Abstract
We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Burst 051117A as observed by Swift . The superb quality of the early X-ray light-curve and spectra of this source, one of the brightest seen by the X-ray Telescope at such early times, allows an unprecedented look at the spectral and temporal evolution of the prompt and early afterglow emission for this GRB and allows us to place stringent limits on the detection of lines. GRB 051117A displays a highly complex light-curve, with an apparent initial slow decline of slope () dominated by numerous superposed flares of varying amplitude and duration. Between orbits 2 and 3, the X-ray light-curve drops abruptly, highlighting the dominance of flaring activity at early times, and indicating that the central engine for this burst remains active for several kiloseconds after the initial explosion. The late time slope ( s) also decays relatively slowly with a powerlaw index of , breaking to a steeper slope of 1.1, 170 ks after the BAT trigger. The X-ray light-curve at early times is characteristic of a noise process, consisting of random shots superposed on an underlying powerlaw decay, with individual shots well-modelled by a fast-rise and exponential decay spanning a broad range in rise-times and decay rates. A temporal spectral analysis of the early light-curve shows that the photon index and source intensity are highly correlated with the spectrum being significantly harder when brighter, consistent with the movement of the peak of the Band function to lower energies following individual flares. The high quality spectrum obtained from the first orbit of WT mode data, enables us to place a upper limit on the strength of any emission line features of eV, assuming a narrow emission-line of 100 eV at the peak of the effective area.
Highlights
The Swift Gamma-Ray Burst Explorer (Gehrels et al 2004), approaching the end of its second year of operations, is routinely observing the prompt gamma-ray and early afterglow emission of Gamma-Ray Bursts (GRBs) in the astrophysically important minutes to hours timescale after the onset of the burst
The superb quality of the early X-ray light-curve and spectra of this source, one of the brightest seen by the X-ray Telescope at such early times, allows an unprecedented look at the spectral and temporal evolution of the prompt and early afterglow emission for this GRB and allows us to place stringent limits on the detection of lines
The X-ray light-curve at early times is characteristic of a noise process, consisting of random shots superposed on an underlying powerlaw decay, with individual shots well-modelled by a fast-rise and exponential decay spanning a broad range in rise-times and decay rates
Summary
The Swift Gamma-Ray Burst Explorer (Gehrels et al 2004), approaching the end of its second year of operations, is routinely observing the prompt gamma-ray and early afterglow emission of Gamma-Ray Bursts (GRBs) in the astrophysically important minutes to hours timescale after the onset of the burst. If the central engine activity ceases abruptly, an external observer will continue to see emission from increasingly large angles (θ > Γ−1) with respect to their line-of-sight, the so-called “curvature effect” (e.g. Kumar & Panaitescu 2000; Dermer 2004; Fan & Wei 2005) This steep phase is typically followed by a shallower decay phase which is spectrally harder (e.g. Goad et al 2006b; Nousek et al 2006; Willingale et al 2007) and is thought to be associated with late time energy injection (refreshed shocks) which suggests the central engine activity may last for far longer than had previously been thought.
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