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Nature 423, 725-727 (12 June 2003) | doi:10.1038/nature01703; Received 3 February 2003; Accepted 6 May 2003

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The magnetic field of an isolated neutron star from X-ray cyclotron absorption lines

G. F. Bignami1,2, P. A. Caraveo3, A. De Luca3,4 & S. Mereghetti3

  1. Centre d'Etude Spatiale des Rayonnements, CNRS-UPS, 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
  2. Università degli Studi di Pavia, Dipartimento Fisica Nucleare e Teorica, Via Bassi 6, 27100 Pavia, Italy
  3. Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano "G. Occhialini", Via Bassini 15, 20133 Milano, Italy
  4. Università di Milano Bicocca, Dipartimento Fisica, Piazza della Scienza 3, 20126 Milano, Italy

Correspondence to: G. F. Bignami1,2 Correspondence and requests for materials should be addressed to G.F.B. (Email: bignami@cesr.fr).

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Isolated neutron stars are highly magnetized, fast-rotating objects that form as an end point of stellar evolution. They are directly observable in X-ray emission, because of their high surface temperatures. Features in their X-ray spectra could in principle reveal the presence of atmospheres1, or be used to estimate the strength of their magnetic fields through the cyclotron process2, as is done for X-ray binaries3, 4. Almost all isolated neutron star spectra observed so far appear as featureless thermal continua5, 6. The only exception is 1E1207.4–5209 (refs 7–9), where two deep absorption features have been detected10, 11, but with insufficient definition to permit unambiguous interpretation. Here we report a long X-ray observation of the same object in which the star's spectrum shows three distinct features, regularly spaced at 0.7, 1.4 and 2.1 keV, plus a fourth feature of lower significance, at 2.8 keV. These features vary in phase with the star's rotation. The logical interpretation is that they are features from resonant cyclotron absorption, which allows us to calculate a magnetic field strength of 8 times 1010 G, assuming the absorption arises from electrons.

  1. Centre d'Etude Spatiale des Rayonnements, CNRS-UPS, 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
  2. Università degli Studi di Pavia, Dipartimento Fisica Nucleare e Teorica, Via Bassi 6, 27100 Pavia, Italy
  3. Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano "G. Occhialini", Via Bassini 15, 20133 Milano, Italy
  4. Università di Milano Bicocca, Dipartimento Fisica, Piazza della Scienza 3, 20126 Milano, Italy

Correspondence to: G. F. Bignami1,2 Correspondence and requests for materials should be addressed to G.F.B. (Email: bignami@cesr.fr).