Correlation between the luminosity and spin-period changes during outbursts of 12 Be binary pulsars observed by the MAXI/GSC and the Fermi/GBM

Abstract

Abstract To study observationally the spin-period changes of accreting pulsars caused by the accretion torque, the present work analyzes X-ray light curves of 12 Be binary pulsars obtained by the MAXI Gas-Slit Camera all-sky survey and their pulse periods measured by the Fermi Gamma-ray Burst Monitor pulsar project, both covering more than six years, from 2009 August to 2016 March. The 12 objects were selected because they are accompanied by clear optical identification and accurate measurements of surface magnetic fields. The luminosity L and the spin-frequency derivatives $\dot{\nu }$, measured during large outbursts with L ≳ 1 × 1037 erg s−1, were found to follow approximately the theoretical relations in the accretion torque models, represented by $\dot{\nu } \propto L^{\alpha }$ (α ≃ 1), and the coefficient of proportionality between $\dot{\nu }$ and Lα agrees, within a factor of ∼3, with that proposed by Ghosh and Lamb (1979b, ApJ, 234, 296). In the course of the present study, the orbital elements of several sources were refined.