Discovery of the transient X - ray pulsar SAX J 2103 . 5 + 4545

We report the discovery of the X-ray transient SAX J2103.5+4545 which was active from Febru-ary to September 1997. The observed peak intensity of 20 mCrab (2 to 25 keV) occurred on April 11. An analysis of data obtained around the time of the peak revealed a pulsed signal with a period of 358.61 ± 0.03 s on MJD 50569. The pulse profile has a pulsed fraction of ∼ 40%. No change in the pulse period was detected, with an upper limit of 6 s yr −1. The energy spectrum complies to a power law function with a photon index of 1.27 ± 0.14 and low-energy absorption equivalent to a hydrogen column density of (3.1 ± 1.4) × 10 22 atoms cm −2 of cold gas of cosmic abundances. In analogy to other X-ray pulsars with similar characteristics we propose this object to be a neutron star in close orbit around a mass-losing star of early spectral type. The B star HD 200709 is a marginal candidate optical counterpart.