Observation of a Long‐Term Spin‐up Trend in 4U 1538−52

Abstract

The high-mass X-ray pulsar 4U 1538-52 has been continuallymonitored by the BATSE (Burst and Transient Source Experiment) on ComptonGamma-Ray Observatory. These observations permit the construction oflong-term pulse frequency and intensity histories. The frequencyhistory reveals that a reversal of the long-term trend in the accretiontorque, from spinning down to spinning up, has occurred, probably in1988. This is the first time a long-term change is known to haveoccurred for this source. The magnitude of the average | nu dot / nu|~10^-11 s-1 over an interval of ~5 yr is similar during spin-downand spin-up. Shorter term pulse frequency variations of either signare also observed. The power density spectrum of fluctuations inangular acceleration is consistent with white noise on timescalesfrom 16 to 1600 days. It can be well fit with a power law with apower-law index of 0.10 +/- 0.21 and white noise strength of (7.6 +/-1.6) x 10-21(Hz s-1)2 Hz-1. These observations are also used to set95% confidence level lower and upper limits on the rate of orbitalperiod change, -3.9 <2.1 , in units of 10-6 yr-1.