  The RXTE observed four outbursts of the accreting X-ray binary transient
source, GX 304-1 in 2010 and 2011. We present results of detailed 3-100 keV
spectral analysis of 69 separate observations, and report a positive
correlation between cyclotron line parameters, as well as other spectral
parameters, with power law flux. The cyclotron line energy, width and depth
versus flux, and thus luminosity, correlations show a flattening of the
relationships with increasing luminosity, which are well described by
quasi-spherical or disk accretion that yield the surface magnetic field to be
~60 keV. Since HEXTE cluster A was fixed aligned with the PCA field of view and
cluster B was fixed viewing a background region 1.5 degrees off of the source
direction during these observations near the end of the RXTE mission, the
cluster A background was estimated from cluster B events using HEXTEBACKEST.
This made possible the detection of the ~55 keV cyclotron line and an accurate
measurement of the continuum. Correlations of all spectral parameters with the
primary 2-10 keV power law flux reveal it to be the primary driver of the
spectral shape. The accretion is found to be in the collisionless shock braking
regime.

High Energy Astrophysical Phenomena