MASTER OT J030227.28+191754.5: a dwarf nova at a massive oxygen-neon white-dwarf system ?

We present timing and spectral analysis results of the {\it NICER} and {\it NuSTAR} observations of the dwarf nova MASTER OT J030227.28$+$191754.5 during the 2021--2022 outburst. The soft X-ray component was found to be dominated by blackbody radiation with a temperature of $\sim$30 eV and also showed prominent oxygen and neon emission lines. The blackbody luminosity exceeded 10$^{34}$ ergs s$^{-1}$, which is consistent with theoretical predictions, and then decreased more than an order of magnitude in 3.5 days. The inferred abundances of oxygen and neon in the optically-thin coronal region surrounding the central white dwarf (WD) are several times higher than the respective solar values. Although inconclusive, the abundance enrichment may originate from the WD, indicating that it may be mainly composed of oxygen and neon. Assuming that the blackbody radiation comes from the belt-shaped boundary layer between the WD and the accretion disk, we estimated the WD radius to be $(2.9\pm1.1)\times10^{8}$ cm, which corresponds to the WD mass range of 1.15--1.34 $M_{\odot}$. If the accretion continues for another $\sim$Gyr, the WD may experience an accretion-induced collapse into a neutron star and form a so-called black-widow pulsar system.