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 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.