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 ( CGRO ) and with the Rossi X-ray Timing Explorer ( RXTE ) .
Two outbursts from Cep X–4 were observed with BATSE in 1993 June-July and 1997 July .
Pulse frequencies of \nu = 15.0941 \pm 0.0002 mHz on 1993 June 25 ( MJD 49,163 ) and \nu = 15.0882 \pm 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 \dot { \nu } = ( -4.14 \pm 0.08 ) \times 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 & 25 with the Proportional Counter Array ( PCA ) on RXTE .
A pulse frequency of \nu = 15.088 \pm 0.004 mHz was measured from observations on 1997 July 18 ( MJD 50,647 ) .
Significant aperiodic noise , with an rms variance of \sim 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 4 outbursts observed from Cep X–4 are assumed to occur at the same orbital phase , we find that the orbital period is between 23 days and 147.3 days .