Abstract
The majority of short gamma-ray bursts (SGRBs) are thought to originate from the merger of compact binary systems collapsing directly to form a black hole. However, it has been proposed that both SGRBs and long gamma-ray bursts (LGRBs) may, on rare occasions, form an unstable millisecond pulsar (magnetar) prior to final collapse. GRB 090515, detected by the Swift satellite was extremely short, with a T_90 of 0.036 +/- 0.016 s, and had a very low fluence of 2 x 10^-8 erg cm^-2 and faint optical afterglow. Despite this, the 0.3 - 10 keV flux in the first 200 s was the highest observed for a SGRB by the Swift X-ray Telescope (XRT). The X-ray light curve showed an unusual plateau and steep decay, becoming undetectable after ~500 s. This behaviour is similar to that observed in some long bursts proposed to have magnetars contributing to their emission. In this paper, we present the Swift observations of GRB 090515 and compare it to other gamma-ray bursts (GRBs) in the Swift sample. Additionally, we present optical observations from Gemini, which detected an afterglow of magnitude 26.4 +/- 0.1 at T+ 1.7 hours after the burst. We discuss potential causes of the unusual 0.3 - 10 keV emission and suggest it might be energy injection from an unstable millisecond pulsar. Using the duration and flux of the plateau of GRB 090515, we place constraints on the millisecond pulsar spin period and magnetic field.
Highlights
Thirty years after the discovery of Gamma-ray Bursts (GRBs) by the Vela satellites (Klebesadel et al 1973), the first X-ray afterglow was detected for GRB 970228 by the Beppo-SAX satellite (Costa et al 1997)
Swift has enabled the detection of Short GRB (SGRB) X-ray afterglows, allowing them to be directly compared to Long GRB (LGRB) afterglows (Gehrels et al 2005; Barthelmy et al 2005a; Hjorth et al 2005a)
Alongside the prediction by Troja et al (2007) and Lyons et al (2010) of a plateau followed by a steep decay for the lightcurve of a millisecond pulsar collapsing to a black hole, which matches the observed light curve for GRB 090515, this analysis provides a consistent case for GRB 090515 forming a millisecond pulsar irrespective of the two initial progenitor models considered
Summary
Thirty years after the discovery of Gamma-ray Bursts (GRBs) by the Vela satellites (Klebesadel et al 1973), the first X-ray afterglow was detected for GRB 970228 by the Beppo-SAX satellite (Costa et al 1997). O’Brien et al (2006) showed that not all X-ray light curves for GRBs are of the “cannonical” variety They and Willingale et al (2007) suggested that the X-ray light curve comprises two main components, the prompt emission and the afterglow. It has been suggested that both LGRB and SGRB progenitors could produce an unstable millisecond pulsar This is expected to contribute a small fraction of the GRB population (Usov 1992; Duncan & Thompson 1992; Dai & Lu 1998a,b; Zhang & Meszaros 2001). Troja et al (2007) and Lyons et al (2010) found examples of LGRBs that have an observable plateau and steep decay in the X-ray light curve, which have been intepreted as caused by energy injection by an unstable millisecond pulsar which collapses. Errors are quoted at 90% confidence for X-ray data and at 1σ for optical data
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