Discovery of Optical Emission in the Hotspots of Three 3cr Quasars: High-energy Particle Acceleration in Powerful Radio Hotspots

Archival Hubble Space Telescope WFPC2 images were used to search for optical emission associated with the radio jets of a number of luminous quasars. From this search, we report new optical hotspot detections in the wellknown blazar 3C 454.3 and the lobe-dominated quasars 3C 275.1 and 3C 336. We also find optical emission in the vicinity of the hotspot in 3C 208, but believe this is a chance alignment. Optical emission from the arcsecond-scale jet in 3C 454.3 is also detected. Multi-frequency archival radio data from the VLA and MERLIN are analyzed, and the synchrotron spectra of these high-power hotspots are presented. We estimate that their break frequencies are in the range of 1010 − 1011 Hz, with large uncertainties due to the wide gap in frequency coverage between the radio and optical bands. We also calculate their equipartition magnetic fields, and find that the anti-correlation between break frequency and magnetic field found by Brunetti et al. for lower power hotspots extends to these high power hotspots. This supports their model of hot-spots based on shock acceleration and synchrotron losses. Subject headings: Galaxies: active — galaxies: jets — quasars: general — quasars: individual (3C 208, 3C 275.1, 3C 336, 3C 454.3) — radio continuum: galaxies