[vsnet-grb-info 24405] LIGO/Virgo S191213g: VLT spectrum of AT2019wxt (PS19hgw) - a probable Ib supernova

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26504
SUBJECT: LIGO/Virgo S191213g: VLT spectrum of AT2019wxt (PS19hgw) - a probable Ib supernova
DATE:    19/12/19 17:23:41 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt at qub.ac.uk>

C. Vogl, A. Floers, S. Taubenberger, W. Hillebrandt, S. Suyu (MPA
Garching), R. Kotak (Turku), S. J. Smartt, M. Dobson (QUB), B.
Leibundgut, J. Spyromilio (ESO), R. Bruch, A. Gal-Yam (Weizmann)

The adH0cc programme for an "accurate determination of H0 with
core-collapse supernovae" reports the classification of the optical
transient AT019wxt (PS19hgw - McLaughlin et al. AstroNote 2019-154 and
McBrien et al., GCN 26485). 

We selected this target as a possible young type II, at the right
magnitude in our target redshift range for II-P SNe in the Hubble flow
(irrespective of its position in the s191213g sky map; LVC GCN 26402).
We generally report our classifications in IAU AstroNotes, where the
classification spectra can be found - see Vogel et al. (AstroNote
2019-157) and repeat the information here for the GW follow-up
community.

Observations were performed on the night of 2019 December 18 with
the ESO Very Large Telescope UT1, equipped with the FORS2 spectrograph
and grism 300V, covering a wavelength range from 3400 to 9600A at a
resolution of 10A.

The FORS2 spectrum of SN 2019wxt is dominated by a blue continuum (see
also Dutta et al., GCN 26490; Izzo et al., GCN 26491; Srivastav &
Smartt, GCN 26493; Muller Bravo et al., GCN 26294 and AstroNote
2019-156), with a few broad undulations and with narrow H II-region
lines superimposed. From the narrow emission lines, we derive a
redshift of z = 0.036, in agreement with the spectroscopic redshift of
the host galaxy.

The shallow, broad features can be attributed to helium lines, with
the 5876A and 7065A lines of He I providing the cleanest
identifications, but plausible identifications of 4471, 4922 and 6678
are visible. The velocity of minima of the absorption troughs appears
to be between 7,000- 10,000 km/s.

Broad hydrogen lines are not unambiguously detected. Based on these
properties, we classify SN 2019wxt as a young supernova of type Ib,
but note similarities to some SNe IIb which show a hot blue continuum
such as SN 2011fu at comparably early phases.