Characterization of the western Pictor A hotspot in the hard X-rays with NuSTAR

The origin of X-ray emission from the resolved kiloparsec-scale jets of many active galactic nuclei (AGN) remains uncertain, particularly where the X-ray emission is separate from the radio-optical synchrotron component. Possible explanations include inverse-Compton upscattering of the Cosmic Microwave Background (IC-CMB) and synchrotron emission from a second electron population, alternatives which imply very different physical conditions within the jet. Until now, X-ray studies of resolved jets have been restricted to the soft X-rays (below ~8 keV), often showing a hard spectral index indicating a spectral peak at higher energies. Here we present NuSTAR and Chandra observations of the nearby powerful radio galaxy Pictor A, in which we clearly detect the western hot spot at 4' from the host galaxy, the first detection of a kpc-scale jet above 10 keV. The NuSTAR 3$-$20 keV spectrum is best fit by a powerlaw of index Γ = 2.03±0.04. Using a maximum likelihood model, we confirm previous reports of variations in the soft X-ray flux detected by Chandra over the 2000 to 2015 period, at a 6.7σ level, higher than the 3.4σ significance level. This rises to >8σ in the common 3-8 keV band using the full 22-year span of Chandra and NuSTAR observations. The variability of Pictor A rules out IC-CMB as the source of the X-ray emission, and the NuSTAR observations are in keeping with a synchrotron origin for the X-ray emission from the western hotspot of Pictor A, with no sign of a cutoff to the flat spectrum.