X-ray flare in XRF 050406: evidence for prolonged engine activity

Abstract

We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at t peak ∼ 210 s after the BAT trigger, a flux variation of δF/F ∼ 6 in a very short time δt/t peak 1 was observed. Its measured fluence in the 0.2−10 keV band was ∼1.4 × 10 −8 erg cm −2 , which corresponds to 1−15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210 s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ∼0.5 after ∼4400 s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (∼10 −7 erg cm −2 in the 15−350 keV band, E iso ∼ 10 51 erg), a soft spectrum (photon index 2.65), no significant flux above ∼50 keV and a peak energy E p < 15 keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.

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

“…Our results ( Fig. 1 and Table 1) suggest that usually the t 0 values are near the beginning of the rising segment of the last pulse of the prompt emission or the corresponding X-ray flare, which is consistent with the expectation of the internal dissipation models for the prompt emission and X-ray flares (e.g., Burrows et al 2005;Zhang et al 2006a;Fan & Wei 2005;Ioka et al 2005;Falcone et al 2006;Romano et al 2006). This suggests that the GRB central engine reactivates after the early prompt emission is over, sometimes up to days after the trigger (e.g., GRB 050502B and GRB 050724).…”

“…Zhang et al (2006a) performed more detailed analysis of various possible scenarios and concluded that the late internal dissipation model is the correct interpretation of X-ray flares. Similar conclusions have been also drawn by Ioka et al (2005), Fan & Wei (2005), Falcone et al (2006), and Romano et al (2006) (see Piro et al 2005; C. D. Dermer 2006, in preparation).…”

“…C. D. Dermer, 2006, in preparation). The distinct X-ray flares typically show rapid rise and fall, with the ratio of the variability timescale and the epoch of the flare typically much less than unity, i.e., t/tT1 (Burrows et al 2005;Falcone et al 2006;Romano et al 2006). Burrows et al (2005) proposed in their discovery paper that the flares are also produced in internal shocks at later times, which requires reactivation of the GRB central engine.…”

Panchromatic study of GRB 060124: from precursor to afterglow

“…Quite often an erratic flaring behaviour is observed superimposed on these phases Falcone et al 2006;Romano et al 2006). The most accredited explanation for the steep phase is that we are seeing high-latitude emission due to the termination of the central engine activity (Kumar & Panaitescu 2000;Zhang et al 2006), while flares are probably due to the reactivation of the central engine.…”

“…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.…”