Physical Properties of the AXP 4U 0142+61 from X-Ray Spectral Analysis

We analyze archival Chandra and XMM-Newton data of 4U 0142+61 within the context of the surface thermal emission and magnetospheric scattering model. We show that the 4U 0142+61 spectrum can be fit very well with this physical model that contains only four parameters. The system parameters can be tightly constrained from the fits, yielding a surface magnetic field strength of B = (4.75 +/- 0.02) × 10¹⁴ G, a surface temperature of kT = 0.309 +/- 0.001 keV, and a scattering optical depth of a few in the magnetosphere. These values do not vary between observations due to the stability of the source within the window of the observations. The detailed fits yield χ² values that are statistically much better than the traditionally employed blackbody+power-law and two blackbody fits. The spectroscopically measured surface magnetic field strength is higher than, but within, the theoretical uncertainties of the value inferred from the dipole spin-down formula.