Accretion Column Disruption in GX 1+4

Daily observations of the binary X-ray pulsar GX 1+4 were made with the RossiX-ray Timing Explorer (RXTE) satellite between 1997 May 16-20 as part of afour-month monitoring program. On May 17 the sharp dips normally observed inthe lightcurve were all but absent, resulting in a pulse fraction f_p approx.0.5 instead of the more typical value of approx. 0.8 measured before and after.Also observed was a dramatic hardening of the 2-40 keV phase-averaged spectrum.The power-law photon index was 1.16 +/- 0.02, whereas values of 1.6-2.0 aremore typical. In terms of a Comptonization continuum component, the opticaldepth for scattering was tau approx 19, with 4-6 the usual range for RXTEspectra (Galloway 2000). Pulse-phase spectrosopy indicates that tau isdecreased relative to the phase-averaged value around the primary minimum,where an increase is normally observed. The reduced depth of the dip isinterpreted as disruption of the accretion column, and the accompanyingspectral variation suggests a substantially different accretion regime than isusual for this source.Comment: 10 pages, 2 figures, minor abstract typo and wording of final paragraph correcte