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

We report on X-ray observations of the 66 s period transient X-ray pulsar CepX-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on theCompton Gamma-Ray Observatory (CGRO) and with the Rossi X-ray Timing Explorer(RXTE). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-Julyand 1997 July. Pulse frequencies of 15.0941 +/- 0.0002 mHz on 1993 June 25 (MJD49,163) and 15.0882 +/- 0.0002 mHz on 1997 July 12 (MJD 50,641) were eachmeasured from 2 day spans of BATSE data near each outburst's peak. Cep X-4showed an average spin down rate of (-4.14 +/- 0.08)*10^(-14) Hz/s between the1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, publicobservations were performed on 1997 July 18 & 25 with the Proportional CounterArray (PCA) on RXTE. A pulse frequency of 15.088 +/- 0.004 mHz was measuredfrom 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 onboth days. Energy and intensity dependent pulse shape variations were also seenin these data. Recently published optical observations associate Cep X-4 with aBe companion star. If all 4 outbursts observed from Cep X-4 are assumed tooccur at the same orbital phase, we find that the orbital period is between 23days and 147.3 days.Comment: 19 pages (LaTeX) including 9 figures. Accepted for publication in the Astrophysical Journa