Abstract
Swift-detected GRB 080307 showed an unusual smooth rise in its X-ray light curve around 100 s after the burst, at the start of which the emission briefly softened. This ‘hump’ has a longer duration than is normal for a flare at early times and does not demonstrate a typical flare profile. Using a two-component power-law-to-exponential model, the rising emission can be modelled as the onset of the afterglow, something which is very rarely seen in Swift-X-ray light curves. We cannot, however, rule out that the hump is a particularly slow early-time flare, or that it is caused by upscattered reverse shock electrons.
Highlights
Many gamma-ray burst (GRB) X-ray light curves roughly follow a ‘canonical’ decay of initially steep and flat, before settling into a classical power-law decline phase, some are noticeably different
Examples include GRB 060105, which had an unusually shallow initial decay (Godet et al 2008); GRB 060218, which was clearly associated with supernova SN 2006aj and showed the breakout of a shock wave from the exploding progenitor (Campana et al 2006); GRB 061007, where the X-ray decay continued with no breaks until at least 106 s after the trigger (Mundell et al 2007; Schady et al 2007); GRB 070110, which showed an abrupt drop in the X-ray emission at the end of the ‘plateau’ stage (Troja et al 2008)
The absence of a supernova makes it unlikely that this burst occurred at low redshift [very high foreground extinction (AV ∼ 4–5 for a burst at z = 0.1) would be required], despite the resemblance between this light curve and that of the low-z GRB 060218 (Campana et al 2006), whilst the magnitude of the host galaxy is entirely consistent with many other GRB hosts at z 2
Summary
Many gamma-ray burst (GRB) X-ray light curves roughly follow a ‘canonical’ decay of initially steep and flat (the socalled ‘plateau’ phase), before settling into a classical power-law decline phase (see e.g. Nousek et al 2006; Zhang et al 2006), some are noticeably different. Examples include GRB 060105, which had an unusually shallow initial decay (Godet et al 2008); GRB 060218, which was clearly associated with supernova SN 2006aj and showed the breakout of a shock wave from the exploding progenitor (Campana et al 2006); GRB 061007, where the X-ray decay continued with no breaks until at least 106 s after the trigger (Mundell et al 2007; Schady et al 2007); GRB 070110, which showed an abrupt drop in the X-ray emission at the end of the ‘plateau’ stage (Troja et al 2008) It is examples such as these, whose behaviour differs from those of more typical afterglows, which provide further insight into the physics of the bursts, allowing the testing of models for the early stages of GRB afterglows.
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