[vsnet-grb-info 5508] Detailed Spectroscopic ID of SN 2008D (Transient in NGC 2770)

[GCN Circulars]

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  <mmodjaz at astro.berkeley.edu>

 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.