  We present commissioning data from the OSIRIS integral field spectrograph
(IFS) on the Keck II 10 m telescope that demonstrate the utility of adaptive
optics IFS spectroscopy in studying faint close-in sub-stellar companions in
the haloes of bright stars. Our R~2000 J- and H-band spectra of the sub-stellar
companion to the 1-10 Myr-old GQ Lup complement existing K-band spectra and
photometry, and improve on the original estimate of its spectral type. We find
that GQ Lup B is somewhat hotter (M6-L0) than reported in the discovery paper
by Neuhauser and collaborators (M9-L4), mainly due to the surface-gravity
sensitivity of the K-band spectral classification indices used by the
discoverers. Spectroscopic features characteristic of low surface gravity
objects, such as lack of alkali absorption and a triangular H-band continuum,
are indeed prominent in our spectrum of GQ Lup B. The peculiar shape of the
H-band continuum and the difference between the two spectral type estimates is
well explained in the context of the diminishing strength of H2 collision
induced absorption with decreasing surface gravity, as recently proposed for
young ultra-cool dwarfs by Kirkpatrick and collaborators. Using our updated
spectroscopic classification of GQ Lup B and a re-evaluation of the age and
heliocentric distance of the primary, we perform a comparative analysis of the
available sub-stellar evolutionary models to estimate the mass of the
companion. We find that the mass of GQ Lup B is 0.010-0.040 Msun. Hence, it is
unlikely to be a wide-orbit counterpart to the known radial-velocity extrasolar
planets, whose masses are < 0.015 Msun. Instead, GQ Lup A/B is probably a
member of a growing family of very low mass ratio widely separated binaries
discovered through high-contrast imaging.
