The optical and infrared afterglow of GRB031203 and the associated hypernova SN 2003lw

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Abstract

Optical and near-infrared photometric observations of the host galaxy of GRB 031203 allowed us to detect a bump in its light curves, peaking about 18 rest-frame days after the burst. We interpreted this as due to the emergence of an associated SN. Subsequent spectroscopic observations allowed us to confirm the presence of a SN, named SN 2003lw. The presence of extremely broad features in these optical spectra, very similar to the ones seen in SN 1998bw, indicate that this is a very energetic hypernova. Moreover, it was even brighter than SN 1998bw. Therefore, the three best confirmed associations between a GRB (980425, 030329, 031203) and a SN involve hypernovae. However, while GRB030329 has properties “typical” of most GRBs, the other two are quite peculiar, in particular they are very weak and nearby. Thus, it is remarkable that the three hypernovae associated to these GRBs are so similar, while the GRBs themselves are very different. Thus, a lot of open questions still remain in the associations between SNe and long-duration GRBs, questions that will soon find an answer thanks to the launch of the Swift satellite.

Introduction

The idea that gamma-ray bursts (GRBs) were somehow associated with supernovae (SNe) has been present in the community for a long time. However, no evidence could be found to confirm this idea. As for many other discoveries in the GRB field, things changed drastically with the advent of the BeppoSAX satellite. Thanks to discovery of the X-ray and optical afterglows associated with GRBs (Costa et al., 1997, van Paradijs et al., 1997) it has been possible to establish that GRBs are cosmological, are the most powerful explosion in the Universe after the Big-Bang, are associated with irregular star forming galaxies and, very recently, that long-duration GRBs are associated with core-collapse SN explosions. To this end people have monitored the decay of the optical and infrared light curve of afterglows looking for a re-brightening that would indicate the emergence of a SN. Various bumps have been found, peaking at ∼20 days after the burst, which have been attributed to the presence of a SN (e.g., Bloom et al., 1999). However, the lack of a spectroscopic confirmation made these findings only suggestive. The real breakthrough came with the discovery of the bright SN 1998bw, spatially and temporally coincident with GRB 980425. This SN showed broad bumps in its spectra, indicating very large expansion velocities and was extremely bright. These SNe are now designed as hypernovae (Iwamoto et al., 1998). The spectroscopic identification of SN 2003dh, with properties similar to that of SN 1998bw, in the afterglow of GRB 030329 (Stanek et al., 2003, Hjorth et al., 2003), seems to confirm that long-duration GRBs are associated with hypernovae. However, in the case of GRB 021211, Della Valle et al. (2003) found a spectroscopic confirmation that the bump detected in the light curve was due to a typical Ic SN or to a dimmed hypernova, as 2002ap (Mazzali et al., 2002). Similar conclusions have been recently achieved by Fynbo et al. (2004) and Levan et al. (2004) after studying GRB 030723 and GRB 020410, respectively. Thus, although the association between long-duration GRBs and SNe seems to be common, there are still many open points. Here, we report the discovery of the bright hypernova SN 2003lw (Tagliaferri et al., 2004, Malesani et al., 2004) associated with the faint GRB 031203 (Götz et al., 2003, Sazonov et al., 2004).

Section snippets

Observations and results

GRB 031203 was detected by the INTEGRAL satellite with a duration of ∼30 s and a peak flux of 1.3 × 10−7 erg cm−2 s−1 in the 20–200 keV band (Götz et al., 2003, Mereghetti and Götz, 2003). The X-ray and radio afterglow were soon discovered (Santos-Lleo and Calderon, 2003, Freil, 2003). Although this field is on the galactic plane and is rather crowded, the small error boxes provided for the X-ray and radio afterglow allowed to immediately identify the host galaxy of this GRB. This is an emission line

Discussion

The distance of GRB031203, the second nearest GRB to date, implies that it is a very faint one, with an isotropic energy release of ∼1 × 1050 erg, much fainter than most GRBs. Moreover, it does not follow the correlations that have been found for “standard” GRBs between their peak energy and energy release (Amati et al., 2002, Ghirlanda et al., 2004). The afterglow is also rather peculiar: it is the faintest NIR afterglow so far detected (Malesani et al., 2004), showing an unusually slow decay in

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