------------------------------------------------------------------- THE ASTRONOMER Electronic Circular No 3168 2016 May 18 20.16UT Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise, Basingstoke, Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074 INTERNET: GUY@TAHQ.DEMON.CO.UK Backup: gmh@wdcc1.bnsc.rl.ac.uk WORLD WIDE WEB http://www.theastronomer.org ------------------------------------------------------------------- GAIA16AMW, PROBABLE MISSED SUPERNOVA IN NGC 7339 M. Fraser et. al. (Warsaw/SRON) report on ATEL 9061 that they obtained a low resolution spectrum of Gaia16amw using the robotic Liverpool Telescope + SPRAT 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 they 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. The quoted position is: RA 22h 37m 45.4s DEC +23 47' 14.0" (2000) 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 2016 Jan 5, 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 it is assumed that this was the case, and using the light curve of the Type Ic SN 2007gr 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. They comment that the late detection of Gaia16amw demonstrates that even in nearby galaxies, supernovae may be missed due to an unfortunate combination of fast evolution and observing seasons. They acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts) Guy M Hurst