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On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars

Nelson, R. W. and Bildsten, L. and Chakrabarty, D. and Finger, M. H. and Koh, D. T. and Prince, Thomas A. and Rubin, B. C. and Scott, D. M. and Vaughan, B. A. and Wilson, R. B. (1997) On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars. Astrophysical Journal Letters, 488 (1). L117-L120. ISSN 2041-8205.

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Dramatic torque reversals between spin-up and spin-down have been observed in half of the persistent X-ray pulsars monitored by the BATSE all-sky monitor on the Compton Gamma Ray Observatory. Theoretical models developed to explain early pulsar timing data can explain spin-down torques via a disk-magnetosphere interaction if the star nearly corotates with the inner accretion disk. To produce the observed BATSE torque reversals, however, these equilibrium models require the disk to alternate between two mass accretion rates, with ˙M5 producing accretion torques of similar magnitude but always of opposite sign. Moreover, in at least one pulsar (GX 114) undergoing secular spin-down, the neutron star spins down faster during brief (~20 day) hard X-ray flares—this is opposite the correlation expected from standard theory, assuming that BATSE pulsed flux increases with mass accretion rate. The 10 day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or viscous timescale near the inner disk boundary and are more suggestive of a global disk phenomenon. We discuss possible explanations of the observed torque behavior. Despite the preferred sense of rotation defined by the binary orbit, the BATSE observations are urprisingly consistent with an earlier suggestion for GX 1+4: the disks in these systems somehow alternate between episodes of prograde and retrograde rotation. We are unaware of any mechanism that could produce a stable retrograde disk in a binary undergoing Roche lobe overflow, but such flip-flop behavior does occur in numerical simulations of wind-fed systems. One possibility is that the disks in some of these binaries are fed by an X-ray–excited wind.

Item Type:Article
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URLURL TypeDescription
Chakrabarty, D.0000-0001-8804-8946
Prince, Thomas A.0000-0002-8850-3627
Additional Information:© 1997. The American Astronomical Society. Received 1997 June 19; accepted 1997 August 18; published 1997 September 16. This work was funded in part by NASA grants NAG 5-3119, NAG 5-3109, NAG 5-1458, NAG 5-3293, NAGW-4517, and NGT-51184 and the Alfred P. Sloan Foundation
Group:Space Radiation Laboratory
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Alfred P. Sloan FoundationUNSPECIFIED
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Space Radiation Laboratory1997-60
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ID Code:52372
Deposited By: Deborah Miles
Deposited On:05 Dec 2014 23:44
Last Modified:09 Mar 2020 13:18

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