title: A variable absorption feature in the X-ray spectrum of a magnetar.
abstract: 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-rays. This behaviour can be understood in the magnetar model, 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 properties 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 parameters, whereas a strong field has been proposed to reside in the stellar interior and in multipole components on the surface. 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 × 10(14) gauss to more than 10(15) gauss.
authors: Istituto Universitario di Studi Superiori, piazza della Vittoria 15, I-27100 Pavia, Italy. andrea.tiengo@iusspavia.it
authors: Andrea
authors: A
authors: Tiengo
authors: Paolo
authors: P
authors: Esposito
authors: Sandro
authors: S
authors: Mereghetti
authors: Roberto
authors: R
authors: Turolla
authors: Luciano
authors: L
authors: Nobili
authors: Fabio
authors: F
authors: Gastaldello
authors: Diego
authors: D
authors: Götz
authors: Gian Luca
authors: GL
authors: Israel
authors: Nanda
authors: N
authors: Rea
authors: Luigi
authors: L
authors: Stella
authors: Silvia
authors: S
authors: Zane
authors: Giovanni F
authors: GF
authors: Bignami
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