[vsnet-grb-info 5504] Transient in NGC 2770: spectroscopic evidence for a SN

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7169
SUBJECT: Transient in NGC 2770: spectroscopic evidence for a SN
DATE:    08/01/11 21:01:14 GMT
FROM:    Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center  <malesani at astro.ku.dk>

D. Malesani, J. Hjorth (DARK), P. Jakobsson (Univ. Hertfordshire), P.M. 
Vreeswijk, C.C. Thoene, J.P.U. Fynbo, D.J.  Watson, J. Sollerman (DARK), 
N.R. Tanvir (Univ. Leicester), T. Stanke (ESO), report on behalf of a 
larger collaboration:

We observed the optical transient in NGC 2770 (Berger & Soderberg, GCN 
7159; Deng & Zhu, GCN 7160), denoted SN 2008D (Li & Filippenko, IAUC 
1202), with the ESO VLT equipped with FORS2, starting on 2008 Jan 11, 
07:17 UT. Two spectra, at low and moderate resolution, were acquired, 
covering the wavelength ranges ~3700-6350 and ~3800-9000 AA. The 
exposure times were 10 and 15 minutes, respectively. The two spectra, 
after flux calibration, agree very well in the overlap region, and the 
peak S/N is ~60 and ~40, respectively.

We detect a strong, red continuum revealing several broad features and 
bumps. The afterglow spectrum does not display a power-law shape. We 
also detect Na I D in absorption at z = 0.0069. This redshift is very 
similar to that of the host z=0.0071, as measured for the Halpha and 
Hbeta lines in emission superimposed on the transient. The detection of 
Na I D confirms that the object is of extragalactic origin. It also 
implies that it is subject to significant extinction. Although 
uncertain, using the relation by Munari & Zwitter (1997, A&A, 318, 269), 
we estimate E(B-V) to be larger that ~0.5 mag based on the equivalent 
width of the 5890 A line EW = 0.67 AA. Such relatively large extinction 
is consistent with the detection of excess column density in the X-ray 
spectra of the flare (Berger & Soderberg, GCN 7159).

The spectral shape of the transient rules out a significant nonthermal 
afterglow component. The presence of broad features is reminiscent of SN 
spectra, although it is not straightforward to identify the involved 
transitions. The lack of prominent Silicon and Hydrogen lines suggests 
the SN is not of type Ia or II. By exclusion, we suggest the transient 
is a SN of type Ib/c. The features are not as broad as in the earliest 
spectra of GRB-associated SNe such as SN1998bw and SN2006aj.

The luminosity of the event is also comparable to that of other 
core-collapse SNe associated with GRBs. The object had V~19.3 
approximately 1.5 days after the X-ray transient (Thoene et al., GCN 
7161). Taking into account the large extinction (> 1.5 mag in V), the 
luminosity is comparable with that of core-collapse SNe soon after the 
explosion. As noted by several authors, the available light curve is 
also consistent with a SN (Valenti et al., GCN 7163; Kong et al., GCN 
7164, Li & Filippenko, IAUC 1202).

In conclusion, the bright absolute magnitude of the object, M_V < -14,  
the broad spectral features, and the lack of H and Si strongly suggest 
that the transient in NGC 2770 is a SN of type Ib/c.

Lastly, we note that the transient does not seem to lie in an intense 
star forming region of its host, as suggested by the modest nebular 
features detected in our spectrum and the absence of prominent emission 
at the position of the optical transient (D'Elia et al., GCN 7162) in 
the archival SDSS images. We caution that strong extinction may affect 
this arguments.

We acknowledge excellent support of the observing staff at Paranal, in 
particular Cedric Ledoux, Linda Schmidtobreick, and Antonio de Ugarte 
Postigo.