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 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 similar to 24.5 (V-0 similar to 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. ASCAs 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.