The synchrotron spectrum of the M87 jet

Abstract

We present 1998 HST observations which yield the first single-epoch spectrum of the M87 jet from 0.3 to 2.0 microns. The flattest optical spectra ( α o∼0.6, where F ν ∝ ν −α ; comparable to α ro) are found in two inner jet knots which contain the fastest superluminal components. Other knots have somewhat steeper optical ( α o=0.8–1.1) than radio-optical ( α ro≈0.65) spectra. We do not find significant evidence of near-IR to UV steepening for any component. We combine the HST data with published X-ray and radio data to explore synchrotron emission models and the nature of the jet’s X-ray emission. We find break frequencies ∼10 15–16 Hz for most jet regions, considerably higher than predicted by previous workers. The highest break frequencies are in ‘strong shock’ regions, consistent with possible particle injection or acceleration. Because ‘continuous injection’ synchrotron models overpredict the X-ray emissions by large factors, it is likely that X-ray emitting regions take up only a small fraction of the jet volume.