Chandra/HETGS Spectroscopy of the anomalous X-ray pulsar 4U 0142+61

We report on a 25 ks observation of the anomalous X-ray pulsar 4U 0142+61 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. The continuum spectrum is consistent with previous measurements and is well fit by an absorbed power-law + blackbody model with photon index Gamma = 3.3 +/- 0.4 and blackbody temperature kT = 0.418 +/- 0.013 keV. The pulsar frequency was 0.1150966 +/- 0.0000017 Hz and the pulse fractions were between 8.7% and 21%, which are also consistent with past measurements. No evidence was found for emission or absorption lines with an upper limit of approximate to50 eV on the equivalent width of a broad feature in the 2.5-13 A (0.95-4.96 keV) range. The absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. For the energy range given above, the allowed magnetic field strengths of 4U 0142+61 are B < 1.9 X 10(14) G and B > 9.8 x 10(14) G.