  We present both timing and spectral analysis of the outburst of 4U 0115+63 in
April -- May 2008 with INTEGRAL and RXTE observations. We have determined the
spin period of the neutron star at $\sim 3.61430 \pm 0.00003$ s, and a spin up
rate during the outburst of $\dot{P}=(-7.24 \pm 0.03)\times10^{-6} {\rm s
d^{-1}}$, the angle of periapsis $\omega=48.67^\circ \pm 0.04^\circ$ in 2008
and its variation (apsidal motion) $\dot{\omega} = 0.048^\circ \pm 0.003^\circ
{\rm yr}^{-1}$. We also confirm the relation of spin-up torque versus
luminosity in this source during the giant outburst. The hard X-ray spectral
properties of 4U 0115+63 during the outburst are studied with INTEGRAL and
RXTE. Four cyclotron absorption lines are detected using the spectra from
combined data of IBIS and JEM-X aboard INTEGRAL in the energy range of 3 -- 100
keV. The 5 -- 50 keV luminosities at an assumed distance of 7 kpc are
determined to be in the range of $(1.5-12)\times 10^{37} {\rm ergs s^{-1}}$
during the outburst. The fundamental absorption line energy varies during the
outburst: around 15 keV during the rising phase, and transiting to $\sim 10$
keV during the peak of the outburst, and further coming back to $\sim 15$ keV
during the decreasing phase. The variations of photon index show the
correlation with the fundamental line energy changes: the source becomes harder
around the peak of the outburst and softer in both rising and decreasing
phases. This correlation and transition processes during the outburst need
further studies in both observations and theoretical work. The known relation
of the fundamental line energy and X-ray luminosity is confirmed by our
results, however, our discoveries suggest that some other factors besides
luminosity play the important role in fundamental line energy variations and
spectral transitions.
