SCYON Abstract

Received on November 23 2001

A Search for the optical counterpart to PSR B1821-24 in M28

Authors	Aaron Golden, Raymond F. Butler, Andrew Shearer
Affiliation	National University of Ireland, University Road, Galway, Ireland
Accepted by	Astronomy & Astrophysics
Contact	ray@physics.nuigalway.ie
URL	http://www.it.nuigalway.ie/resear/papers/index.html
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Abstract

We have analysed archival HST/WFPC2 images in both the F555W & F814W bands of the core field of the globular cluster M28 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 M28, 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.