Simultaneous X-ray and IR variability in the quasar 3C 273

From a combination of high-quality X-ray observations from the NASA Rossi X-ray Timing Explorer (RXTE) and IR observations from the UK Infrared Telescope (UKIRT) we show that the medium-energy X-ray (3-20 keV) and near-IR fluxes in the quasar 3C 273 are highly correlated. It is widely believed that the X-ray emission in quasars like 3C 273 arises from Compton scattering of low-energy seed photons, and our observations provide the first reliable detection of correlated variations in 3C 273 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 wavebands 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 IR leads the X-rays by 0.75 +/- 0.25 d. These observations rule out the 'External Compton' emission process for the production of the X-rays.