GCN Circular 7169
Subject
Transient in NGC 2770: spectroscopic evidence for a SN
Date
2008-01-11T21:01:14Z (18 years ago)
From
Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center <malesani@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, CBET 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.
[GCN OPS NOTE(13jan08): Per author's request, the "IAUC 1202" citation
was changed to "CBET 1202" (at the end of the 4th paragraph).]