Probing the spectrum of the magnetar 4U 0142+61 with JWST

TL;DR

JWST observations of magnetar 4U 0142+61 reveal a nonthermal IR spectrum inconsistent with a fallback disk, suggesting different emission regions for IR and X-ray emissions and challenging previous disk models.

Contribution

First JWST IR spectrum of 4U 0142+61 showing nonthermal emission, refuting the fallback disk hypothesis from earlier Spitzer data.

Findings

IR spectrum fits an absorbed power-law with spectral slope 0.96

Data do not support the passive disk model from prior studies

X-ray spectrum is significantly harder than IR spectrum

Abstract

JWST observed the magnetar 4U 0142+61 with the MIRI and NIRCam instruments within a 77 min 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 $\mu$m range can be satisfactorily described by an absorbed power-law model, $f_{\nu}\propto \nu^{-\alpha}$, with a spectral slope $\alpha =0.96\pm0.02$, interstellar extinction $A_V= 3.9\pm0.2$, and normalization $f_0 = 59.4\pm 0.5$ $\mu$Jy at $\lambda = 8$ $\mu$m. These observations do not support the passive disk model proposed by Wang et al. (2006), based on the 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 power-law fit to the hard X-ray spectrum above 10 keV, measured by 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.