  The X-ray emission from quasars such as 3C273 is generally agreed to arise
from Compton scattering of low energy seed photons by relativistic electrons in
a relativistic jet oriented close to the line of sight. However there are a
number of possible models for the origin of the seed photons. In Paper I
(McHardy et al 1999) we showed that the X-ray and IR variability from 3C273 was
highly correlated in 1997, with the IR flux leading the X-rays by ~0.75 +/-
0.25 days. The strong correlation, and lag, supports the Synchrotron
Self-Compton (SSC) model, where the seed photons are synchroton photons from
the jet itself. The previous correlation was based on one moderately well
sampled flare and another poorly sampled flare, so the possibility of chance
correlated variability exists. Here we report on further X-ray and IR
observations of 3C273 which confirm the behaviour seen in Paper I. During a 2
week period of observations we see a flare of amplitude ~25%, lasting for ~5
days, showing a high correlation between IR and X-ray variations, with the
X-rays lagging by ~1.45+/- 0.15 days. These observations were not scheduled at
any special time, implying that the same mechanism - almost certainly SSC -
dominates the X-ray emission on most occasions and that the structure of the
emission region is similar in most small flares.
