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 \pm 0.4 and blackbody temperature kT = 0.418 \pm 0.013 keV .
The pulsar frequency was 0.1150966 \pm 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 \approx 50 eV on the equivalent width of a broad feature in the 2.5–13 Å ( 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 \times 10 ^ { 14 } G and B > 9.8 \times 10 ^ { 14 } G .