  We present one of the most ultraviolet (UV) luminous Lyman Break Galaxies
(LBGs) (J1432+3358) at z=2.78, discovered in the NOAO Deep Wide-Field Survey
(NDWFS) Bootes field. The R-band magnitude of J1432+3358 is 22.29 AB, more than
two magnitudes brighter than typical L* LBGs at this redshift. The deep z-band
image reveals two components of J1432+3358 separated by 1.0" with flux ratio of
3:1. The high signal-to-noise ratio (S/N) rest-frame UV spectrum shows Lya
emission line and interstellar medium absorption lines. The absence of NV and
CIV emission lines, the non-detection in X-ray and radio wavelengths and
mid-infrared (MIR) colors indicate no or weak active galactic nuclei (AGN)
(<10%) in this galaxy. The galaxy shows broader line profile with the full
width half maximum (FWHM) of about 1000 km/s and larger outflow velocity (~500
km/s) than those of typical z~3 LBGs. The physical properties are derived by
fitting the spectral energy distribution (SED) with stellar synthesis models.
The dust extinction, E(B-V)=0.12, is similar to that in normal LBGs. The star
formation rates (SFRs) derived from the SED fitting and the dust-corrected UV
flux are consistent with each other, ~300 Msun/yr, and the stellar mass is
1.3e11 Msun. The SFR and stellar mass in J1432+3358 are about an order of
magnitude higher than those in normal LBGs. The SED-fitting results support
that J1432+3358 has a continuous star formation history with the star formation
episode of 630 Myr. The morphology of J1432+3358 and its physical properties
suggest that J1432+3358 is in an early phase of 3:1 merger process. The unique
properties and the low space number density (~1e-7 Mpc^{-3})are consistent with
the interpretation that such galaxies are either found in a short unobscured
phase of the star formation or that small fraction of intensive star-forming
galaxies are unobscured.
