Discovery of a Cyclotron Resonance Feature in the X-ray Spectrum of GX 304-1 with RXTE and Suzaku during Outbursts Detected by MAXI in 2010

TL;DR

This paper reports the first detection of a cyclotron resonance scattering feature in GX 304-1, revealing a high magnetic field and flux-dependent spectral variation during 2010 outbursts observed by RXTE and Suzaku.

Contribution

It presents the discovery of a CRSF in GX 304-1, measuring its magnetic field and analyzing its flux-dependent energy variation, which is novel for this source.

Findings

CRSF detected at ~54 keV in GX 304-1

Surface magnetic field estimated at 4.7 x 10^12 G

CRSF energy varies with X-ray flux, showing positive correlation

Abstract

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.