The outburst of HST-1 in the M87 jet

The X-ray intensity of knot HST-1, 0.85 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.