TITLE:   GCN CIRCULAR
NUMBER:  20721
SUBJECT: LIGO/Virgo G274296:  PS17bek is a superluminous supernova at z=0.31
DATE:    17/02/22 12:51:03 GMT
FROM:    Giorgos Leloudas at Weizmann Institute of Science  <giorgos@dark-cosmology.dk>

A. Gal-Yam, G. Leloudas, P. Vreeswijk (Weizmann), K. C. Chambers
(IfA), M. E. Huber (IfA), S. J. Smartt, K. W. Smith, D. Wright,
D. Young (QUB), F. Taddia, C. Barbarino, A. Nyholm, C. Fremling,
J. Sollerman (OKC), M. T. Botticella (INAF-Capodimonte), M. Fraser
(UCD), C. Inserra, E. Kankare, K. Maguire, M. Sullivan (Southampton),
S. Valenti (UC Davis), O. Yaron, I. Manulis (Weizmann), E. Cappellaro
(INAF-Padova),, M. Coughlin (Harvard), T.-W. Chen (MPE), L. Denneau,
H. Flewelling, A. Heinze, T. Lowe, E. A. Magnier (IfA), A. Rest
(STScI), B. Stalder, A. S. B. Schultz (IfA), C. W. Stubbs (Harvard)
J. Tonry, C. Waters, R. J. Wainscoat, H. Weiland, M. Willman (IfA) report:


We have reanalysed the spectrum of PS17bek obtained by PESSTO (Taddia
et al. GCN 20708), an object in the sky map of G274296.  The spectrum
is blue but shows weak broad features and a weak emission line that
was previously identified with Halpha at z=0. The object was therefore
thought to be Galactic.

Cross-correlating the spectrum with supernova template spectra in SNID
(Blondin & Tonry, 2007), we find a good match to the spectra of
super-luminous supernovae (SLSNe type I).  In particular, we find a
good match with the spectra of SN 2010gx at -5 days before peak if
PS17bek is at a redshift of z~0.31. In this context, the weak emission
line at 6559.4 is consistent with [O III] 5007 at z = 0.31, and we
also detect [O III] 4959 at a consistent redshift but lower
significance. The [O III] lines are typically the strongest emission
lines in SLSN dwarf, starforming hosts (Leloudas et al. 2015).  There
is a faint, uncatalogued source in the Pan-STARRS1 3Pi r-band image
which is likely the host at r~23.5 (Chambers et al. arXiv:1612.05560).

We therefore classify PS17bek as a SLSN I at z = 0.31. 
The Pan-STARRS lightcurve is flat at i=19.8 over two days : 
MJD             mag     filter 
57802.30629	19.85	i	
57804.58218	19.81	i

This indicates that the object is likely close to maximum light
and has a rest frame absolute magnitude of M_r ~ -21.0.

Although these superluminous events are relatively rare by sky
density, they have rise times from explosion to peak of around 20-40
days (e.g. Nicholl et al. 2015). Therefore the explosion probably
occurred at least 20 days before the GW detection of G274296
(57801.254 ; Shawhan et al. GCN20689) and is likely unrelated.