Session 92 - Pulsars.
Oral session, Wednesday, January 15
Harbour C,

## [92.02] Detection and Timing of Accretion-Powered Pulsars with BATSE

D. Chakrabarty (MIT), T. A. Prince, D. T. Koh, B. A. Vaughan (Caltech), L. Bildsten (U.C. Berkeley), M. H. Finger (NASA/MSFC)

The BATSE all-sky monitor on the Compton Gamma Ray Observatory is a superb tool for the study of accretion-powered pulsars. BATSE's one-day sensitivity to the 20--60 keV pulsed emission from accreting pulsars with spins in the range 2 s \lesssim P_spin \lesssim 400 s is 15 mCrab. In continuous daily observations since 1991 April, more than half of the 43 known accreting pulsars have been detected by BATSE and are monitored whenever they are active. Except for a few systems whose spin periods lie outside of our sensitivity range, the known undetected systems are all transients which may eventually be detected in outburst. The detected systems include five new transients discovered by BATSE. Our observations have also provided binary orbit determinations for nearly a dozen accreting pulsars, including the discovery of regular X-ray eclipses in 38-s pulsar OAO 1657--415.

The long, uninterrupted timing histories provided by BATSE are a valuable probe for studing accretion torques X-ray pulsars. In particular, we have observed dramatic and abrupt torque reversals in 4U 1626--67 and GX 1+4, systems which normally undergo prolonged intervals of steady spin-up or spin-down. In the torque reversal of 4U 1626--67, the similarly abrupt change in flux predicted by standard magnetic accretion torque theory was not observed. The torque reversals of GX 1+4 did show roughly the expected flux correlation. However, during most of the spin-down intervals of GX 1+4 monitored by BATSE, there was a clear anticorrelation of torque and hard X-ray flux, the opposite of what standard accretion torque theory predicts. The long-term BATSE observations are providing strong constraints on (and challenges to) the standard picture of accreting neutron stars.