  We report the first half-year monitoring of the new Galactic black hole
candidate MAXI J1348-630, discovered on 2019 January 26 with the Gas Slit
Camera (GSC) on-board MAXI. During the monitoring period, the source exhibited
two outburst peaks, where the first peak flux (at T=14 day from the discovery
of T =0) was ~4 Crab (2-20 keV) and the second one (at T =132 day) was ~0.4
Crab (2-20 keV). The source exhibited distinct spectral transitions between the
high/soft and low/hard states and an apparent "q"-shape curve on the
hardness-intensity diagram, both of which are well-known characteristics of
black hole binaries. Compared to other bright black hole transients, MAXI
J1348-630 is characterized by its low disk-temperature (~0.75 keV at the
maximum) and high peak flux in the high/soft state. The low peak-temperature
leads to a large innermost radius that is identified as the Innermost Stable
Circular Orbit (ISCO), determined by the black hole mass and spin. Assuming the
empirical relation between the soft-to-hard transition luminosity (Ltrans) and
the Eddington luminosity (LEdd), Ltrans/LEdd ~ 0.02, and a face-on disk around
a non-spinning black hole, the source distance and the black hole mass are
estimated to be D ~ 4 kpc and ~7 (D/4 kpc) Mo, respectively. The black hole is
more massive if the disk is inclined and the black hole is spinning. These
results suggest that MAXI J1348-630 may host a relatively massive black hole
among the known black hole binaries in our Galaxy.
