TITLE: GCN CIRCULAR NUMBER: 11579 SUBJECT: GRB 101219B: tentative redshift and spectroscopic supernova detection DATE: 11/01/19 18:33:31 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst Antonio de Ugarte Postigo (DARK/NBI), Paolo Goldoni (APC/Univ. Paris 7 and SAp/CEA), Bo Milvang-Jensen, Daniele Malesani, Martin Sparre, Johan P.U. Fynbo, Giorgos Leloudas (DARK/NBI), Stefano Covino (INAF/OABr), Hector Flores (GEPI/Obs. de Paris), Valerio D'Elia (ASI/ASDC and INAF/Roma), Andrew Levan (Univ. Warwick), report on behalf of the X-shooter GTO GRB collaboration: We observed the afterglow of GRB 101219B (Gelbord et al., GCN 11473) with the X-shooter spectrograph mounted on the ESO-VLT UT2. Observations were taken on two epochs, 2010 Dec 20.17 and 2011 Jan 5.09 UT (11.6 hr and 16.4 days after the burst, respectively). Exposure times were 80 and 120 min, respectively. In the first-epoch spectrum, the afterglow is detected over the full spectral range from about 3200 to 22,000 AA. A careful analysis provides a tentative redshift z=0.5519 based on detection of weak Mg II and Mg I. No other clear features can be found at the same redshift either in absorption or emission. The second-epoch spectrum was taken 16.4 days after the GRB, which corresponds to 10.6 days in the rest frame at z=0.55. The continuum is again detected, over the wavelength range 3500 to 22,000 AA, and is substantially redder. No narrow emission lines can be distinguished, but broad SN-like undulations are apparent, particularly around 8000 AA. We carried out a comparison of our spectrum with those of SN 1998bw (Patat et al. 2001, ApJ, 555, 900) transformed at various redshifts. We find a convincing match at z~0.55 after comparing with a spectrum taken on 1998 May 4 (i.e., 12 days after the GRB). The observed flux is consistent assuming a roughly equal contribution from a SN as bright as SN 1998bw and an underlying host galaxy. The match of the spectral shape, brightness, and spectral epoch together strenghten both the redshift determination and the existence of a SN associated with GRB 101219B. Our findings are in good agreement with the GROND observations reported by Olivares et al. (GCN 11578). Further observations are planned. We acknowledge the assistence of the ESO observing staff at Paranal, in particular Dimitri Gadotti, Leonel Rivas, Thomas Rivinius, Maja Vuckovic, and Thomas Szeifert.