Evidence for a Very Slow X‐Ray Pulsar in 2S 0114+650 fromRossi X‐Ray Timing ExplorerAll‐Sky Monitor Observations

Abstract

Rossi X-Ray Timing Explorer (RXTE) all-sky monitor (ASM) observations of the X-ray binary 2S 0114+650 show modulations at periods close to both the optically derived orbital period (11.591 days) and proposed pulse period (~2.7 hr). The pulse period shows frequency and intensity variability during the more than 2 years of ASM observations analyzed. The pulse properties are consistent with this arising from accretion onto a rotating neutron star, and this would be the slowest such period known. The shape of the orbital light curve shows modulation over the course of the entire orbit, and a comparison is made with the orbital light curve of Vela X-1. However, the expected phase of eclipse, based on an extrapolation of the optical ephemeris, does not correspond with the observed orbital minimum. The orbital period derived from the ASM light curve is also slightly longer than the optical period.