Torque Reversals in Disc Accreting Pulsars

Jianke Li & Dayal T. Wickramasinghe

ANU Astrophysical Theory Centre,
Department of Mathematics,
Faculty of Science & the Mount Stromlo and Siding Spring Observatories,
The Australian National University, ACT 0200, AUSTRALIA
ljk@maths.anu.edu.au dayal@maths.anu.edu.au

Abstract:

X-ray binaries in which the accreting component is a neutron star commonly exhibit significant changes in their spin. In the system Cen X-3, a disc accreting binary system, the pulsar was observed to spin up at a rate Hz s when averaged over the past twenty years, but significant fluctuations were observed above this mean. Recent BASTE observations have disclosed that these fluctuations are much larger than previously noted, and appeared to be a system characteristic. The change in the spin state from spin-up to spin-down or vice-versa occurs on a time scale that is much shorter than the instrument can resolve ( d), but appears always to be similar amplitude, and to occur stochastically. These observations have posed a problem for the conventional torque-mass accretion relation for accreting pulsars, because in this model the spin rate is closely related to the accretion rate, and the latter needs to be finely tuned and to change abruptly to explain the observations. As a review presented at the ANUATC workshop, we summarise recent work in this direction and present a coherent picture that explains these observations. We also draw attention to some outstanding problems for future studies.

Keywords:

Magnetic accretion - pulsar spin

© Copyright Astronomical Society of Australia 1997