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

JWST observed the magnetar 4U 0142+61 with the MIRI and NIRCam instrumentswithin a 77 min time interval on 2022 September 20-21. The low-resolution MIRIspectrum and NIRCam photometry show that the spectrum in the wavelength range1.4-11 μm range can be satisfactorily described by an absorbed power-lawmodel, 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 notsupport the passive disk model proposed by Wang et al. (2006), based on theSpitzer photometry, which was interpreted as evidence for a fallback disk fromdebris formed during the supernova explosion. We suggest a nonthermal originfor this emission and source variability as the most likely cause ofdiscrepancies between the JWST data and other IR-optical observing campaigns.However, we cannot firmly exclude the presence of a large disk with a differentdependence of the effective disk temperature on distance from the magnetar.Comparison with the power-law fit to the hard X-ray spectrum above 10 keV,measured by NuSTAR contemporaneously with JWST, shows that the X-ray spectrumis significantly harder. This may imply that the X-ray and IR nonthermalemission come from different sites in the magnetosphere of the magnetar.