The Optical-Near-Infrared Spectrum of the M87 Jet fromHubble 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 015 resolution. We find αro ≈ 0.67, comparable to previous measurements, and αo ≈ 0.9 (Fν ∝ 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 (αo ~ 0.5-0.6, comparable to or flatter than αro) 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 αo ~ 0.7-0.9. The maps of αo and αro appear poorly correlated. In knots A, B, and C, αo and αro are essentially anticorrelated with one another. Near the flux maxima of two inner jet knots (HST-1 and F), changes in αro appear to lag changes in αo, 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 tacc tcool for optically emitting electrons in knots HST-1 and F, and tacc ~ tcool for optically emitting electrons in knots D and E. Synchrotron models fit to the radio-optical data yield νB 1016 Hz for knots D, A, and B, and somewhat lower values, νB ~ 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.

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