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We present optical, radio and X-ray observations of a rapidly-evolvingtransient AT2019wxt (PS19hgw), discovered during the search for anelectromagnetic (EM) counterpart to the gravitational-wave (GW) triggerS191213g (LIGO Scientific Collaboration & Virgo Collaboration 2019a). AlthoughS191213g was not confirmed as a significant GW event in the off-line analysisof LIGO-Virgo data, AT2019wxt remained an interesting transient due itspeculiar nature. The optical/NIR light curve of AT2019wxt displayed adouble-peaked structure evolving rapidly in a manner analogous to currentlyknow ultra-stripped supernovae (USSNe) candidates. This double-peaked structuresuggests presence of an extended envelope around the progenitor, best modelledwith two-components: i) early-time shock-cooling emission and ii) late-timeradioactive $^{56}$Ni decay. We constrain the ejecta mass of AT2019wxt at$M_{ej} \approx{0.20 M_{\odot}}$ which indicates a significantly strippedprogenitor that was possibly in a binary system. We also followed-up AT2019wxtwith long-term Chandra and Jansky Very Large Array observations spanning$\sim$260 days. We detected no definitive counterparts at the location ofAT2019wxt in these long-term X-ray and radio observational campaigns. Weestablish 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 radioflux over the course of our observations. A substantial host galaxycontribution to the measured radio flux is likely. The discovery and early-timepeak capture of AT2019wxt in optical/NIR observation during EMGW follow-upobservations highlights the need of dedicated early, multi-band photometricobservations to identify USSNe.

AT2019wxt: An ultra-stripped supernova candidate discovered in electromagnetic follow-up of a gravitational wave trigger