Outburst of GX 304-1 monitored with INTEGRAL: positive correlation
  between the cyclotron line energy and flux
  X-ray spectra of many accreting pulsars exhibit significant variations as a
function of flux and thus of mass accretion rate. In some of these pulsars, the
centroid energy of the cyclotron line(s), which characterizes the magnetic
field strength at the site of the X-ray emission, has been found to vary
systematically with flux. GX 304-1 is a recently established cyclotron line
source with a line energy around 50 keV. Since 2009, the pulsar shows regular
outbursts with the peak flux exceeding one Crab. We analyze the INTEGRAL
observations of the source during its outburst in January-February 2012. The
observations covered almost the entire outburst, allowing us to measure the
source's broad-band X-ray spectrum at different flux levels. We report on the
variations in the spectral parameters with luminosity and focus on the
variations in the cyclotron line. The centroid energy of the line is found to
be positively correlated with the luminosity. We interpret this result as a
manifestation of the local sub-Eddington (sub-critical) accretion regime
operating in the source.
