NuSTAR Hard X-Ray Studies of the Pulsar Wind Nebula 3C 58

We report on new NuSTAR and archival Chandra observations of the pulsar wind nebula (PWN) 3C 58. Using the X-ray data, we measure energy-dependent morphologies and spatially resolved spectra of the PWN. We find that the PWN size becomes smaller with increasing energy and that the spectrum is softer in outer regions. In the spatially integrated spectrum of the PWN, we find a hint of a spectral break at ∼25 keV. We interpret these findings using synchrotron-radiation scenarios. We attribute the size change to the synchrotron burn-off effect. The radial profile of the spectral index has a break at R∼ 80^\prime\prime , implying a maximum electron energy of ∼200 TeV, which is larger than a previous estimate, and the 25 keV spectral break corresponds to a maximum electron energy of ∼140 TeV for an assumed magnetic field strength of 80 μG. Combining the X-ray data and a previous radio-to-IR spectral energy distribution, we measure a cooling break frequency to be ∼10¹⁵ Hz, which constrains the magnetic field strength in 3C 58 to be 30-200 μG for an assumed age range of 800-5000 yr.