Electronic Telegram No. 3529 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION CBAT Director: Daniel W. E. Green; Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network SUPERNOVA 2013cq = GRB 130427A 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; D. Xu, Dark Cosmology Centre; G. Leloudas, Oskar Klein Centre (OKC), Stockholm University, and Dark Cosmology Centre; T. Kruehler and D. Malesani, Dark Cosmology Centre; J. Gorosabel, IAA/CSIC and Universidad del Pais Vasco University of the Basque Country; C. C. Thoene and R. Sanchez-Ramirez, IAA/CSIC; S. Schulze, Pontificia Universidad Catolica de Chile and Millennium Center for Supernova Science; J. P. U. Fynbo and J. Hjorth, Dark Cosmology Centre; Z. Cano and P. Jakobsson, University of Iceland; and A. Cabrera-Lavers, Instituto de Astrofisica de Canarias, Universidad de La Laguna, on behalf of a larger collaboration, report that they have been monitoring the optical counterpart of GRB 130427A (discovered with the Swift spacecraft; cf. Maselli et al., GCN Circular 14448, posted at website URL http://gcn.gsfc.nasa.gov/gcn3/14448.gcn3; and Elenin et al., GCN 14450). The authors obtained spectroscopy of the optical counterpart and the burst's host galaxy with the 10.4-m Gran Telescopio Canarias (GTC) at Roque de los Muchachos Observatory in La Palma, Spain, 16.7 days after the onset of the gamma-ray outburst; this is 12.5 days in the host-galaxy rest frame (z = 0.34; cf. Levan et al., GCN 14455; Xu et al., GCN 14478; Flores et al., GCN 1449). Observations consisted of four 1200-s exposures with the R500R grism, covering the range 480-1000 nm with a resolution of about 600. The slit was oriented to cover both the afterglow and the host-galaxy center. The spectrum has a strong contribution from the host galaxy. To overcome this, they built a synthetic host-galaxy spectrum based on the Sloan Digital Sky Survey (DR9) photometry using LePhare (version 2.2; Arnouts et al. 1999, MNRAS 310, 540; Ilbert et al. 2006, A.Ap. 457, 841). They then subtracted this host-galaxy template from the GTC spectrum to obtain a "clean" spectrum of the counterpart associated with GRB 130427A. The resulting spectrum is that of a broad-lined type-Ic supernova, with a prominent bump at about 680.0 nm in the observer frame. In particular, they obtained an excellent match with the spectrum of SN 2010bh at 12.7 (rest-frame) days after GRB 100316D (Bufano et al. 2012, Ap.J. 753, 67). The authors stress that this conclusion is independent of the host-galaxy model adopted. By running SNID (Blondin and Tonry 2007, Ap.J. 666, 1024) on the original spectrum (i.e., including host contamination), they still obtain good matches with a series of broad-lined type-Ic supernovae, including with 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 around 34000 km/s at similar phases (Bufano et al. 2012, Ap.J. 753, 67), which the present suggest is also the case for the supernova associated with GRB 130427A, here designated SN 2013cq. A figure of their preliminary analysis can be seen at website URL http://www.iaa.es/~deugarte/GRBs/130427A/130427A_GTC.jpg. The authors acknowledge excellent support from the GTC staff. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2013 CBAT 2013 May 16 (CBET 3529) Daniel W. E. Green