Abstract
GRB 090618 was an extremely bright burst, detected across the electromagnetic spectrum. It has a redshift of 0.54 and a supernova (SN) was identified in ground-based photometry. We present a thorough analysis of the prompt and early afterglow emission using data from Swift, Fermi Gamma-ray Burst Monitor and ROTSE, in which we track the evolution of the synchrotron spectral peak during the prompt emission and through the steep decay phase. We find evidence of a thermal X-ray component alongside the expected non-thermal power-law continuum. Such a component is rare among gamma-ray bursts (GRBs), with firm data for only GRBs 060218 and 100316D so far, and could potentially originate from an SN shock breakout, although there remains doubt regarding this explanation for any of the bursts. However, in contrast to these other Swift GRB–SNe with similar thermal signatures, GRB 090618 is a much more ‘typical’ burst: GRB–SNe 060218 and 100316D were both low-luminosity events, with long durations and low peak energies, while GRB 090618 was more representative of the wider population of long GRBs in all of these areas. It has been argued, based both on theory and observations, that most long GRBs should be accompanied by SNe. If this thermal X-ray component is related to the SN, its detection in GRB 090618, a fairly typical burst in many ways, may prove an important development in the study of the GRB–SN connection.
Highlights
Gamma-ray bursts (GRBs) are intrinsically luminous objects, allowing them to be detected right across the Universe, from relatively nearby locations [e.g. GRB 980425 at a redshift, z, of 0.0085 (Tinney et al 1998) and GRB 060218 at z = 0.0331 (Campana et al 2006)] out to great distances [e.g. GRB 090423 with z = 8.2 (Tanvir et al 2009; Salvaterra et al 2009) and GRB 090429B at z = ∼9.4 (Cucchiara et al 2011)]
The γ -ray data are best fitted by a power law with a changing exponential cutoff signifying the peak energy of the spectrum moving through the band
There is a late-time break in the X-ray and optical temporal decay which is consistent with being achromatic, the final decay is shallower than usually expected for a post-jet-break decay and the fit improves if the break time is allowed to vary between the two data sets
Summary
Gamma-ray bursts (GRBs) are intrinsically luminous objects, allowing them to be detected right across the Universe, from relatively nearby locations [e.g. GRB 980425 at a redshift, z, of 0.0085 (Tinney et al 1998) and GRB 060218 at z = 0.0331 (Campana et al 2006)] out to great distances [e.g. GRB 090423 with z = 8.2 (Tanvir et al 2009; Salvaterra et al 2009) and GRB 090429B at z = ∼9.4 (Cucchiara et al 2011)]. Cano et al (2011) present optical data for this burst, revealing a probable SN-bump in the light curves, peaking a few tens of days after the GRB trigger. Very bright bursts such as GRB 090618 permit analysis of exceptional detail, helping to reveal the underlying physics of the processes involved.
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