Gaia16amw is a previously missed, old core-collapse SN at 20 Mpc
ATel #9061; M. Fraser, S. Mattila, S. T. Hodgkin, D. Harrison (IoA), L. Wyrzykowski (Warsaw), Z. Kostrzewa-Rutkowska (Warsaw/SRON), N. Blagorodnova (Caltech)
on 18 May 2016; 16:32 UT
Credential Certification: Morgan Fraser (mf@ast.cam.ac.uk)
Subjects: Optical, Supernovae
We obtained a low resolution spectrum of Gaia16amw using the robotic Liverpool Telescope + SPRAT (R~350; 400-800 nm) on the night of 2016 May 17. The spectrum reveals broad [O I] emission at 6300,6364, and [Ca II] at 7291,7324. Na D is present, with a P-Cygni line profile. The [O I] line is of comparable strength to the [Ca II] line. No clear H emission is present in the SN, and while we cannot exclude a weak, broad Halpha contribution, it would have to be significantly weaker than the O or Ca lines. This would suggest that the spectrum is that of a stripped envelope (i.e. Types IIb, Ib or Ic) core-collapse supernova at the nebular phase.
Gaia16amw was first detected by Gaia on 6 May 2016 at a magnitude of 17.8. At the distance of NGC 7339 (20 Mpc), this corresponds to an absolute magnitude of -13.75.
NGC 7339 was last visible from the ground at the start of the night in mid January, and only became re-observable in early May. Gaia observed NGC 7339 on 5th Jan 2016, and did not see any bright source at the position of Gaia16amw on this occasion. It is likely that Gaia16amw exploded soon after this in mid January 2016. If we assume that this was the case, and using the lightcurve of the Type Ic SN 2007gr (Hunter et al. 2009) as a template, then Gaia16amw would have had a peak absolute magnitude of ~-17. The Liverpool spectrum would then be from ~120 days after explosion, at which phase we would expect a Type Ic SN to exhibit a nebular spectrum.
There are pre-explosion HST images available of NGC 7339, however none of these cover the explosion site.
We note that the late detection of Gaia16amw demonstrates that even in nearby galaxies, we may still miss SNe due to an unfortunate combination of fast evolution and observing seasons.
We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts).
