Using JWST to Investigate the Crab Nebula’s Synchrotron Emission

Abstract

This work takes advantage of the James Webb Space Telescope’s (JWST) 1.6 − 30 μm observing range and provides one of the first looks at the variations in the Crab Nebula’s synchrotron index in the mid infrared. We used new, synchrotron-dominated images of the Crab Nebula taken at 4.8 μm and 11.3 μm to search for evidence of a cooling break and multiple particle populations in the Crab Nebula’s infrared synchrotron spectrum. We created a high resolution, spectral index map between 4.8 μm and 11.3 μm to investigate the spectral and morphological distribution of ultra-relativistic particles in the Crab Nebula. We found that the JWST 4.8 μm to 11.3 μm spectral index, α, steepens from ∼ 0.4 ± 0.02 in the torus and ∼ 0.5 ± 0.02 in the jet structures to ∼ 0.65 ± 0.04 in the outer parts of the nebula, indicating the synchrotron cooling persists in the infrared. The spectral index also steepens faster towards the southwest (bottom right) edge of the nebula compared to the other edges, potentially indicating the existence of multiple particle populations within the nebula. We filled in the Crab Nebula’s synchrotron spectrum in the mid-infrared by finding the total flux through the two JWST images and using the index map to derive the synchrotron flux at 5.6 μm, 18 μm, and 21 μm. We found that the Spitzer 3.6 μm and 4.5 μm, JWST 4.8 μm and 11.3 μm, and derived 5.6 μm, 18.0 μm, and 21.0 μm fluxes follow a power law relationship of gradually changing slope that increases with increasing frequency within the uncertainties. This change in slope along with synchrotron cooling indicated by the JWST 4.8 μm to 11.3 μm spectral index map indicates a cooling break between the radio and near infrared.