A tale of two GRB-SNe at a common redshift of z = 0.54

TL;DR

This study presents detailed optical observations of two GRB-associated supernovae at z=0.54, comparing their properties with known supernovae and analyzing afterglow characteristics to understand their nature.

Contribution

It provides the first detailed optical analysis of two GRB-SNe at the same redshift, comparing their light curves and spectra with known supernovae, and discusses afterglow properties within a jet model.

Findings

Both SNe resemble SN1998bw more than SN1994I.

The afterglow of GRB 090618 shows a jet-break at >0.5 days.

Peak magnitudes of GRB/XRF-SNe are similar to local type Ibc SNe.

Abstract

We present ground-based and HST optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z = 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broadband spectral energy distributions (SEDs) of the OT resemble those of local type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB-supernova, and SN1994I, a typical type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly-available SWIFT -XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 days. We then compare the rest-frame, peak V -band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe.