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

摘要

A report is made on the luminosity and pulse-period evolution of the Bebinary X-ray pulsar, GX 304$-$1, during a series of outbursts from 2009 to 2013observed by MAXI/GSC, RXTE/PCA, and Fermi/GBM. In total, twelve outburstsrepeated by $\sim$ 132.2 days were observed, which is consistent with the X-rayperiodicity of this object observed in the 1970s. These 12 outbursts, togetherwith those in the 1970s, were found to all recur with a well defined period of132.189$\pm$0.02 d, which can be identified with the orbital period. The pulseperiod at $\sim 275$ s, obtained from the RXTE/PCA and Fermi/GBM data,apparently exhibited a periodic modulation synchronized with the outburstperiod, suggesting the pulsar orbital motion, which is superposed on a secularspin-up trend throughout the entire active phase. The observed pulse-periodchanges were successfully represented by a model composed of the binary orbitalmodulation and pulsar spin up caused by mass accretion through an accretiondisk. The orbital elements obtained from the best-fit model, including theprojected orbital semi-major axis $a_{\rm x}\sin i \simeq 500-600$ light-s andan eccentricity $e \simeq 0.5$, are typical of Be binary X-ray pulsars.