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

6. Conclusions

The data shown in Fig. 1 leave no room for doubt that a wide emission feature exists in the optical region of Geminga's thermal continuum. The feature falls in the wavelength region where the atmospheric ion-cyclotron emission will be located close to the surface of a magnetic neutron star. Since Geminga is a magnetic neutron star, to wit its periodic -ray emission, and most probably has an atmosphere, to wit its soft X-ray emission, we have provided here a semi-quantitative interpretation for such feature. It is based on the reasonable assumption that the polar cap regions of the NS are covered by a thin plasma layer heated to a temperature higher that the global surface atmosphere by, e.g., infalling particles. This is not a new scenario per se. It was foreseen both in the case of INS accretion of ionized matter funnelled towards the poles by the B-field configuration and of magnetospheric particles drawn back to the polar surface by the strong E field induced by the oblique rotator.

The plausibility of this emission model in the visible range frequencies is also supported by estimate of power balance performed along the line proposed by Halpern & Ruderman. In the case of Geminga a pair flux in excess of can release in the emitting plasma layer a linear power density (Jackson 1975). This power is sufficient to compensate plasma losses mainly due to the ion cyclotron emission and Bremsstrahlung over the whole star surface.

What is new here is the excellent fit obtained to the multiple experimental data by our physical model using a minimum of assumption. In particular, we have shown that the feature could not originate over the whole star surface, because global B-field variations would induce a feature wider than observed. The only free parameter is the geometry of the emission with respect to the observer; note, however, that our geometry is fully compatible with the oblique rotator proposed for Geminga by Halpern & Ruderman (1993). The assumption that the composition of the outer emitting layer is either H or a light, fully ionized element mixture is supported by the estimated value for the magnetic field. Such a value is in good agreement with the standard pulsar magnetic field prediction. It represents, in fact, the first independent measurement of the surface magnetic field of an INS.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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