  We present the observation of an extraordinary luminous soft X-ray transient,
MAXI J0158-744, by the Monitor of All-sky X-ray Image (MAXI) on 2011 November
11. This transient is characterized by a soft X-ray spectrum, a short duration
(1.3 x 10^3 s < \Delta T_d < 1.10 x 10^4 s), a very rapid rise (< 5.5 x 10^3
s), and a huge peak luminosity of 2 x 10^40 erg s^-1 in 0.7-7.0 keV band. With
Swift observations and optical spectroscopy from the Small and Moderate
Aperture Research Telescope System (SMARTS), we confirmed that the transient is
a nova explosion, on a white dwarf in a binary with a Be star, located near the
Small Magellanic Cloud. An extremely early turn-on of the super-soft X-ray
source (SSS) phase (< 0.44 d), the short SSS phase duration of about one month,
and a 0.92 keV neon emission line found in the third MAXI scan, 1296 s after
the first detection, suggest that the explosion involves a small amount of
ejecta and is produced on an unusually massive O-Ne white dwarf close to, or
possibly over, the Chandrasekhar limit. We propose that the huge luminosity
detected with MAXI was due to the fireball phase, a direct manifestation of the
ignition of the thermonuclear runaway process in a nova explosion.
