The 2009 Fermi Symposium

New Torque Reversal of the Ultracompact Binary System 4U1626-67 Discovered By Fermi-GBM

Ascension Camero-Arranz

Abstract:

After about 18 years of steadily spinning down, the accretion-powered pulsar 4U 1626-67, experienced a torque reversal at the beginning of 2008. We have used all available Fermi/GBM data since its launch in 2008 June 11 and over 5 yr of hard X-ray Swift/BAT observations (since 2004 October) to trace in detail the evolution of the source spin frequency. This second detected torque reversal was centered near MJD 54500 (2008 Feb 4) and it lasted approximately 150 days with a simultaneous increased of the flux (a ~ 2.5 factor). The spin-up and spin-down torques were again almost identical in magnitude before and after the transition (~ 5 x 10^-13 Hz s^-1). As in the 1990 reversal, the lack of correlation between the X-ray flux and the torque applied to the neutron star challenges our understanding of the physical mechanisms operating in this system.