1995A&A...295..413S

An outburst of Cep X-4 (GS 2138+56) was witnessed by ROSAT in one of two pointed observations. We locate Cep X-4 at a position of α(2000)=21h39m31.8s±1.6s and δ(2000)=56d59'00.8"±7.6". The pulsar appeared in two extreme intensity states, in an outburst and a quiescent state in which the source flux was reduced by a factor of ∼1000. X-ray pulsations were observed during the outburst with a period of 66.2552 ± 0.0007 s. Compared to the latest observed pulse period with GINGA this leads to a spin down between the two observed outbursts of {dot}(p)/p=+1.8x10^–5yr^–1. The pulse profile between 0.1 and 2.5keV varies with energy indicating a transition from a single peak main pulse above 1keV to a double peaked shape below 1keV. The energy spectrum could be fitted very well with a power law of photon index 1.0, which is typical for accretion powered pulsars. The analysis of pulse phase resolved energy spectra indicates a significant softening of the spectra between the main and the interpulse. We set a lower limit of 1kpc to the distance.