  We present optical, radio and X-ray observations of a rapidly-evolving
transient AT2019wxt (PS19hgw), discovered during the search for an
electromagnetic (EM) counterpart to the gravitational-wave (GW) trigger
S191213g (LIGO Scientific Collaboration & Virgo Collaboration 2019a). Although
S191213g was not confirmed as a significant GW event in the off-line analysis
of LIGO-Virgo data, AT2019wxt remained an interesting transient due its
peculiar nature. The optical/NIR light curve of AT2019wxt displayed a
double-peaked structure evolving rapidly in a manner analogous to currently
know ultra-stripped supernovae (USSNe) candidates. This double-peaked structure
suggests presence of an extended envelope around the progenitor, best modelled
with two-components: i) early-time shock-cooling emission and ii) late-time
radioactive $^{56}$Ni decay. We constrain the ejecta mass of AT2019wxt at
$M_{ej} \approx{0.20 M_{\odot}}$ which indicates a significantly stripped
progenitor that was possibly in a binary system. We also followed-up AT2019wxt
with long-term Chandra and Jansky Very Large Array observations spanning
$\sim$260 days. We detected no definitive counterparts at the location of
AT2019wxt in these long-term X-ray and radio observational campaigns. We
establish the X-ray upper limit at $9.93\times10^{-17}$ erg cm$^{-2}$ s$^{-1}$
and detect an excess radio emission from the region of AT2019wxt. However,
there is little evidence for SN1993J- or GW170817-like variability of the radio
flux over the course of our observations. A substantial host galaxy
contribution to the measured radio flux is likely. The discovery and early-time
peak capture of AT2019wxt in optical/NIR observation during EMGW follow-up
observations highlights the need of dedicated early, multi-band photometric
observations to identify USSNe.
