  We report on an outburst of the high mass X-ray binary 4U 0115+63 with a
pulse period of 3.6s in 2008 March/April as observed with RXTE and INTEGRAL.
During the outburst the neutron star's luminosity varied by a factor of 10 in
the 3--50\,keV band. In agreement with earlier work we find evidence for five
cyclotron resonance scattering features at ~10.7, 21.8, 35.5, 46.7, and
59.7keV. Previous work had found an anti-correlation between the fundamental
cyclotron line energy and the X-ray flux. We show that this apparent
anti-correlation is probably due to the unphysical interplay of parameters of
the cyclotron line with the continuum models used previously, e.g., the
negative and positive exponent power law (NPEX). For this model, we show that
cyclotron line modeling erroneously leads to describing part of the exponential
cutoff and the continuum variability, and not the cyclotron lines. When the
X-ray continuum is modeled with a simple exponentially cutoff power law
modified by a Gaussian emission feature around 10keV, the correlation between
the line energy and the flux vanishes and the line parameters remain virtually
constant over the outburst. We therefore conclude that the previously reported
anti-correlation is an artifact of the assumptions adopted in the modeling of
the continuum.
