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

Abstract

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 ?=0.11510039(3) Hz, with an upper limit of | ##IMG## [http://ej.iop.org/icons/Entities/dotnu.gif] {dot nu} |?4?10 ?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 ##IMG## [http://ej.iop.org/icons/Entities/dotnu.gif] {dot nu} =-3.0?0.1?10 ?14 Hz s ?1 . Searches for orbital modulations in pulse arrival times yielded an upper limit of a x sin i ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] {lesssim} 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.