Search for Soft X-ray Flashes at Fireball Phase of Classical/Recurrent   Novae using MAXI/GSC data
  We searched for precursive soft X-ray flashes (SXFs) associated with optically-discovered classical or recurrent novae in the data of five-years all-sky observations with   Gas Slit Camera (GSC) of the Monitor of All-sky X-ray Image (MAXI). We first developed a tool to measure fluxes of point sources by fitting the event distribution with the model that incorporates the point-spread function (PSF-fit) to minimize the potential contamination from nearby sources. Then we applied the PSF-fit tool to 40 classical/recurrent novae that were discovered in optical observations from 2009 August to 2014 August. We found no precursive SXFs with significance above $3 \sigma$ level in the energy range of 2$-$4 keV between $t_{d}-10$d and $t_{d}$, where $t_{d}$ is the date when each nova was discovered. We obtained the upper limits for the bolometric luminosity of SXFs, and compared them with the theoretical prediction and that observed for MAXI J0158$-$744. This result could constrain the population of massive white dwarfs with a mass of roughly 1.40 solar mass, or larger, in binary systems. 
Mikio Morii, Hitoshi Yamaoka, Tatehiro Mihara, Masaru Matsuoka and   Nobuyuki Kawai
astro-ph.HE