2008CBET.1602....1D

M. Della Valle, Istituto Nazionale di Astrofisica (INAF), Napoli; S. Benetti and P. Mazzali, INAF, Padova; J. Fynbo, Dark Cosmology Centre, Copenhagen; E. Pian, INAF, Trieste; R. Filgas, J. Greiner, and T. Kruehler, Max-Planck-Institute (MPI) fuer Extraterrestrische Physik, Garching; D. Fugazza, P. D'Avanzo, and S. Covino, INAF, Milano; and L. Amati, INAF, Bologna – on behalf of a larger collaboration – report the discovery of an apparent supernova at R.A. = 22h39m50s, Decl = -40d08'49" (equinox 2000.0), which is coincident (within the errors) with the position of the REM-robotic-telescope optical counterpart (Covino et al. 2008, GCN 8331, posted at website URL http://gcn.gsfc.nasa.gov/gcn3/8331.gcn3) of GRB 081007 (Baumgarther et al. 2008, GCN 8330) at z = 0.53 (Berger et al. 2008, GCN 8335). A preliminary analysis of a 2-hr Very Large Telescope FORS2 spectrum (range 600-1000 nm, resolution 1.2 nm), obtained at the European Southern Observatory (ESO), Paranal, on Nov. 2 (17 days after the Swift/BAT trigger in the gamma-ray-burst rest-frame), reveals – after subtraction of a starburst galaxy template (sb1 of Kinney et al. 1996, Ap.J. 467, 38) and redshift correction – broad bumps at about 460, 540, and 640 nm, which are very similar to those exhibited by the spectrum of SN 1998bw (Patat et al. 2001, Ap.J. 555, 900) about one week past maximum. Further analysis shows similarities with post-maximum spectra of other two "broad-line" type-Ic supernovae: 1997ef (Mazzali et al. 2000, Ap.J. 545, 407) and 2004aw (Taubenberger et al. 2006, MNRAS 371, 1459). The photometric follow-up carried out with GROND (Greiner et al. 2008, PASP 120, 405) at the MPI/ESO 2.2-m telescope (La Silla, Chile), starting from about 1000 s after the Swift/BAT trigger, reveals a flattening starting at about 7 days after the gamma-ray burst (rest frame). The broad-band spectral-energy distribution at 10 restframe days, constructed with GROND photometric measurements, shows a distincly different shape than at early times; in particular, it has a spectral maximum in the i' band, and no detections at near-infrared wavelengths, supporting the presence of a supernova superposed on the afterglow.