EDP Sciences
Free Access
Volume 371, Number 1, May III 2001
Page(s) 198 - 204
Section Formation, structure and evolution of stars
DOI https://doi.org/10.1051/0004-6361:20010340
Published online 15 May 2001

A&A 371, 198-204 (2001)
DOI: 10.1051/0004-6361:20010340

A search for the optical counterpart to PSR B1821-24 in M 28

A. Golden, R. F. Butler and A. Shearer

National University of Ireland, Galway, Ireland

(Received 18 September 2000 / Accepted 7 March 2001)

We have analysed archival HST/WFPC2 images in both the F555W & F814W bands of the core field of the globular cluster M 28 in an attempt to identify the optical counterpart of the magnetospherically active millisecond pulsar PSR B1821-24. Examination of the radio derived error circle yielded several potential candidates, down to a magnitude of V ~ 24.5 (V0 ~ 23.0). Each were further investigated, both in the context of the CMD of M 28, and also with regard to phenomenological models of pulsar magnetospheric emission. The latter was based on both luminosity-spindown correlations and known spectral flux density behaviour in this regime from the small population of optical pulsars observed to date. None of the potential candidates exhibited emission expected from a magnetospherically active pulsar. The fact that the magnetic field & spin coupling for PSR B1821-24 is of a similar magnitude to that of the Crab pulsar in the vicinity of the light cylinder has suggested that the millisecond pulsar may well be an efficient nonthermal emitter. ASCA's detection of a strong synchrotron-dominated X-ray pulse fraction encourages such a viewpoint. We argue that only future dedicated 2-d high speed photometry observations of the radio error-circle can finally resolve this matter.

Key words: pulsars: general -- pulsars: individual PSR B1821-24 -- globular clusters: general -- globular clusters: individual M 28/NGC 6626 -- Techniques: image processing -- Astrometry

Offprint request: A. Golden, agolden@itc.nuigalway.ie

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© ESO 2001

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