Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...468l..29m&link_type=abstract
Astrophysical Journal Letters v.468, p.L29
Astronomy and Astrophysics
Astronomy
13
Accretion, Accretion Disks, Stars: Neutron, Stars: Oscillations, Stars: Pulsars: Individual Alphanumeric: Gro J1744-, 8, X-Rays: Stars
Scientific paper
The X-ray pulsar GRO J1744-28 accretes so rapidly that the gas flow onto its magnetic poles should be decelerated by radiation forces rather than gas pressure gradients. The vertical scale of the deceleration region, where the bulk of the flow's kinetic energy is transformed to radiation, is large compared to the size of the accretion flow's footprint on the stellar surface, and radiation escapes in a fan beam through the sides of the decelerating gas column. If one assumes that the X-ray bursts observed from GRO J1744-28 are triggered in the accretion disk, this picture of the accretion column geometry and a consideration of the interaction between the disk and the stellar magnetic field at the magnetospheric boundary lead to a natural explanation for the phase lags of the X-ray pulses observed during bursts: The accretion column is not round but flattened, so its footprint forms an arc on the stellar surface close to the magnetic pole. The broad side of the arc faces the magnetic pole. When the accretion rate rises, the gas flow shifts to a different set of field lines, and the accretion footprint pivots through a moderate angle around the magnetic pole. Although the footprint has remained close to the magnetic pole, the direction it faces has changed. The time of pulse maximum, when an observer sees the broad side of the accretion column most face-on, changes accordingly, and a phase-lag results.
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