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A variable absorption feature in the X-ray spectrum of a magnetar


Soft-γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays1,2. This behaviour can be understood in the magnetar model3,4,5, according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties6,7,8 of SGRs and AXPs are greater than—or at the high end of the range of—those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters9, whereas a strong field has been proposed to reside in the stellar interior10,11 and in multipole components on the surface12. Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star’s rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2 × 1014 gauss to more than 1015 gauss.

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Figure 1: Phase-dependent spectral feature in the EPIC data of SGR 0418+5729.
Figure 2: Results of the phase-resolved spectroscopy of SGR 0418+5729.
Figure 3: Example of a phase-resolved EPIC pn spectrum and its residuals with respect to different models.


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We thank G. Goggi and C. Paizis for discussions. This work is based on data and software provided by the ESA XMM-Newton Science Archive (XSA) and the NASA/GSFC High Energy Astrophysics Science Archive Research Center (HEASARC). We acknowledge partial funding from INAF through a PRIN 2010 grant and ASI through contract I/032/10/0.

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A.T., P.E. and S.M. processed and analysed the data, with contributions by F.G. and D.G.; theoretical interpretation was provided by R.T., L.N. and A.T., with contributions by S.Z. and L.S.; A.T., P.E., S.M. and R.T. composed the text, using inputs from the other co-authors; all authors discussed the results and commented on the manuscript.

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Correspondence to Andrea Tiengo.

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The authors declare no competing financial interests.

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Tiengo, A., Esposito, P., Mereghetti, S. et al. A variable absorption feature in the X-ray spectrum of a magnetar. Nature 500, 312–314 (2013).

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