RXTE Observations of Positive Correlations between the Cyclotron Line Parameters and Luminosity in GX 304-1

The Rossi X-ray Timing Explorer observed four outbursts of the accreting X-ray binary transient source GX 304-1 in 2010 and 2011. During the 2010-2011 observations, the HEXTE cluster A viewing direction was fixed aligned with the PCA field of view and HEXTE cluster B was fixed viewing a background region 1.5 degrees off of the source direction. The cluster A background was successfully estimated from cluster B events, and this made possible the measurement of the ~55 keV cyclotron line and an accurate measurement of the continuum. The cyclotron line energy spans 50 to 60 keV throughout each outburst, implying magnetic fields ranging from 4-5 teraGauss as the scattering region reacts to the varying mass accretion rate. We present results of a detailed 3-100 keV spectral analysis of 69 separate observations, and report a greater than 7 sigma measurement of a positive correlation between cyclotron line parameters (energy, width, and depth) and luminosity, as well as other spectral parameters' correlations with luminosity. The three cyclotron line parameters’ correlations with luminosity show a flattening of the relationships with increasing luminosity, and have been fitted by quasi-spherical accretion and disk accretion models. The width and depth correlation exponents follow directly from the energy correlation exponent with only the assumption that the accretion column is in the subcritical (Coulomb-braking) regime and the energy changes in proportion to the characteristic stopping length of protons. Correlations of all spectral parameters with primary 2-10 keV power law flux reveal the mass accretion rate to be the primary driver of the spectral shape. A large enhancement in the line of sight column density lasting about three days is seen just before periastron in one outburst and a smaller enhancement of similar duration at the same orbital phase is seen in a second outburst, suggesting the presence of a dense structure in the stellar wind.