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

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An atmospheric model for the ion cyclotron line of Geminga

A. Jacchia 1, F. De Luca 1,2, E. Lazzaro 1, P.A. Caraveo 3,4, R.P. Mignani 5 and G.F. Bignami 6,7

1 Istituto di Fisica del Plasma "P. Caldirola", CNR, Via Cozzi 53, I-20125 Milan, Italy (jacchia@ifp.mi.cnr.it)
2 Universitá degli studi di Milano, Dipartimento di Fisica, Via Celoria 16, I-20133 Milan, Italy
3 Istituto di Fisica Cosmica "G. Occhialini", CNR, Via Bassini 15, I-20133 Milan, Italy
4 Istituto Astronomico, Via Lancisi 29, I-00161 Rome, Italy
5 STECF-ESO, Karl-Schwarzschild-Strasse 2, D-85740 Garching, Germany
6 Agenzia Spaziale Italiana, Via di Villa Patrizi 13, I-00161 Rome, Italy
7 Universitá di Pavia, Via Bassi 6, I-27100 Pavia, Italy

Received 29 October 1998 / Accepted 29 April 1999

Abstract

Recent optical/UV data of Geminga (Mignani et al. 1998) have strengthen the significance of the emission feature present over the thermal continuum best fitting the EUV/soft X-ray data. Here we present a phenomenological model interpreting the data in terms of an ion cyclotron feature, arising from a thin outer layer of hot plasma covering Geminga's polar caps. The width of the feature favours for Geminga the strongly oblique rotator configuration, as already deduced from the soft X-ray data (Halpern & Ruderman 1993). Our interpretation implies a magnetic field of [FORMULA], consistent with the value deduced from the dynamical parameters of the pulsar. This would represent the first case of an independent measurement of the surface magnetic field of an isolated neutron star.

The expected time modulation of Geminga's feature is also discussed.

Key words: radiation mechanisms: thermal – stars: pulsars: individual: Geminga = 1E 0630+1

Send offprint requests to: A. Jacchia

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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