Luminosity dependent changes in the X-ray pulse profile of the transient pulsar Cepheus X-4 during its declining phase of the 1997 outburst

The transient X-ray pulsar Cepheus X-4 underwent its latest outburst in 199 7, during July-August, which lasted for about 30 days. The Pointed mode Pro portional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE) o n board IRS-P3 satellite observed the source in its declining phase during 1997 July 28 to July 30. The timing analysis of the data confirms the 66 se conds pulsation of the neutron star. The Xray pulse profile obtained in two energy bands between 2 and 18 keV, shows energy dependent variations. The pulse profile obtained by us in the declining phase of the outburst when th e X-ray luminosity of the source was about 6x10(35) ergs s(-1), is distinct ly different from the one observed with the RXTE in the earlier phase of th e outburst. It is found that near the end of the outburst, the relative str ength of the two pulses of the double-pulse profile got reversed and the in ter-pulse became more dominant compared to the main pulse. The observations can be interpreted in terms of a luminosity dependent emission profile of the pulsar, where depending on the pulsar geometry with respect to line of sight, one of the emission patterns, either a pencil-beam or a fan-beam, be comes more dominant. This is due to the changes in the pulsar magneto-spher e below a specific luminosity, which may cause relative changes in accretio n process onto the two poles of the neutron star.