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

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 th e optical counterpart of the magnetospherically active millisecond pulsar P SR B1821-24. Examination of the radio derived error circle yielded several potential candidates, down to a magnitude of V similar to 24.5 (V-0 similar to 23.0). Each were further investigated, both in the context of the CMD o f M 28, and also with regard to phenomenological models of pulsar magnetosp heric emission. The latter was based on both luminosity-spindown correlatio ns and known spectral flux density behaviour in this regime from the small population of optical pulsars observed to date. None of the potential candi dates exhibited emission expected from a magnetospherically active pulsar. The fact that the magnetic field & spin coupling for PSR B1821-24 is of a s imilar magnitude to that of the Crab pulsar in the vicinity of the light cy linder has suggested that the millisecond pulsar may well be an efficient n onthermal emitter. ASCA's detection of a strong synchrotron-dominated X-ray pulse fraction encourages such a viewpoint. We argue that only future dedi cated 2-d high speed photometry observations of the radio error-circle can finally resolve this matter.