X-ray Variability of Jets on kpc scales — Results of a comprehensive Chandra archival survey

One of the major achievements of the Chandra observatory has been the surprise discovery of strong X-ray emission from radio jets on scales of tens to 100s of kiloparsecs. The origin of this X-ray emission, which appears as a second spectral component from that of the radio emission, has been debated for over two decades, with the most commonly assumed mechanism being inverse Compton upscattering of the Cosmic Microwave Background (IC-CMB) by very low-energy electrons in a still highly relativistic jet. Under this mechanism no variability in the X-ray emission is expected. Significant X-ray variability on large scales has been observed in three well-studied jets (M87, Cen A, and Pictor A) but two of these are likely sampling the tail of the radio synchrotron spectrum, leaving only the 4-sigma detection of years-long variability in Pictor A as a serious challenge to the steady IC-CMB model. Here we report the detection of ubiquitous variability in the large-scale jet population, using a novel statistical analysis of 54 jets with multiple Chandra observations. Taken as a population, we find that the distribution of p-values for the observed counts over time in 150 independent jet regions are strongly inconsistent with steady emission, and instead imply that jets are commonly variabile at the tens of percent level over timescales of months to a few years. These results suggest 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.