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Session 113 - Old Worn-Out Stars.
Oral session, Thursday, January 16
The standard theory of disk accretion by a magnetic neutron star (Ghosh amp; Lamb 1979) predicts that the accretion produces a torque that depends only on the mass accretion rate and parameters that are intrinsic to the neutron star. BATSE measurements of the pulse frequency derivative of the Bursting Pulsar, GRO J1744-28, during its 1996 outburst show that the scaling of the frequency derivative with pulsed X-ray flux is in excellent agreement with the scaling with accretion rate predicted by the standard theory. The theory also predicts that at low accretion rates the accretion torque should become negative, causing the neutron star to spin down. We have observed the Bursting Pulsar with the Proportional Counter Array on the Rossi X-ray Timing Explorer since January 1996, when it was near the peak of its outburst and had a luminosity between bursts near the Eddington critical luminosity for a neutron star. We have made more than 100 simultaneous measurements of the X-ray flux and pulse frequency from 18 January to 20 October 1996. During this interval the X-ray flux decreased by a factor greater than 200, providing an opportunity to test the standard theory of disk accretion over an unprecedented range of accretion rate. Although the pulse frequency derivative decreases with decreasing X-ray flux and may vanish at the lowest fluxes seen, there is no significant evidence in this data that the accretion torque is ever negative. This implies that the dipole component of the pulsar's magnetic field is less than 10^11 Gauss at the pole. The field cannot be much smaller than this or the scaling of the frequency derivative with X-ray flux at the height of the outburst should have been different from what was observed.
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