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We investigate the variability timescales in the jet of M87 with two goals.The first is to use the rise times and decay times in the radio, ultravioletand X-ray lightcurves of HST-1 to constrain the source size and the energy lossmechanisms affecting the relativistic electron distributions. HST-1 is thefirst jet knot clearly resolved from the nuclear emission by Chandra and is thesite of the huge flare of 2005. We find clear evidence for afrequency-dependent decrease in the synchrotron flux being consistent withE-squared energy losses. Assuming that this behavior is predominantly caused bysynchrotron cooling, we estimate a value of 0.6 mG for the average magneticfield strength of the HST-1 emission region, a value consistent with previousestimates of the equipartition field. In the process of analyzing the firstderivative of the X-ray light curve of HST-1, we discovered a quasi-periodicoscillation which was most obvious in 2003 and 2004 prior to the major flare in2005. The four cycles observed have a period of order 6 months. The second goalis to search for evidence of differences between the X-ray variabilitytimescales of HST-1 and the unresolved nuclear region (diameter <0.6"). Thesefeatures, separated by more than 60 pc, are the two chief contenders for theorigin of the TeV variable emissions observed by HESS in 2005 and by MAGIC andVERITAS in 2008. The X-ray variability of the nucleus appears to be at leasttwice as rapid as that of the HST-1 knot. However, the shortest nuclearvariability timescale we can measure from the Chandra data (<= 20 days) isstill significantly longer than the shortest TeV variability of M87 reported bythe HESS and MAGIC telescopes (1-2 days).

Variability Timescales in the M87 Jet: Signatures of E-Squared Losses, Discovery of a Quasi-period in HST-1, and the Site of TeV Flaring