TITLE: GCN CIRCULAR NUMBER: 26508 SUBJECT: LIGO/Virgo S191213g: LBT spectrum of AT2019wxt (PS19hgw) reveals a type IIb supernova DATE: 19/12/20 04:30:21 GMT FROM: Peter Garnavich at U of Notre Dame P. Vallely (OSU), C. M. Wood (ND), C. Phillips (OSU), R. M. Wagner (LBTO and OSU), P. Garnavich (ND), P. Kelly (Minnesota), C. Kochanek (OSU), J. Neustadt (OSU), K. Stanek (OSU), D. Thompson (LBTO), T. Thompson (OSU), C. E. Woodward (Minnesota) We obtained an optical spectrum on 2019 December 19.238 UT of the optical transient AT2019wxt discovered by Pan-STARRS1 (GCN 26485) that might be associated with the BNS merger event S191213g (GCN 26402). The observations were performed using the 2 x 8.4 m Large Binocular Telescope (LBT) and the Multi-Object Double Spectrograph (MODS1 and MODS2; Pogge et al. 2010, SPIE, 7335, 9). The spectra cover the wavelength range 3200 to 9750 Ang at a spectral resolution of about 4 Ang. Our spectrum shows narrow nebular emission line arising from H-alpha, H-beta, [N II] 6548, 6584 Ang; [S II] 6717, 6731 Ang; [O III] 4959, 5007 Ang; and [O II] 3727 Ang superposed on a blue continuum. The wavelengths of the nebular emission lines are consistent with the z = 0.036 redshift of the host galaxy. In addition, broad lines arising from He I 7065 Ang and He I 5875 Ang are present, confirming the detection by Vogl et al. (GCN 26504). The displacement of the weak absorption troughs is difficult to measure but is about -7500 km/s. Our spectrum is in substantial agreement with those described by Vogl et al. (GCN 26504), Dutta et al. (GCN 26490), Izzo et al. (GCN 26491), Srivastav and Smartt (GCN 26493), and Muller Bravo et al. (GCN 26494). Vogl et al. described the spectrum of AT2019wxt as similar to a young type Ib supernova on the basis of the presence of broad He I features but also noted that it was similar to some type IIb supernovae such as SN 2011fu that also show a blue continuum. Our spectrum of AT2019wxt is very similar to the early spectra of SN 2011fu from Kumar et al. (2013, MNRAS, 431, 308, Fig 7). If the spectral evolution of AT2019wxt is similar to 2011fu, the He I lines should strengthen over the next few weeks. Additional optical spectroscopy of AT2019wxt is encouraged.