APJ, ACCEPTED Preprint typeset using LATEX style emulateapj v. 6/22/04 THE OUTBURST OF HST-1 IN THE M87 JET

The X-ray intensity of knot HST-1, 085 from the nucleus of the radio galaxy M87, has increased by more than a factor of 50 during the last 5 yr. The optical increase is similar, and our more limited radio data indicate a commensurate activity. We give the primary results of our Chandra X-Ray Observatory monitoring program and consider some of the implications of this extreme variability in a relativistic jet. We find that the data support a "modest beaming synchrotron" model as indicated in our earlier papers. Based on this model, the decay of the X-ray light curve appears to be dominated by the light-travel time across the emitting region of HST-1, rather than synchrotron loss timescales.