[Previous] | [Session 22] | [Next]
D.K. Galloway (MIT), A.B. Giles (University of Tasmania/LHEA/USRA), K. Wu (RCfTA, University of Sydney), J.G. Greenhill (University of Tasmania)
Sharp dips observed in the pulse profiles of the X-ray pulsars GX~1+4, RX~J0812.4-3114 and A~0535+26 have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy of RXTE data from GX~1+4 and RX~J0812.4-3114, as well as Monte-Carlo simulations based on Compton scattering in a simplified column geometry which confirms this interpretation. The dip phase corresponds with the closest approach of the column (magnetic) axis to the line of sight, and the additional optical depth experienced by photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted \tau measured at this phase. Analysis of the arrival time of individual dips in GX~1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with standard deviation 2--6\arcdeg depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and individual eclipses demonstrates that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate \dot{M}.
The author(s) of this abstract have provided an email address for comments about the abstract: duncan@space.mit.edu