind environments revisited

Abstract

When a cold shell sweeps up the ambient medium, a forward shock and a reverse shock will form. We analyse the reverse‐forward shocks in a wind environment, including their dynamics and emission. An early afterglow is emitted from the shocked shell, e.g. an optical flash may emerge. The reverse shock behaves differently in two approximations: the relativistic and Newtonian cases, which depend on the parameters, e.g. the initial Lorentz factor of the ejecta. If the initial Lorentz factor is much less than 114E1/453Δ−1/40,12A−1/4*,−1, the early reverse shock is Newtonian. This may take place for the wider of a two‐component jet, an orphan afterglow caused by a low initial Lorentz factor and so on. The synchrotron self‐absorption effect is significant especially for the Newtonian reverse shock case, as the absorption frequency νa is larger than the cooling frequency νc and the minimum synchrotron frequency νm for typical parameters. For the optical to X‐ray band, the flux is nearly unchanged with time during the early period, which may be a diagnostic for the low initial Lorentz factor of the ejecta in a wind environment. We also investigate the early light curves with different wind densities and compare them with those in the interstellar medium model.

Very Bright Prompt and Reverse Shock Emission of GRB 140512a

“…In the framework of the RS models for an ISM scenario, the expected RS emission lightcurve increases as F ∝ t 5 and F ∝ t 0.5 for the thin and thick shell cases, respectively, and decays as t ∼−2 after the peak time for the two cases. In a wind medium, the decay slope of the RS emission is steeper (about -3 in standard parameters) (Kobayashi & Zhang 2003a;Zou et al 2005). The rising and decaying slopes of the optical peak of GRB 140512A are 3.04±0.09 and −1.93±0.07, respectively.…”

“…Since bright RS emission was firstly detected in GRB 990123 (Akerlof et al 1999), extensive studies on reverse shock emission in the optical/IR bands have been made with the early optical afterglow data (e.g., Sari & Piran 1999b;Meszaros & Rees 1999;Fan et al 2002;Kobayashi & Zhang 2003b;Zhang et al 2003a;Kumar & Panaitescu 2003;Wu et al 2003;Fan et al 2004;Nakar & Piran 2004;Zhang & Kobayashi 2005;Zou et al 2005;Jin & Fan 2007;Harrison & Kobayashi 2013;Yi et al 2013;Japelj et al 2014;Gao et al 2015;Zhang et al 2015). Motivated by the extremely bright RS emission detected in GRB 990123, it is expected that the Swift optical-UV telescope (UVOT) and ground-based rapid follow-up optical telescopes can detect the RS emission for a large sample of GRBs with XRT prompt and precise localization capacity (e.g.…”

New Galactic Wolf-Rayet stars, and candidates

“…Some of them could be the possible progenitors of core-collapse supernovae and γ-ray bursts, especially in a low metallicity environment (e.g., Hirschi et al 2005;Petrovic et al 2005;Langer & Norman 2006;Woosley & Heger 2006;Fruchter et al 2006). Where ∼35% of the Galactic WR stars have wind blown bubbles, visible as ring nebulae (Marston 1997), they provide the ideal environment for a γ-ray burst afterglow (e.g., Chevalier 2005;Dwardakas 2005;Zou et al 2005;Eldridge & Vink 2006;Hammer et al 2006). For all practical purposes, it is important to know as many WR stars as possible.…”

GRB 120711A: an intense INTEGRAL burst with long-lasting softγ-ray emission and a powerful optical flash

“…Therefore, GRB 120711A seems to be in an intermediate state between the so-called thin-shell case for which t peak > T 90 , and the thick-shell case for which t peak < T 90 (e.g. Sari & Piran 1999;Zou et al 2005;Mészáros 2006). The time of the peak of the forward shock (t peak ∼ T 0 + 240 s) can be interpreted as the onset of the afterglow emission and thus it can also be used to estimate t dec (e.g.…”