Luminosity and spin-period evolution of GX 304−1 during outbursts from 2009 to 2013 observed with the MAXI/GSC, RXTE/PCA, and Fermi/GBM

A report is made on the luminosity and pulse period evolution of the Be binary X-ray pulsar GX 304−1 during a series of outbursts from 2009 to 2013 observed by MAXI/GSC, RXTE/PCA, and Fermi/GBM. In total, 12 outbursts repeated by ∼ 132.2 d were observed, which is consistent with the X-ray periodicity of this object observed in the 1970s. These 12 outbursts, together with those in the 1970s, were all found to recur with a well-defined period of 132.189 ± 0.02 d, which can be identified with the orbital period. The pulse period of ∼ 275 s, obtained from the RXTE/PCA and Fermi/GBM data, apparently exhibited a periodic modulation synchronized with the outburst period, suggesting the pulsar orbital motion, which is superposed on a secular spin-up trend throughout the entire active phase. The observed pulse period changes were successfully represented by a model composed of the binary orbital modulation and pulsar spin up caused by mass accretion through an accretion disk. The orbital elements obtained from the best-fit model, including the projected orbital semi-major axis a x sin i ≃ 500–600 light-s and an eccentricity e ≃ 0.5, are typical of Be binary X-ray pulsars.