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Astron. Astrophys. 332, L37-L40 (1998)

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Letter to the Editor

HST/FOC observations confirm the presence of a spectral feature in the optical spectrum of Geminga *

R.P. Mignani 1, P.A. Caraveo 2, 3 and G.F. Bignami 4, 5

1 Max-Plack-Institute für Extraterrestrische Physik, Postfach 1603 Giessenbachstrasse, D-85740 Garching, Germany
(e-mail: rmignani@xray.mpe.mpg.de)
current address STECF-ESO, Karl Schwarzschild Strasse 2, D-85740 Garching, Germany
2 Istituto di Fisica Cosmica del CNR, Via Bassini 15, I-20133 Milan, Italy (e-mail: pat@ifctr.mi.cnr.it)
3 Istituto Astronomico, Via Lancisi 29, I-00161 Rome, Italy
4 Agenzia Spaziale Italiana, Via di Villa Patrizi 13, I-00161 Rome, Italy (e-mail: gfb@asirom.rm.asi.it)
5 Universitá di Pavia, Via Bassi 6, I-27100 Pavia, Italy

Received 6 January 1998 / Accepted 6 February 1998


New optical and near-UV HST observations of Geminga are presented. When compared with previous ground-based and HST imaging, the data confirm and better define the presence of an emission feature centered at [FORMULA] and superimposed on the thermal continuum best fitting the extreme-UV/soft X-ray data. This feature may be interpreted in terms of cyclotron emission originated from a mixture of H/He ions in the neutron star's atmosphere. In the case of pure Hydrogen, the feature wavelength would imply a magnetic field of order [FORMULA], consistent with the value deduced from the dynamical parameters of the pulsar. If due to cyclotron emission, the observation of this feature would represent the first case of an in situ measurement of the surface magnetic field of an isolated neutron star.

Key words: optical, pulsar Geminga

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555

Send offprint requests to: R.P. Mignani

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998