[vsnet-grb-info 24409] LIGO/Virgo S191213g: LBT spectrum of AT2019wxt (PS19hgw) reveals a type IIb supernova

[GCN Circulars]

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  <pgarnavi at nd.edu>

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.