[ITAL]Chandra[/ITAL] High-Resolution Spectrum of the Anomalous X-Ray Pulsar 4U 0142+61

Abstract

We report on a 25 ks observation ofthe 8.7 s anomalous X-ray pulsar 4U 0142+61 with the High EnergyTransmission Grating Spectrometer on the Chandra X-Ray Observatory. Thecontinuum spectrum is consistent with previous measurements and iswell fitted by an absorbed power law+blackbody with a photon indexGamma=3.3+/-0.4 and kT=0.418+/-0.013 keV. No evidence was found foremission or absorption lines, with an upper limit of ~50 eV on theequivalent width of broad features in the 2.5-13 � (0.95-5.0 keV)range and an upper limit of ~10 eV on the equivalent width of narrowfeatures in the 4.1-17.7 � (0.7-3.0 keV) range. If the source isa magnetar, then the absence of a proton cyclotron line stronglyconstrains magnetar atmosphere models and hence the magnetic fieldstrength of the neutron star. We find no strong features that areindicative of cyclotron absorption for magnetic field strengths of(1.9-9.8)�10 14 G. This is still consistent with the dipole field strength ofB=1.3�10 14 G (at the polar cap) estimated from the pulsar's spin-down.