Variability of extragalactic X-ray jets on kiloparsec scales

Super-massive black holes residing at the centres of galaxies can launch powerful radio-emitting plasma jets which reach scales of hundreds of thousands of light-years, well beyond their host galaxies. The advent of Chandra, the only X-ray observatory capable of sub-arcsecond-scale imaging, has lead to the surprising discovery of strong X-ray emission from radio jets on these scales. The origin of this X-ray emission, which appears as a second spectral component, separate from the radio synchrotron emission, has been debated for over two decades. The most commonly assumed mechanism is inverse Compton upscattering of the Cosmic Microwave Background (IC-CMB) by very low-energy electrons in a highly relativistic jet. Under this mechanism, we expect the the X-ray emission to be non-variable. We report here the detection of X-ray variability in the large-scale jet population, using a novel statistical analysis of 54 jets with multiple Chandra observations. Individually 13/54 jets have at least one feature which is variable at the p<0.05 level. Taken as a population, we find that the distribution of p-values from a Poisson model is strongly inconsistent with steady emission, with a global p-value of 9.4×10-4 under a Kolmogorov-Smirnov test against the expected Uniform (0,1) distribution. The inconsistency significantly increases when we exclude high-redshift core-dominated quasars and jet regions with high background emission.

These results strongly imply that the dominant mechanism of X-ray production in kpc-scale jets is synchrotron emission by a second population of electrons reaching multi-TeV energies. X-ray variability on the time-scale of months to a few years implies extremely small emitting volumes much smaller than the cross-section of the jet. Simple Monte Carlo simulations of the X-ray jet population suggest a wide range of possible values for the fraction of variable sources and the average amplitude of the flux variations.