  We report on a long-term monitoring of a newly discovered X-ray nova, MAXI
J1910-057 (= Swift J1910.2-0546), by MAXI and Swift. The new X-ray transient
was first detected on 2012 May 31 by MAXI Gas Slit Camera (GSC) and Swift Burst
Alert Telescope (BAT) almost simultaneously. We analyzed X-ray and UV data for
270 days since the outburst onset taken by repeated MAXI scans and Swift
pointing observations. The obtained X-ray light curve for the inital 90 days is
roughly represented by a fast-rise and exponential-decay profile. However, it
re-brightened on the ~110 days after the onset and finally went down below both
GSC and BAT detec- tion limits on the 240 day. All the X-ray energy spectra are
fitted well with a model consisting of a multi-color-disk blackbody and its
Comptonized hard tail. During the soft-state periods, the inner-disk radius of
the best-fit model were almost constant. If the radius represents the innermost
stable circular orbit of a non-spinning black hole and the soft-to-hard
transitions occur at 1-4% of the Eddington luminosity, the mass of the compact
object is estimated to be > 2.9Mo and the distance to be > 1.70 kpc. The
inner-disk radius became larger in the hard / hard-intermediate state. This
suggests that the accretion disk would be truncated. We detected an excess of
the UV flux over the disk blackbody component extrapolated from the X-ray data,
which can be modelled as reprocessed emission irradiated by the inner disk. We
also found that the UV light curve mostly traced the X-ray curve, but a short
dipping event was observed in both the UV and the X-ray bands with a 3.5-day
X-ray time lag. This can be interpreted as the radial inflow of accreting
matter from the outer UV region to the inner X-ray region.
