Probing the Spectrum of the Magnetar 4U 0142+61 with JWST

JWST observed the magnetar 4U 0142+61 with the Mid-Infrared Instrument (MIRI) and Near Infrared Camera (NIRCam) instruments within a 77 minute time interval on 2022 September 20-21. The low-resolution MIRI spectrum and NIRCam photometry show that the spectrum in the wavelength range 1.4-11 μm range can be satisfactorily described by an absorbed power-law (PL) model, f _ν ∝ ν ^−α , with a spectral slope α = 0.96 ± 0.02, interstellar extinction A _V = 3.9 ± 0.2, and normalization f ₀ = 59.4 ± 0.5 μJy at λ = 8 μm. These observations do not support the passive disk model proposed in 2006 by Wang, Chakrabarty and Kaplan, based on Spitzer photometry, which was interpreted as evidence for a fallback disk from debris formed during the supernova explosion. We suggest a nonthermal origin for this emission and source variability as the most likely cause of discrepancies between the JWST data and other IR-optical observing campaigns. However, we cannot firmly exclude the presence of a large disk with a different dependence of the effective disk temperature on distance from the magnetar. Comparison with the PL fit to the hard X-ray spectrum above 10 keV, measured by the NuSTAR contemporaneously with JWST, shows that the X-ray spectrum is significantly harder. This may imply that the X-ray and IR nonthermal emission come from different sites in the magnetosphere of the magnetar.