X-ray observations of the slowest known Be/X-ray pulsars RX J0146.9+6121 and X Persei

Abstract

We present ASCA and ROSAT observations of the Be/X-ray binary pulsars RX J0146.9+6121 and X Persei between 1990 and 1996. Measuring the neutron star spin period and X-ray luminosity of X Persei shows that the episode of low X-ray luminosity and monotonic spin-down since 1978 continued to at least August 1995. ASCA and ROSAT HRI observations of RX J0146.9+6121 detected the pulsar with the longest known neutron star spin period at low luminosities after the X-ray outburst seen by EXOSAT in August 1984. The large spin period decrease seen between 1984 and 1993 has nearly stopped at a period of 1407.3 s in February 1996. We discuss that the X-ray outburst behaviour of X Persei with respect to its Be star phase changes observed in the optical can be caused by an asymmetry in the matter distribution around the Be star.