Periodicities in rotation and DM of PSR B1557–50

¹ Hartebeesthoek Radio Astronomy Observatory, PO Box 443, Krugersdorp 1740, South Africa e-mail: austine@hartrao.ac.za;aus$_{-}$chukwude@yahoo.com
² Department of Physics and Astronomy, University of Nigeria, Nsukka Enugu State, Nigeria e-mail: EPSEELON@aol.com

Corresponding author: A. E. Chukwude, aus_chukwude@yahoo.com

Received: 15 November 2001
Accepted: 11 November 2002

Abstract

We have analysed the pulse arrival time data of the pulsar B1557-50 collected over a period of nearly 13 years, from 1986 July to 1999 May, in order to investigate the long-term spin behaviour. The results reveal sustained cyclic variations in both the timing residuals and dispersion measure (DM) of this object. Both the timing and DM sequences are well described by two harmonically-related periodicities of ~1600 and 800 days. The variations in spin-down rate and DM are significantly anticorrelated in the sense that maximum corresponds to minimum DM. Interpretation of the data in terms of free precession of an isolated radio pulsar suggests a wobble angle . The data are consistent with at least 6.5% of the stellar moment of inertia being coupled to the crust.