Astron. Astrophys. 319, 515-524 (1997)

The torque and X-ray flux changes of OAO 1657-415

Altan Baykal

Laboratory for High Energy Astrophysics NASA/GSFC Greenbelt, Maryland 20771, USA NAS/NRC Resident Research Associate, Physics Department, Middle East Technical University, Ankara 06531, Turkey

Received 23 May 1996 / Accepted 14 August 1996

Abstract

Combining previously published pulse frequencies and BATSE measurements, we estimate the noise strengths (or power density estimates) of angular accelerations by using the root mean square residuals of angular velocity time series of OAO 1657-415 and present the power spectra. The statistical interpretation of the angular velocity fluctuations is consistent with a random walk model. In order to investigate the short term angular velocity fluctuations in detail, a structure function analysis is applied for a two component neutron star model with a solid crust and a superfluid neutron core which is subjected to external white torque noise. No evidence for core-crust coupling on timescales longer than one day is found. The correlations between X-ray flux and angular acceleration () fluctuations are investigated. These are compared with disk accretion theory (Ghosh & Lamb 1979 a, b) and wind accretion theory (Blondin et al., 1990). It is found that the most natural explanation of X ray flux and angular acceleration fluctuations is the formation of episodic accretion disks in the case of stellar wind accretion.

Key words: stars: OAO 1657-415 accretion X-rays: stars stars: neutron

Online publication: July 3, 1998

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