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
  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, ultraviolet
and X-ray lightcurves of HST-1 to constrain the source size and the energy loss
mechanisms affecting the relativistic electron distributions. HST-1 is the
first jet knot clearly resolved from the nuclear emission by Chandra and is the
site of the huge flare of 2005. We find clear evidence for a
frequency-dependent decrease in the synchrotron flux being consistent with
E-squared energy losses. Assuming that this behavior is predominantly caused by
synchrotron cooling, we estimate a value of 0.6 mG for the average magnetic
field strength of the HST-1 emission region, a value consistent with previous
estimates of the equipartition field. In the process of analyzing the first
derivative of the X-ray light curve of HST-1, we discovered a quasi-periodic
oscillation which was most obvious in 2003 and 2004 prior to the major flare in
2005. The four cycles observed have a period of order 6 months. The second goal
is to search for evidence of differences between the X-ray variability
timescales of HST-1 and the unresolved nuclear region (diameter <0.6"). These
features, separated by more than 60 pc, are the two chief contenders for the
origin of the TeV variable emissions observed by HESS in 2005 and by MAGIC and
VERITAS in 2008. The X-ray variability of the nucleus appears to be at least
twice as rapid as that of the HST-1 knot. However, the shortest nuclear
variability timescale we can measure from the Chandra data (<= 20 days) is
still significantly longer than the shortest TeV variability of M87 reported by
the HESS and MAGIC telescopes (1-2 days).
