We report the discovery of a cyclotron resonance scattering feature ( CRSF ) in the X-ray spectrum of GX 304 - 1 , obtained by RXTE and Suzaku during major outbursts detected by MAXI in 2010 .
The peak intensity in August reached 600 mCrab in the 2–20 keV band , which is the highest ever observed from this source .
The RXTE observations on more than twenty occasions and one Suzaku observation revealed a spectral absorption feature at around 54 keV , which is the first CRSF detection from this source .
The estimated strength of surface magnetic field , 4.7 \times 10 ^ { 12 } G , is one of the highest among binary X-ray pulsars from which CRSFs have ever been detected .
The RXTE spectra taken during the August outburst also suggest that the CRSF energy changed over 50–54 keV , possibly in a positive correlation with the X-ray flux .
The behavior is qualitatively similar to that observed from Her X-1 on long time scales , or from A 0535 + 26 , but different from the negative correlation observed from 4U 0115+63 and X 0331+53 .