4 research outputs found
Evidence of a Change in the Long Term Spin-down Rate of the X-ray Pulsar 4U 1907+09
We analyzed RXTE archival observations of 4U 1907+09 between 17 February 1996
and 6 March 2002. The pulse timing analysis showed that the source stayed at
almost {\bf{constant}} period around August 1998 and then started to spin-down
at a rate of Hz s which is 0.60
times lower than the long term ( years) spin-down rate (Baykal et al.
2001). Our pulse frequency measurements for the first time resolved significant
spin-down rate variations since the discovery of the source. We also presented
orbital phase resolved X-ray spectra during two stable spin down episodes
during November 1996 - December 1997 and March 2001 - March 2002. The source
has been known to have two orbitally locked flares. We found that X-ray flux
and spectral parameters except Hydrogen column density agreed with each other
during the flares.We interpreted the similar values of X-ray fluxes as an
indication of the fact that the source accretes not only via transient
retrograde accretion disc (in't Zand et al. 1998) but also via the stellar wind
of the companion (Roberts et al. 2001), so that the variation of the accretion
rate from the disc does not cause significant variation in the observed X-ray
flux. Lack of significant change in spectral parameters except Hydrogen column
density was interpreted as a sign of the fact that the change in the spin-down
rate of the source was not accompanied by a significant variation in the
accretion geometry.Comment: Revised version. Accepted for publication in MNRA