2001ApJ...551..206P

We present 1998 Hubble Space Telescope observations of M87 that yield the first single-epoch optical and radio-optical spectral index image of the jet at 0".15 resolution. We find <α_ro>~0.67, comparable to previous measurements, and <α_o>~0.9 (F_ν∝ν^–α), slightly flatter than previous workers. Reasons for this discrepancy are discussed. These observations reveal a large variety of spectral slopes. Bright knots exhibit significantly flatter spectra than interknot regions. The flattest spectra (α_o∼0.5-0.6, comparable to or flatter than α_ro) are found in the two inner jet knots (D-East and HST-1), which contain the fastest superluminal components. The flux maximum regions of other knots have α_o∼0.7-0.9. The maps of α_o and α_ro appear poorly correlated. In knots A, B, and C, α_o and α_ro are essentially anticorrelated with one another. Near the flux maxima of two inner jet knots (HST-1 and F), changes in α_ro appear to lag changes in α_o, but in two other knots (D and E), the opposite relationship is observed. This is further evidence that the radio and optical emissions of the M87 jet come from substantially different physical regions. The delays observed in the inner jet are consistent with localized particle acceleration in the knots, with t_acc≪t_cool for optically emitting electrons in knots HST-1 and F, and t_acc∼t_cool for optically emitting electrons in knots D and E. Synchrotron models fit to the radio-optical data yield ν_B≳10¹⁶ Hz for knots D, A, and B, and somewhat lower values, ν_B∼10¹⁵-10¹⁶ Hz, in other regions of the jet. If the X-ray emissions from knots A, B, and D are cospatial with the optical and radio emission, we can strongly rule out the ``continuous injection'' model, which overpredicts the X-ray emissions by large factors. Because of the short lifetimes of X-ray synchrotron-emitting particles, the X-ray emission likely traces sites of particle acceleration and fills volumes much smaller than the optical emission regions.