In early 2008, the young low-mass star EX Lupi, the prototype of the EXor
class of eruptive variables, optically brightened by over five magnitudes for a
period of 7 months. The previous time a change of such amplitude had been
observed in EX Lup was over 50 years ago. In this Letter we present new optical
and near-IR high resolution spectroscopy of EX~Lup during the 2008 outburst. We
investigate the physical characteristics of the outburst both soon after it
began and some four months later, and consider the energetics and kinematics
observed. Emission line strengths, widths, and profiles changed significantly
between the two observations. Also, modeling of the 2.2935 um CO overtone
bandhead emission suggests that an inner gap in the circumstellar gas disk
around the star may be present and it is from the inner edge of the gas disk
that the CO overtone emission probably arises. We derive a mass accretion
luminosity and rate during the extreme outburst of ~2+-0.5~Lsun and
~2+-0.5x10^-7 Msun yr^-1, respectively, which suggests that this outburst was
indeed one of the strongest witnessed in EX Lup, yet not as strong as those
observed in FU Orionis stars.Comment: 5 pages, 4 figure
