A&A 366, 930-934 (2001)
DOI: 10.1051/0004-6361:20000283

Crab pulsar photometry and the signature of free precession

A. Cadez¹, S. Vidrih¹, M. Galicic¹ and A. Carramiñana<sup>1

1</sup>  Department of Physics, FMF University in Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia I.N.A.O.E., Luis Enrique Erro 1, Tonantzintla, Puebla 72840, Mexico

(Received 3 July 2000 / Accepted 16 November 2000 )

Abstract
Optical photometry for the pulsar PSR 0531+21 has been extended with new observations that strengthen evidence for a previously observed 60 s periodicity. This period is found to be increasing with time at approximately the same rate as the rotational period of the pulsar. The observed period and its time dependence fit a simple free precession model.

Key words: stars: neutron -- pulsars: individual: PSR 0531+21 -- rotation -- oscillations

Offprint request: A. Cadez, andrej.cadez@uni-lj.si

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