  Our goal is to investigate the X-ray timing and spectral variability of the
high-mass X-ray binary 1A 1118-615 during a type-II outburst. We performed a
detailed color, spectral and timing analysis of a giant outburst from 1A
1118-615, using RXTE data. Results. We report the discovery of a variable
quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615,
with a centroid frequency of ~0.08 Hz. The centroid frequency of the QPO
correlates with the X-ray flux, as expected according to the most accredited
models for QPO production. For energies above ~4 keV, the QPO rms variability
decreases as the energy increases. Pulse profiles display energy dependence,
with a two-peak profile at lower energies, and a single peak at higher
energies. From spectral analysis, we confirm the presence of a cyclotron
absorption feature at ~60 keV, the highest value measured for an X-ray pulsar.
We find that the spectral parameters (photon index, cutoff energy, iron
fluorescence line strength) display a marked dependence with flux. We detect
two different levels of neutral hydrogen column density, possibly due to the Be
companion activity. We report for the first time a correlation between the
timing and spectral parameters in an X-ray pulsar. All the correlations found
between spectral/timing parameters and X-ray flux are present up to a flux of
~6x10^-9 erg cm^-2 s^-1, when a saturation level is reached. We propose that
the saturation observed corresponds to the minimum extent of the neutron star
magnetosphere. We estimate the magnetic field of the neutron star from two
independent ways, using results from spectral (cyclotron line energy) and
timing (QPO frequency) analysis, obtaining consistent values, of ~7-8x10^12 G.
Results from the comprehensive spectral and timing analysis are discussed in
comparison with other X-ray pulsars.
