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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.

From a combination of high quality X-ray observations from the NASA Rossi X-ray Timing Explorer (RXTE), and infrared observations from the UK Infrared Telescope (UKIRT) we show that the medium energy X-ray (3-20 keV) and near infrared fluxes in the quasar 3C273 are highly correlated. It is widely believed that the X-ray emission in quasars like 3C273 arises from Compton scattering of low energy seed photons and our observations provide the first reliable detection of correlated variations in 3C273 between the X-ray band and any lower energy band. For a realistic electron distribution we demonstrate that it is probable that each decade of the seed photon distribution from the mm to IR waveband contributes roughly equally to the medium energy X-ray flux. However the expected mm variations are too small to be detected above the noise, probably explaining the lack of success of previous searches for a correlation between X-ray and mm variations. In addition we show that the infrared leads the X-rays by 0.75+/-0.25 days. These observations rule out the `External Compton' emission process for the production of the X-rays., 7 pages, 5 figures. Uses mn style file and epsf. Accepted for publication in the MNRAS

ABSTRA C T We present details of a large X-ray flare in the blazar 3C 279 detected during a 3-week period of daily observations by the satellite RXTE in early 1996. The flare lasted for a total of 7 d. The shape of the flare is well described by a symmetrical, exponential rise and fall with e-folding time-scales in each case of 1.1 d. The peak measured flux is three times the quiescent level. The flare is superimposed on a well-defined quiescent level and appears to represent a separate emission component. There is no statistically significant variability of the X-ray spectral index during the observations, but the errors are large and the data hint at a hardening of about 0.1 at the peak of the flare. Such a hardening is required to allow the X-ray flare spectrum to join on, in a single power law, to the g-ray flare that occurred at the same time. The exponential rise and decay most likely correspond to a variation in the acceleration of relativistic electrons or in the flux of seed photons, if the latter do not originate in the jet, as the energy loss time-scales are shorter than the rise and decay time-scales and the flare profile does not match that expected if light-travel delays determine the light curve.