TITLE: GCN CIRCULAR NUMBER: 14646 SUBJECT: GRB 130427A: Spectroscopic detection of the SN from the 10.4m GTC DATE: 13/05/14 21:21:33 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D. Xu (DARK/NBI), G. Leloudas (OKC, Stockholm, DARK/NBI), T. Kruehler, D. Malesani (DARK/NBI), J. Gorosabel (IAA-CSIC, UPV/EHU), Z. Cano (U. Iceland), C.C. Thoene, R. Sanchez-Ramirez (IAA-CSIC), S. Schulze (PUC and MCSS), J.P.U. Fynbo, J. Hjorth (DARK/NBI), P. Jakobsson (U. Iceland) and A. Cabrera-Lavers (IAC-ULL) report on behalf of a larger collaboration: We obtained spectroscopy of the optical counterpart and host galaxy of GRB 130427A (Maselli et al., GCN 14448; Elenin et al., GCN 14450) with the 10.4m GTC telescope, 16.7 days after the GRB onset. This is 12.5 days in the host galaxy rest-frame (z = 0.34; Levan et al. GCN 14455, Xu et al. GCN 14478 and Flores et al. GCN 1449). Observations consisted of 4x1200s with the R500R grism, covering the range between 4800 and 10000 AA with a resolution of ~600. The slit was oriented to cover both the afterglow and the host galaxy centre. The spectrum has a strong contribution from the host galaxy. To overcome this, we built a synthetic host galaxy spectrum based on the SDSS (DR9) photometry using LePhare (version 2.2, Arnouts et al. 1999, MNRAS, 310, 540; Ilbert et al. 2006, A&A, 457, 841). We then subtracted this host galaxy template from the GTC spectrum to obtain a "clean" spectrum of the counterpart associated to GRB 130427A. The resulting spectrum is that of a broad-lined Ic SN, with a prominent bump at ~6800 A observer frame. In particular, we obtain an excellent match with the spectrum of SN 2010bh at 12.7 (rest-frame) days after GRB 100316D (Bufano et al. 2012, ApJ 753, 67). We stress that this conclusion is independent of the host galaxy model adopted. By running SNID (Blondin & Tonry 2007, ApJ, 666, 1024) on the original spectrum (i.e. including host contamination), we still obtain good matches with a series of broad-lined Type Ic SNe, including SNe 1998bw, 1997ef, 2002ap and 2006aj, albeit at a lower redshift. The fact that SNID suggests a lower redshift is explained by the fact that SN 2010bh had high expansion velocities, reaching ~34000 km/s at similar phases (Bufano et al. 2012, ApJ 753, 67), which we suggest is also the case for the SN associated with GRB 130427A. A figure of our preliminary analysis can be seen at: http://www.iaa.es/~deugarte/GRBs/130427A/130427A_GTC.jpg We acknowledge excellent support from the GTC staff. [GCN OPS NOTE(14may13): Per author's request, ZCwas added to the author list.]