Pulse characteristics of the X-ray pulsar 4U 1907+09

The X-ray pulsar 4U 1907+09 was observed in the 2-18 keV energy band with t he Indian X-Ray Astronomy Experiment (IXAE) during 1996 August and again in 1998 June. From the timing measurements, the spin-down rate of the neutron star is measured to be +0.23 +/- 0.01 s yr(-1). A straight-line fit to the historical pulse period data indicates that the pulsar has been in a monot onic phase of spin-down since its discovery. The day-to-day pulse profile i ndicates some variations during its 8.4 day binary period. The average prof ile obtained from these observations shows a double-pulse shape with a pron ounced asymmetric primary separated by a dip from a relatively weaker but b road secondary. These profiles show a closer resemblance to the pulse shape obtained with the recent observations with the Rossi X-Ray Timing Explorer (IXAE) during 1996 February, as compared to the earlier observations carri ed out six to 13 years earlier. A secondary flare of 88 mcrab intensity was detected during the IXAE observations in 1996 August. A detailed analysis of the flare data shows the presence of transient 14.4 s oscillations, whic h may be quasi-periodic during the flaring activity but have a period diffe rent from the earlier reported oscillations of 18.2 s as detected by RXTE d uring the flare of 1996 February. These results, therefore, strengthen the evidence for the presence of a transient accretion disk around the neutron star during the flare, which may be responsible for the continuous slowing down of the pulsar. The studies of such transient quasi-periodic oscillatio ns during flaring activities of 4U 1907+09 thus provide opportunities for u nderstanding the transient behavior of the accretion disk and its physical characteristics.