Correlation Between Luminosity and Accretion Torque in 4U 1626-67 and GX 301-2

Abstract

We present X-ray light curves and energy spectra for the persistent accreting pulsars 4U 1626-67 and GX 301-2 measured by the All-Sky Monitor (ASM) on Ginga from 1987 March - 1991 October. We compare these with simultaneous and near simultaneous measurements of spin frequency and flux by other instruments, principally the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). A dramatic change in the shape of the X-ray spectrum and a 20% decrease in the 1-20 keV X-ray flux accompany the 1990 transition from steady spin up to steady spin down in 4U 1626-67. The Ginga ASM is the only instrument to observe 4U 1626-67 during both spin up and spin down. We show that the distance to 4U 1626-67 is greater than 5 kpc. If 4U 1626-67 is a near-equilibrium rotator and if the 0.04 Hz Quasi-Period Oscillations seen during spin up are magnetospheric beat-frequency oscillations, then the distance to the source is 5 kpc, assuming a neutron-star mass of 1.4 solar masses, radius 10 km, and moment of inertia 10^45 g cm^2. The X-ray flux of GX 301-2 measured with the ASM varies with orbital phase. The flux peaks shortly before periastron, with a secondary maximum near apastron. Such variations were seen previously in the 20-50 keV pulsed flux with BATSE. The ASM observations confirm that the 20-50 keV pulsed flux in GX 301-2 is a good tracer of the bolometric flux. The X-ray flux in GX 301-2 was a factor of 2 larger than average during the periastron passage prior to an episode of persistent spin up in 1991 July observed with BATSE that lasted half an orbit and resembled outbursts seen in transient X-ray pulsars.