2016ApJ...817..121B

Many powerful radio quasars are associated with large-scale jets, exhibiting bright knots as shown by high-resolution images from the Hubble Space Telescope (HST) and the Chandra X-ray Observatory. The radio-optical flux component from these jets can be attributed to synchrotron radiation by accelerated relativistic electrons while the IC/CMB model, by far, has been the most popular explanation for the observed X-ray emission from these jets. Recently, the IC/CMB X-ray mechanism has been strongly disfavored for 3C 273 and PKS 0637-752 since the anomalously hard and steady gamma-ray emission predicted by such models violates the observational results from Fermi-LAT. Here we propose the proton synchrotron origin of the X-ray-gamma-ray flux from the knots of PKS 0637-752 with a reasonable budget in luminosity, by considering synchrotron radiation from an accelerated proton population. Moreover, for the source 3C 273, the optical data points near 10¹⁵ Hz could not be fitted using electron synchrotron. We propose an updated proton synchrotron model, including the optical data from HST, to explain the common origin of optical-X-ray-gamma-ray emission from the knots of quasar 3C 273 as an extension of the work done by Kundu & Gupta. We also show that TeV emission from large-scale quasar jets, in principle, can arise from proton synchrotron, which we discuss in the context of knot wk8.9 of PKS 0637-752.