Supernova 2013ez = GRB 130215A

A. de Ugarte Postigo, Instituto de Astrofisica de Andalucia (IAA), Consejo Superior de Investigaciones Cientificas (CSIC), and Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen; Z. Cano, University of Iceland; C. C. Thoene, IAA/CSIC; J. Gorosabel, IAA/CSIC and University of the Basque Country; R. Sanchez-Ramirez, IAA/CSIC; G. Leloudas, Oskar Klein Centre, Stockholm University, and Dark Cosmology Centre; D. Xu, Dark Cosmology Centre; K. Wiersema, University of Leicester; J. P. U. Fynbo, D. Malesani, and J. Hjorth, Dark Cosmology Centre; P. Jakobsson, University of Iceland; and O. E. Hartoog, University of Amsterdam, on behalf of a larger collaboration, report that they have been monitoring the optical counterpart of GRB 130215A (discovered with the Swift satellite on Feb. 15.064 UT; cf. D'Elia et al., posted at URL http://gcn.gsfc.nasa.gov/gcn3/14204.gcn3, GCN Circular 14204, citing position R.A. = 2h53m55s, Decl. = +13d22'14", equinox 2000.0, uncertainty 3'; also, Zheng et al., GCN Circ. 14205, at position end figures 54m00s.7, 23'43".7, and unfiltered CCD mag 14.2 on Feb. 15.072). The present authors obtained spectroscopy of the optical counterpart with the 10.4-m Gran Telescopio Canarias (GTC) at Roque de los Muchachos Observatory in La Palma, Spain, around Mar. 12.87, at 25.8 days after the onset of the gamma-ray burst, which is 16.2 days after the burst in the host-galaxy rest frame (z = 0.597; cf. Cucchiara et al., GCN Circ. 14207). Observations consisted of three 1200-s exposures with the R500R grism, covering the range 480-1000 nm, with a resolution of about 600. Due to the variable's low altitude at the time of the observations, the data had to be obtained at high airmass (between 1.6 and 2.2), and there were thin cirrus clouds, but the seeing was very good (0".6). At this epoch, the spectrum presents undulations typical of supernovae spectra, including a prominent bump at about 850 nm. The spectrum has been compared to a series of supernovae templates using SNID version 5.0 (Blondin and Tonry 2007, Ap.J. 666, 1024), and a good match is obtained when compared to a number of type-Ic supernovae, including broad- lined and normal events such as SN 2002ap and SN 1994I around maximum light and up to +10 days. When leaving the redshift unconstrained, the fit returns a value of z = 0.60 +/- 0.02, fully consistent with that obtained from absorption-line spectroscopy. A comparison of the spectrum of GRB 130215A with that of SN 2002ap has been posted by the authors at the following URL: http://www.iaa.es/~deugarte/GRBs/130215A/130215A.jpg.