Astron. Astrophys. 347, 494-499 (1999)

1. Introduction

The panorama of optical observations of Isolated Neutron Stars (INS) is limited by the faintness of the vast majority of them. Only one object, the Crab pulsar, has good, medium resolution optical (Nasuti et al. 1996) and near-UV spectral data (Gull et al. 1998), while for PSR0540-69 the synchrotron continuum has been measured by HST (Hill et al. 1997). For a few more cases acceptable multicolour photometry exists, while the rest of the data base (a grand total of less than 10 objects currently) consists of one/two - wavelengths detections (see Caraveo 1998 for a summary of the observational panorama).

Apart from the very young objects, characterized by flat, synchrotron-like spectra arising from energetic electron interactions in their magnetosphere, of particular interest are the middle-aged ones ( old). The non-thermal, magnetospheric emission should have faded enough (in the X-ray waveband at least) to render visible the thermal emission from the hot INS surface. Standard cooling calculations predict a surface temperature in the range , in excellent agreement with recent X-ray observations of INS with thermal spectra (e.g. Becker & Trümper 1997). It is easy to predict the IR-optical-UV fluxes generated along the Rayleigh-Jeans slope of the Planck curve best fitting the X-ray data, and to compare predictions to observations, where available.

In what follows, we shall concentrate on Geminga, which is certainly the most studied object of its class (Bignami & Caraveo 1996 and refs. therein), and possibly of all INSs (except for the Crab). Its IR-optical-UV data (Bignami et al. 1996; Mignani et al. 1998) show the presence of a well defined emission feature. Such a feature is superimposed on the thermal continuum expected from the extrapolation of the black-body X-ray emission detected by ROSAT (e.g. Halpern & Ruderman 1993). The presence of a clear maximum centered on V has been recently questioned by Martin et al. (1998) on the basis of a spectrum which is at the limit of the capability of the Keck telescope. The spectrum of Geminga, detected at a level of just 0.5% of the dark sky, seems fairly flat and, although broadly consistent with earlier measurements, it is definitely above the flux measured in the B-band by the HST/FOC (Mignani et al. 1998). Therefore, in view of the faintness of the target, we shall concentrate on the photometric measurements, which have been repeated using different instrumental set-ups and appear more reliable than the available spectral data.

Recently, pulsations in the B-band have been tentatively detected by Shearer et al. (1998). Also in this case, the faintness of the source limits quite severely the S/N ratio and thus the statistical significance of the result. Indeed, a pulsed signal at just the 3.5 level was found during only one of the three nights devoted to the project. If confirmed, these measurements would have deep implications on the mechanisms responsible for the optical emission of Geminga. However, in view of the rather low statistical significance of these results, we shall stand by the interpretation of Mignani et al. (1998) and propose a phenomenological model interpreting the feature as an atmospheric cyclotron line emission from Geminga's polar caps.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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