The optical-near-infrared spectrum of the M87 jet from Hubble Space Telescope observations

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 [alpha (ro)] approximate to 0.67 comparable to previous measurements, and [alpha (o)] approximate to 0.9 (F-nu proportional to nu (-alpha)), 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 (alpha (o) similar to 0.5-0.6, comparable to or flatter than alpha (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 alpha (o) similar to 0.7-0.9. The maps of alpha (o) and alpha (ro) appear poorly correlated. In knots A, B, and C, alpha (o) and alpha (ro) are essentially anticorrelated with one another. Near the flux maxima of two inner jet knots (HST-1 and F), changes in alpha (ro) appear to lag changes in alpha (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) similar to t(cool) for optically emitting electrons in knots D and E. Synchrotron models Dt to the radio-optical data yield nu (B) greater than or similar to 10(16) Hz for knots D, A, and B, and somewhat lower values, nu (B) similar to 10(15)-10(16) 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.