GRB 081008: From Burst to Afterglow and the Transition Phase in Between

Abstract

We present a multi-wavelength study of GRB 081008, at redshift 1.967, by Swift, ROTSE-III and GROND. Compared to other Swift GRBs, GRB 081008 has a typical gamma-ray isotropic equivalent energy output (∼ 10 53 erg) during the prompt phase, and displayed two temporally separated clusters of pulses. The early X-ray emission seen by the Swift/XRT was dominated by the softening tail of the prompt emission, producing multiple flares during and after the Swift/BAT detections. Optical observations that started shortly after the first active phase of gamma-ray emission showed two consecutive peaks. We interpret the first optical peak as the onset of the afterglow associated with the early burst activities. A second optical peak, coincident with the later gamma-ray pulses, imposes a small modification to the otherwise smooth lightcurve and thus suggests a minimal contribution from a probable internal component. We suggest the early optical variability may be from continuous energy injection into the forward shock front by later shells producing the second epoch of burst activities. These early observations thus provide a potential probe for the transition from prompt to the afterglow phase. The later lightcurve of GRB 081008 displays a smooth steepening in all optical bands and X-ray. The temporal break is consistent with being achromatic at the observed wavelengths. Our broad energy coverage shortly after the break constrains a spectral break within optical. However, the evolution of the break frequency is not observed. We discuss the plausible interpretations of this behavior.

THE AFTERGLOWS OFSWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFTANDSWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

“…From a multicolor joint fit, we find α 1 = 0.931 ± 0.006, α 2 = 1.132±0.008, t b = 0.048±0.004 days, n = 10 fixed, and a faint host galaxy assumed. Our decay slopes agree very well with Yuan et al (2010), but we find an earlier break. Late upper limits hint that a further break may have occurred.…”

“…The SED shows a bit of scatter, we find it is best fit by a small amount of SMC dust and an intrinsically blue slope. Yuan et al (2010) find a break in the optical SED, with a larger amount of dust and β ≈ 0, we cannot confirm this.…”

“…We constructed the light curve and the SED with data from the following sources: Yuan et al (2010), as well as our own ANDICAM data set. Yuan et al (2010) report an initially rising afterglow with small-scale variability (probably linked to central engine activity) superposed, which then goes over into a smooth decay which later shows a break. From a multicolor joint fit, we find α 1 = 0.931 ± 0.006, α 2 = 1.132±0.008, t b = 0.048±0.004 days, n = 10 fixed, and a faint host galaxy assumed.…”

A Large Catalog of Homogeneous Ultra-Violet/Optical GRB Afterglows: Temporal and Spectral Evolution

“…As in Paper1, we compare a sample of the resultant light curves with those published in the literature to check for consistency: GRBs 050525A (Blustin et al 2006), 050603 (Grupe et al 2006), 050730 (Perri et al 2007), 050801 (De Pasquale et al 2007), 050802 (Oates et al 2007), 060124 (Romano et al 2006), 060313 (Roming et al 2006a), 060729 (Grupe et al 2007), 061007 (Schady et al 2007), 070125 (Updike et al 2008), 080319B (Racusin et al 2008), 080810 (Page et al 2009), 081008 (Yuan et al 2010), 081203A (Kuin et al 2009), 090426 (Xin et al 2011), 090510 (De Pasquale et al 2010, and 090902B (Pandey et al 2010). For each of these bursts, we look for at least three events with comparable exposure times at similar epochs while keeping the normalized and published filters the same whenever possible.…”

A faint optical flash in dust-obscured GRB 080603A: implications for GRB prompt emission mechanisms

“…None the less, we note that no chromatic evolution was required by comparably early-time observations of several other GRBs (e.g. Krühler et al 2009;Nysewander et al 2009;Covino et al 2010;Yuan et al 2010;Perley et al 2011).…”