1999ApJ...513..464W

We observed the anomalous X-ray pulsar 4U 0142+61 using the Proportional Counter Array on board the Rossi X-Ray Timing Explorer in 1996 March. The pulse frequency was measured as ν=0.11510039(3) Hz, with an upper limit of |ν{dot}|≤4x10^–13 Hz.s^–1 on the short-term change in frequency over the 4.6 day span of the observations. A compilation of all historical measurements showed an overall spin-down trend with slope ν{dot}=-3.0±0.1x10^–14 Hz.s^–1. Searches for orbital modulations in pulse arrival times yielded an upper limit of a_xsini≲ 0.26 lt-s (99% confidence) for the period range 70 s to 2.5 days. These limits combined with previous optical limits and evolutionary arguments suggest that 4U 0142+61 is probably not a member of a binary system.