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We observed the anomalous X-ray pulsar 4U 0142+61 using the ProportionalCounter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE) in March1996. The pulse frequency was measured as f = 0.11510039(3) Hz with an upperlimit of df/dt < 4 * 10^(-13) Hz/s upon the short term change in frequency overthe 4.6 day span of the observations. A compilation of all historicalmeasurements showed an overall spin-down trend with slope df/dt = (-3.0 +/-0.1) * 10^(-14) Hz/s. Searches for orbital modulations in pulse arrival timesyielded an upper limit of a_x sin i < 0.26 lt-s (99% confidence) for the periodrange 70 s to 2.5 days. These limits combined with previous optical limits andevolutionary arguments suggest that 4U 0142+61 is probably not a member of abinary system.Comment: 20 pages (LaTeX) including 7 figures. Accepted for publication in the  Astrophysical Journa

Rossi X‐Ray Timing ExplorerObservations of the Anomalous Pulsar 4U 0142+61