Huge explosion in the early Universe

Abstract

Long gamma-ray bursts (GRBs) are bright flashes of high-energy photons that can last for tens of minutes; they are generally associated with galaxies that have a high rate of star formation and probably arise from the collapsing cores of massive stars, which produce highly relativistic jets (collapsar model 1 ). Here we describe ȍ-and X-ray observations of the most distant GRB ever observed (GRB 050904): its redshift 2,3 (z) of 6.29 means that this explosion happened 12.8 billion years ago, corresponding to a time when the Universe was just 890 million years old, close to the reionization era 4 . This means that not only did stars form in this short period of time after the Big Bang, but also that enough time had elapsed for them to evolve and collapse into black holes.GRB 050904 triggered the Burst Alert Telescope (BAT) on board the Swift 5 satellite on 4 September 2005 at 1:51:44 GMT. The spacecraft quickly slewed to allow observations by the X-ray Telescope (XRT) 6,7 , which measured the burst for ten days after its onset. Figure 1 (top panel) shows the history of the burst. We shall present and discuss the GRB phenomenology from the point of view of the rest frame of its source.The BAT light curve shows three main peaks: two of about 2 s at T+3.8 s and T+7.7 s, and a long-lasting one at about T+13.7 s, where T is the time of the burst onset. It also shows a weak peak at about T+64 s. The early XRT light curve shows a steep power-law decay with an index of ǁ2.07DŽ0.03; two flares are superimposed at T+64 s (coincident with the last peak of the BAT light curve) and T+170 s. Although interrupted by the constraints of low-Earthorbit observation, the X-ray light curve reveals highly irregular intensity variations, probably due to the presence of flares for up to T+1.5 hours. At later times, flaring activity is not detected, leaving only a residual emission that is 10 6 times lower than the initial intensity.The flares in the XRT light curve can be interpreted as late internal shocks related to central engine activity. In this scenario, they would have the same origin as the first ȍ-ray emission [8][9][10] , which would require the central engine to remain active for at least 5,000 seconds, consistent with the collapsar model 1 .Spectral analysis was performed by selecting time intervals corresponding to characteristic phases of the light curve evolution. All spectra were well modelled by a single power law, with both galactic and intrinsic absorption components in the case of the XRT spectra. Figure 1 (bottom) shows the evolution with time of the photon index Ǵ. The BAT spectra have Ǵǃ ǁ1.2. If we exclude the spectrum of the first XRT flare at T+64 s, the XRT photon indices show a clear, decreasing trend from about ǁ1.2 to about ǁ1.8 in the first T+200 s. No further spectral evolution is present in later XRT data.The overall phenomenology of GRB 050904 is not peculiar with respect to other GRBs at lower redshift. This suggests that the mechanisms of GRB explosions in the early Universe and today are similar.Based ...

No evidence for dust extinction in GRB 050904 atz~ 6.3

“…The X-ray lightcurve is extremely variable, exhibiting long lasting flaring activity (Watson et al 2006b;Cusumano et al 2006;Gendre et al 2006). The flares suggest two separate components, which may be due to a number of causes, possibly activity of the GRB central engine (e.g.…”

“…Time-resolved X-ray spectroscopy reveals that the column density of metals within the first few hours is highly variable (Campana et al 2007;Cusumano et al 2006;Gendre et al 2006). Due to the rapid changes in the X-ray spectrum this apparently variable column may be an artifact of the changing intrinsic spectrum resulting in a downturn at soft energies that disappears at later times (see Butler et al 2006).…”

Swift multi-wavelength observations of the bright flaring burst GRB 051117A

“…Gehrels et al 2005), the detection of a very high redshift burst (e.g. Cusumano et al 2006), an unexpected rapid decline phase in the prompt X-ray light-curves (Tagliaferri et al 2005;Goad et al 2006b), observations of frequent flaring activity in the early X-ray light-curves of approximately half of all bursts King et al 2005;Falcone et al 2006), and the intriguing discovery of a burst which would have been classified as short by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO), but had a long soft tail in the BAT which lasted over 100 s (e.g. Barthelmy et al 2005a), to name but a few.…”

Introduction: recent developments in the study of gamma-ray bursts

“…Indeed, the majority of Swift GRB detections to date have been of the long-burst variety, and studies of the early afterglows, previously inaccessible, have added to evidence supporting the view that long-duration bursts are produced during the collapse of a massive star. Red shifts have now been measured for over 50 long bursts, including the first GRB at very high red shift (zO6; Cusumano et al 2006;Haislip et al 2006;Kawai et al 2006). These bursts are providing new ways to probe the high-red-shift universe (as reviewed by Ghirlanda 2007;Lamb 2007) and Tanvir & Jakobsson (2007) discuss conditions under which GRBs may be used as a tracer of the star formation rate in high-redshift galaxies.…”