Simultaneous X-ray and infrared variability in the quasar 3C273 - II. Confirmation of the correlation and X-ray lag

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 (M c Hardy et al.), we showed that the X-ray and infrared (IR) variability from 3C273 was highly correlated in 1997, with the IR flux leading the X-rays by ∼0.75 ± 0.25 d. The strong correlation, and lag, supports the synchrotron self-Compton (SSC) model, where the seed photons are synchrotron 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 per cent, lasting for ∼5 d, showing a high correlation between IR and X-ray variations, with the X-rays lagging by ∼1.45 ± 0.15 d. 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.