Recent Outbursts from the Transient X‐Ray Pulsar Cepheus X‐4 (GS 2138+56)

Abstract

We report on X-ray observations of the 66 s period transient X-ray pulsar Cep X-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory and with the Rossi X-Ray Timing Explorer ( RXTE ). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-July and in 1997 July. Pulse frequencies of ?=15.0941?0.0002 mHz on 1993 June 25 (MJD 49,163) and ?=15.0882?0.0002 mHz on 1997 July 12 (MJD 50,641) were each measured from 2-day spans of BATSE data near each outburst's peak. Cep X-4 showed an average spin down rate of ##IMG## [http://ej.iop.org/icons/Entities/dotnu.gif] {dot nu} =(-4.14?0.08)?10 ?14 Hz s -1 between the 1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, public observations were performed on 1997 July 18 and July 25 with the Proportional Counter Array (PCA) on RXTE . A pulse frequency of ?=15.088?0.004 mHz was measured from observations on 1997 July 18 (MJD 50,647). Significant aperiodic noise, with an rms variance of ~18% in the frequency range 0.01-1.0 Hz, was observed on both days. Energy- and intensity-dependent pulse shape variations were also seen in these data. Recently published optical observations associate Cep X-4 with a Be companion star. If all four outbursts observed from Cep X-4 are assumed to occur at the same orbital phase, we find that the orbital period is between 23 and 147.3 days.