TITLE: GCN CIRCULAR NUMBER: 7173 SUBJECT: Detailed Spectroscopic ID of SN 2008D (Transient in NGC 2770) DATE: 08/01/12 00:12:13 GMT FROM: Maryam Modjaz at UC Berkeley S. Blondin (Harvard-Smithsonian Center for Astrophysics, CfA), on behalf of the CfA Supernova Group; T. Matheson (National Optical Astronomy Observatory); and M. Modjaz (University of California at Berkeley), report: A spectrum (range 320-840 nm) of 2008D (cf. CBET 1202), obtained on Jan. 11.41 UT by P. Berlind with the MMT 6.5-m telescope (+ Blue Channel spectrograph), shows it most likely to be a peculiar type-Ic supernova in its early phases, confirming and refining earlier results (Malesani et al, GCN 7169; Valenti et al, GCN 7171). The spectrum consists of a relatively blue continuum, with P-Cyg profiles (FWHM of approximately 10000 km/s) that are characteristic of a supernova spectrum blueward of 550 nm, and an extremely broad bump peaking at approximately 630 nm (with an associated absorption at 590 nm) that is similar to the Si II line (rest 635.5 nm) seen in the spectra of broad-line type-Ic supernovae around maximum light. More specifically, the double-absorption feature centered on 420 nm resembles that seen in the earliest spectrum of the normal type-Ic supernova 1990aa (cf. IAUC 5087, 5111; Matheson et al. 2001, A.J. 121, 1648), while the broad feature centered on 630 nm resembles the Si II bump seen in spectra of the broad-line type-Ic supernova 1998bw (associated with GRB 980425; cf. IAUC 6895, 6901; Patat et al. 2001, Ap.J. 555, 900) around maximum light (roughly three weeks after the gamma-ray-burst detection). Blondin et al. add that their spectrum is not consistent with the description given by Soderberg et al. in GCN 1765, who report on a "smooth, featureless continuum" with "some suggestion of a broad bump near 5500 Angstroms". However, based on other spectra of familiar objects fully reduced from the same night, Blondin et al. are confident that their flux and wavelength calibrations are accurate. The separation in time between the MMT spectrum and the one described in GCN 1765 is only 0.1 day. Blondin et al. clearly detect narrow emission lines at the supernova position, consistent with the NED redshift of 1934 km/s for NGC 2770 (from The Updated Zwicky Catalog, http://tdc-www.harvard.edu/uzc/). Adopting this recession velocity, the maximum absorption in the Si II line (rest 635.5 nm) is blueshifted by roughly 23000 km/s. Also, interstellar Na I D absorption at that redshift is visible in the spectrum with an equivalent width of 0.2 nm. A comparison plot can be seen at the following URL: http://www.cfa.harvard.edu/supernova/spectra/sn2008D_comp.gif; note that, in this plot, the spectrum of SN 1990aa had to be blueshifted by 5500 km/s in order for prominent spectroscopic features to match up with those present in SN 2008D, while that of SN 1998bw has been redshifted by 13000 km/s for the same reason.