Accretion Column Disruption in Gx 1+4

Daily observations of the binary X-ray pulsar GX 1+4 were made with the Rossi Xray Timing Explorer (RXTE ) satellite between 1997 May 16–20 as part of a four-month monitoring program. On May 17 the sharp dips normally observed in the lightcurve were all but absent, resulting in a pulse fraction fp ≈ 0.5 instead of the more typical value of ≈ 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 are more typical. In terms of a Comptonization continuum component, the optical depth for scattering was τ ≈ 19, with 4–6 the usual range for RXTE spectra (Galloway 2000). Pulse-phase spectrosopy indicates that τ is decreased relative to the phase-averaged value around the primary minimum, where an increase is normally observed. The reduced depth of the dip is interpreted as disruption of the accretion column, and the accompanying spectral variation suggests a substantially different accretion regime than is usual for this source. Subject headings: X-rays: stars — pulsars: individual (GX 1+4) — accretion — scattering