Astron. Astrophys. 363, 617-628 (2000)

1. Introduction

As the youngest known isolated rotation powered neutron star, and having the highest known spin-down flux density, the Crab pulsar is not surprisingly the most efficient source of high energy magnetospheric emission. All theoretical models attempting to elucidate the processes of nonthermal emission must satisfy the empirical constraints provided by the extensive observational datasets available for this object from -rays to the infrared.

Contemporary theoretical frameworks in existence to explain the nonthermal emission from this and other isolated rotation-powered neutron stars may be broadly divided into two schools. The first places the source of emission close to the polar cap region, e.g. Sturrock (1971), and the second that place the source at a considerable distance above the neutron star surface, in the outer magnetosphere (e.g. Cheng et al. 1986). In recent times, the development of these models has been to some extent restricted to the problem of -ray emission. These efforts have been predominantly analytical in nature, as in this energy regime primary emission is being sampled, which avoids complications associated with the lower energy sources of emission. In these latter cases, the emission is secondary in nature and a result of interactions that require numerical representation. Thus the polar cap school (e.g. Daugherty & Harding 1982) concluded with correct flux estimates but poor light curves, and the outer gap school (Cheng et al. 1986) with reasonable light curves and approximate flux distributions but requiring extraordinary lines-of-sights and magnetospheric physics to justify the emission.

The group at Stanford University lead by R. Romani is generally credited by being the first to rigourously put the existing models to the test numerically. Their conclusions in many ways matched the intuitive premonitions regarding the two model frameworks, with perhaps the `evolved' outer gap model of Cheng et al. (1986) being the more likely candidate (Chiang & Romani 1992; Romani & Yadagiroglu 1995). Recently Romani (1998) has indicated that it is only by both the development of more advanced numerical simulations that incorporate the full emission physics (from -rays to the IR) in a truly self-consistent manner and the accurate characterisation of pulsar light curves in the lower energy regimes that one can hope to disentangle the problem of nonthermal pulsar emission.

Optically, spectroscopic, integrated and high speed single-pixel photometry have produced some critically important results in this regard. However, true phase-resolved data acquisition with accurate isolation of the pulsar's emission from that of the nebula requires high time resolved ( 1 µs) 2-d photometry to confirm unambiguously earlier conclusions and to determine the true nature of the magnetosphere's emission in the range 1-10 eV.

In a previous paper (Golden et al. 2000), we presented an analysis of such observations of the Crab pulsar in which we isolated and theoretically speculated upon the unpulsed component of the pulsar's optical emission. These observations were made in January 1996 with the TRIFFID 2-d high speed photometer in three colourbands (UBV) using the 6m BTA of the Special Astrophysical Observatory in the Russian Caucasus. Here we present a thorough photometric and temporal analysis of the same data. Following a review of the pulsar's relevant empirical and theoretical characteristics, we briefly outline the observational and data reduction methodology - the reader is encouraged to consult Golden et al. (2000) for a more rigorous treatment of this process. We then detail the results of the photometric and temporal analysis, and conclude with a discussion on the implications of these results for current thinking on this pulsar, with wider ramifications for pulsar emission theory in general.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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