Testing the Curvature Effect and Internal Origin of Gamma‐Ray Burst Prompt Emissions and X‐Ray Flares withSwiftData

Abstract

The X-ray light curves of many GRBs have a steep tail following the gamma-rays and have some erratic flares. We assume that these tails and flares are of ''internal'' origin and that their decline behaviors are dominated by the curvature effect. This effect suggests that the decay slope of the late steep decay part of the light curves is ¼ 2 þ , where is the X-ray spectral index. We present a self-consistency test for this scenario with a sample of 36 prompt emission tails/flares in 22 light curves observed by the Swift XRT. We derive the zero time (t 0 ) for each steep decay component by fitting the light curves with the constraint of ¼ 2 þ . Our results show that the t 0 values of the prompt emission tails and the tails of well-separated flares are self-consistent with the expectation of the internal dissipation models, indicating that each X-ray flare forms a distinct episode of the central engine activity and the central engine remains active after the prompt emission is over, sometimes up to $1 day after the GRB trigger. This challenges the conventional models and calls for new ideas to restart the central engine. We further show that the onset time of the late central engine activity does not depend on the GRB duration. We also identify a minority group of GRBs whose combined BAT-XRT light curves are smoothly connected, without an abrupt transition between the prompt emission and the afterglow. These GRBs may have an external origin for both the prompt emission and the afterglow.

Panchromatic study of GRB 060124: from precursor to afterglow

“…This has become a general feature of both long and short GRBs in view of the commonly detected X-ray flares hundreds of seconds after the burst trigger, which are generally interpreted as late central engine activities Zhang et al 2006;Romano et al 2006;Falcone et al 2006;Barthelmy et al 2005b;Campana et al 2006;Liang et al 2006). The second main peak (excluding the precursor) would have been categorized as an X-ray flare had it been even softer.…”

A Three‐Stage Model for the Inner Engine of Gamma‐Ray Bursts: Prompt Emission and Early Afterglow

“…Note that a possible break in the light curve due to collimation of the outflow producing the GRB and the afterglow will occur at later times than what is shown in this figure. star shell. Finally, we mention recent work by (Liang et al 2006), in which a zero time point (right before the beginning of the flare) was taken as a signature of the reactivation of the engine. If true, this observation suggests that the engine reactivates more than once which cannot be reconciled with our model.…”

Rapid X‐Ray Declines and Plateaus inSwiftGRB Light Curves Explained by a Highly Radiative Blast Wave

“…The number of physical models for this effect is not yet large; there is a preference recently (e.g., Nava et al 2007;Panaitescu 2006;O'Brien et al 2006a;Zhang et al 2007;Liang et al 2006) to quantify the effect and perform empirical phenomenological studies rather than constructing physical models. An exception is Pe'er et al (2006b) who consider scattering of GRB emission in a cocoon formed as the blast wave jet emerges from the stellar photosphere (see also Pe'er et al 2006a).…”

“…This model applies strictly to a collapsar scenario, where the shocks emerge from the stellar photosphere by forming a cocoon (e.g., Ramirez-Ruiz et al 2002), yet rapid decays also occur in short, hard GRBs (e.g., GRB 050724, Barthelmy et al 2005b), which are thought to originate from coalescence events that lack stellar photospheres and cocoons (for recent reviews of short hard GRBs, see Nakar 2007; Lee & Ramirez-Ruiz 2007). 3 The curvature effect (Kumar & Panaitescu 2000;Dermer 2004;Liang et al 2006), which limits the rate at which an impulsively illuminated shell can temporally decay, obviously plays an important role. The curvature limits apply to the case of an active central engine that suddenly shuts itself off, or dramatically reduces its activity, as in the model of Proga & Zhang (2006).…”