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


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 × 1010 G, assuming the absorption arises from electrons.

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Figure 1: Spectra collected by the PN (a) and MOS (b) cameras on the EPIC instrument during the observation of 4–6 August 2002.
Figure 2: Total light curve relative to 208,000 photons collected by the PN camera folded at the best period of P = 0.42413076 s.
Figure 3: Comparison of the four PN spectra for phase intervals as defined in Fig. 2.


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We thank F. Jansen and all the XMM-Newton team in Vilspa for approving and carrying out this target of opportunity observation at very short notice. We also thank W. Becker and M. Turner for helping us to optimize EPIC settings. G.F.B. acknowledges discussions with D. Lamb, G. Ricker and J. Doty about the cyclotron line interpretation of the spectral features of this object. The XMM-Newton data analysis is supported by the Italian Space Agency (ASI). A.D.L. acknowledges an ASI fellowship.

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Bignami, G., Caraveo, P., Luca, A. et al. The magnetic field of an isolated neutron star from X-ray cyclotron absorption lines. Nature 423, 725–727 (2003).

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