Rossi X-ray timing explorer observations of the anomalous pulsar 4U 0142+61

We observed the anomalous X-ray pulsar 4U 0142+61 using the Proportional Co unter Array on board the Rossi X-Ray Timing Explorer in 1996 March. The pul se frequency was measured as v = 0.11510039(3) Hz, with an upper limit of \ (v) over dot\ less than or equal to 4 x 10(-13) Hz s(-1) on the short-term change in frequency over the 4.6 day span of the observations. A compilatio n of all historical measurements showed an overall spin-down trend with slo pe (v) over dot = -3.0 +/- 0.1 x 10(-14) Hz s(-1). Searches for orbital mod ulations in pulse arrival times yielded an upper limit of a(x) sin i less t han or similar to 0.26 It-s (99% confidence) for the period range 70 s to 2 .5 days. These limits combined with previous optical limits and evolutionar y arguments suggest that 4U 0142 + 61 is probably not a member of a binary system.