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E. S. Perlman, J. A. Biretta, W. B. Sparks, F. D. Macchetto (STScI)
We present results from February 1998 HST NICMOS and WFPC2 observations of M87 which yield the first single-epoch spectrum of the jet from 0.3-2 microns, at <0.1'' resolution. These results are analyzed in the light of recent findings that several regions of the M87 jet are generating superluminal components, with speeds ranging from 1-6c (Biretta et al. 1998). Both spectral index maps, as well as spectra of individual jet regions from aperture photometry, are presented.
Our results differ from previous results on the optical-near-IR spectrum of the jet in that for the first time, a large variety of spectral slopes are seen thanks to the high resolution of HST. The flattest spectra are found in the two inner jet knots (D-East and HST-1) which contain the fastest superluminal components. The optical-near-IR spectral indices in these regions (\alpha = 0.6, where F\nu \propto \nu-\alpha) are comparable to their radio-to-optical spectral indices, implying that the peak frequency of synchrotron radiation for these two components must be considerably in excess of 1015 Hz. Other knots tend to have steeper optical-near-IR spectra (\alpha = 0.8 - 1.1) than their radio-optical spectral indices, indicating slow curvature of the spectra. No evidence for sharp spectral breaks are found for any jet component; if present, they must occur at \nu < 1014 Hz or at \nu > 1015 Hz. Interestingly, even for most regions of the jet where some steepening is implied, the \nu L\nu is still increasing. Our data do not require steep synchrotron cutoffs as predicted by previous models of the jet's spectrum (e.g. Meisenheimer et al. 1996). We explore the impact of our results on models of synchrotron radiation from extragalactic jets. In addition, we re-examine hypotheses for the origin of the X-ray emission from the jet.