The Optical-Near-IR Spectrum of the M87 Jet From HST Observations

Abstract

We present 1998 HST observations of M87 which yield the first single-epoch optical and radio-optical spectral index images of the jet at $0.15''$ resolution. We find $<α_{ro} > \approx 0.67$, comparable to previous measurements, and $<α_o > \approx 0.9$ ($F_ν\propto ν^{-α}$), slightly flatter than previous workers. Reasons for this discrepancy are discussed. These observations reveal a large variety of spectral slopes. Bright knots exhibit flatter spectra than interknot regions. The flattest spectra ($α_o \sim 0.5-0.6$; comparable to or flatter than $α_{ro}$) are found in two inner jet knots (D-East and HST-1) which contain the fastest superluminal components. In knots A, B and C, $α_o$ and $α_{ro}$ are essentially anti-correlated. Near the flux maxima of knots HST-1 and F, changes in $α_{ro}$ lag changes in $α_o$, but in knots D and E, the opposite relationship is observed. This is further evidence that radio and optical emissions in the M87 jet come from substantially different physical regions. The delays observed in the inner jet are consistent with localized particle acceleration, with $t_{acc} << t_{cool}$ for optically emitting electrons in knots HST-1 and F, and $t_{acc} \sim t_{cool}$ for optically emitting electrons in knots D and E. Synchrotron models yield $ν_B \gsim 10^{16}$ Hz for knots D, A and B, and somewhat lower values, $ν_B \sim 10^{15}- 10^{16}$ Hz, in other regions. If X-ray emissions from knots A, B and D are co-spatial with optical and radio emission, we can strongly rule out the ``continuous injection'' model. Because of the short lifetimes of X-ray synchrotron emitting particles, the X-ray emission likely fills volumes much smaller than the optical emission regions.