A new X-ray pulsar GS 2138 + 56 (Cepheus X-4)

A transient X-ray pulsar was discovered with the Ginga satellite at the position of alpha(1950) = 21 h 38 m + or - 1 m, delta(1950) = 56 deg 50 arcmin + or - 6 arcmin (designed as GS 2138 + 56). The error box includes the previously reported transient source Cep X-4. An X-ray pulsation with a heliocentric pulse period of 66.2490 + or - 0.0001 sec (at MJD = 47263.5) was discovered. Assuming no intrinsic change of the pulse period during the observation, a lower limit of the orbital period of 23 days and projected semiaxis of 9 lt-sec are obtained. The X-ray spectrum has a typical shape for X-ray pulsars having power-law index of about 1.0 with an exponential cutoff at an energy typical shape for X-ray pulsars having power-law index of about 1.0 with an exponential cutoff at an energy of about 16 keV. It is suggested, based on circumstantial evidence, that GS 2138 + 56 is likely to be Be star binary. 13 refs.