Chandra high-resolution spectrum of the anomalous X-ray pulsar 4U 0142+61

We report on a 25 ks observation of the 8.7 s anomalous X-ray pulsar 4U 0142+61 with the High Energy Transmission Grating Spectrometer on the Chandra X-Ray Observatory. The continuum spectrum is consistent with previous measurements and is well fitted by an absorbed power law+blackbody with a photon index Gamma = 3.3 +/- 0.4 and kT = 0.418 +/- 0.013 keV. No evidence was found for emission or absorption lines, with an upper limit of approximate to50 eV on the equivalent width of broad features in the 2.5-13 Angstrom (0.95-5.0 keV) range and an upper limit of approximate to10 eV on the equivalent width of narrow features in the 4.1-17.7 Angstrom (0.7-3.0 keV) range. If the source is a magnetar, then the absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. We find no strong features that are indicative of cyclotron absorption for magnetic field strengths of (1.9-9.8) x 10(14) G. This is still consistent with the dipole field strength of B = 1.3 x 10(14) G (at the polar cap) estimated from the pulsar's spin-down.