Spectral Modelling of the High Energy Emission of the Magnetar 4 U 0142 + 614

We present an empirical spectral modelling of the high energy emission of the anomalous X-ray pulsar 4U 0142+614, based on simultaneous Swift and INTEGRAL observations from X to γ-ray energies. We adopted models contained in the XSPEC analysis package, as well as models based on recent theoretical studies, and restricted ourselves to those combinations of up to three components which produce a good fit while requiring the lowest number of free parameters. Only three models were found to fit satisfactorily the 0.5–250 keV spectrum of 4U 0142+614: i) a ∼ 0.4 keV blackbody and two power-laws, ii) a resonant cyclotron scattering model plus a power-law and iii) two log-parabolic functions. We found that only the latter two models do not over-predict the infrared/optical emission observed simultaneously from this AXP, and only the log-parabolic functions can naturally account for the upper limits set by COMPTEL in the γ-ray range. A possible interpretation of the two log-parabolae in terms of inverse Compton scattering of soft X-ray photons by very energetic particles is discussed. Soft γ-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are peculiar classes of high energy sources which share a number of properties (among others , the emission of short bursts, slow rotation periods in a narrow range, secular spin down and a X-ray luminos-ity of ≈ 10 34 –10 36 erg s −1 , see e.g. Woods & Thompson 2006 for a recent review). SGRs and AXPs are believed to be powered by ultra-magnetic neutron stars (" magne-tars " ; Duncan & Thompson 1992; Thompson & Duncan 1995). Spectral analysis is an important tool in magnetar astrophysics since it can provide key information on the emission mechanism(s). The first attempts at modelling the spectra of AXPs were based on soft X-ray data (< 10 keV) only. It was found that a blackbody (kT ∼0.3–0.6 keV) plus a power-law (photon index Γ ∼2– 4) or, limited to a few cases, two blackbodies (kT 1 ∼0.1 and kT 2 ∼0.8 keV) could successfully model the 0.5– 10 keV emission. Even though the blackbody plus power-law spectral model has been frequently applied to the X-ray spectra of magnetar candidates, a convincing physical interpretation is still missing (for the power-law component in particular). Recent studies have attempted to overcome this limitation. Thompson, Lyutikov & Kulkarni (2002) proposed that magnetars have twisted magnetospheres threaded by currents, which can efficiently boost via resonant cyclotron scattering …