RXTE Observations of the Be Star X-Ray Transient X0726-260 (4U 0728-25): Orbital and Pulse Periods

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of the transient Be star X-ray source X0726-260 suggest a 34.5 day period. This is apparently confirmed by a serendipitous RXTE Proportional Counter Array (PCA) slew detection of the source on 1997 May 5, near the time of a predicted flux maximum. A subsequent 5 ks pointed observation of X0726-260 with the RXTE PCA detector was carried out on 1997 June 7, when X0726-260 was predicted to be bright again, and this revealed pulsations at a period of 103.2 s. If the 34.5 day period is orbital, then the pulse period is surprisingly long compared with that predicted by the correlation between orbital period and spin period observed for other Be/neutron star systems. A possible similarity with GRO J2058+42 is briefly discussed.