GCN Circular 14994
Subject
GRB130702A: NOT observation - the detection of an emerging supernova
Event
Date
2013-07-08T04:31:28Z (12 years ago)
From
Steve Schulze at U of Iceland <sts30@hi.is>
S. Schulze (PUC, MCSS), G. Leloudas (OKC, Stockholm and DARK/NBI), D. Xu, J. P. U. Fynbo (DARK/NBI), S. Geier (NOT, DARK/NBI) and P. Jakobsson (U Iceland) report on behalf a larger collaboration:
We observed the field of GRB 130702A (Singer et al., GCN 14967; Cheung et al., GCN 14971; Collazzi et al., GCN 14972) with the 2.5-m Nordic Optical Telescope (NOT) equipped with ALFOSC. We obtained 1 x 150 s in r' and 5 x 60 in i'. Observations started at 23:23:10 UT on July 07 (i.e. 5.97 days after the burst).
We measure r' = 20.03 and i' = 20.21 mag. The afterglow became clearly brighter in r'-band with respect to our observation from the night before. Compared to our first observation on 3 July (Schulze et al. GCN 14978), the colour changed from 0.13 to -0.19 mag. This colour evolution is consistent with Butler et al. (GCN 14993), who reported a colour change of -0.08 mag between 4.25 and 5.26 days after the burst. Such a colour evolution is not expected for a decaying optical afterglow but clearly points to an emerging supernova.
We obtained an optical low-resolution spectrum with the ALFOSC camera starting at 23:45:33 UT. We used grism #4 that covers the wavelength range from 3750 to 9000 AA. The observation consisted of three individual spectra with a total exposure time of 3x1200 s. The flux-calibrated spectrum presents clear deviations from a power law. In particular, we identify two broad emission features peaking at 4900 and 5600 AA with a local minimum at 5200 AA (all observer frame). Assuming z=0.145 (Mulcaey et al. ATel 5191, GCN 14985; Leloudas et al. GCN 14983; D'Avanzo et al. GCN 14984), the spectrum resembles SN 1998bw at phase 7-8 days past explosion (Patat et al. 2001, ApJ, 555, 900).