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Over ∼150 resolved, kpc-scale X-ray jets hosted by active galactic nuclei have been discovered with the Chandra X-ray Observatory. A significant fraction of these jets have an X-ray spectrum either too high in flux or too hard to be consistent with the high-energy extension of the radio-to-optical synchrotron spectrum, a subtype we identify as Multiple Spectral Component (MSC) X-ray jets. A leading hypothesis for the origin of the X-rays is the inverse-Compton scattering of the cosmic microwave background by the same electron population producing the radio-to-optical synchrotron spectrum (known as the IC/CMB model). In this work, we test the IC/CMB model in 45 extragalactic X-ray jets using observations from the Fermi Large Area Telescope to look for the expected high level of gamma-ray emission, utilizing observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope (HST) when possible to best constrain the predicted gamma-ray flux. Including this and previous works, we now find the IC/CMB model to be ruled out in a total of 24/45 MSC X-ray jets due to its over-prediction for the observed MeV-to-GeV gamma-ray flux. We present additional evidence against the IC/CMB model, including the relative X-ray-to-radio relativistic beaming in these sources, and the general mismatch between radio and X-ray spectral indexes. Finally, we present upper limits on the large-scale bulk-flow Lorentz factors for all jets based on the Fermi upper limits, which suggest that these jets are at most mildly relativistic.

 

Unexpectedly strong X-ray emission from extragalactic radio jets on kiloparsec scales has been one of the major discoveries of Chandra, the only X-ray observatory capable of sub-arcsecond-scale imaging. 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. The most commonly assumed mechanism is inverse-Compton upscattering of the cosmic microwave background by very low-energy electrons in a still highly relativistic jet. Under this mechanism, no variability in the X-ray emission is expected. Here we report the detection of X-ray variability in the large-scale jet population, using a novel statistical analysis of 53 jets with multiple Chandra observations. 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 1.96 × 10−4 under a Kolmogorov–Smirnov test against the expected uniform (0, 1) distribution. These results strongly imply that the dominant mechanism of X-ray production in kiloparsec-scale jets is synchrotron emission by a second population of electrons reaching multi-teraelectronvolt energies. X-ray variability on the timescale of months to a few years implies extremely small emitting volumes much smaller than the cross-section of the jet. 

Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This view requires a jet that is highly relativistic (bulk Lorentz factor >20–40) on scales of hundreds of kiloparsecs, and a jet that is comparably or more powerful than the black hole’s Eddington luminosity. The second possibility is synchrotron emission from a high-energy population of electrons. This requires a much less powerful jet that does not need to be relativistically beamed, but it imposes other extreme requirements, namely the need to accelerate particles to >100 TeV energies at distances of hundreds of kiloparsecs from the active nucleus. We are exploring these questions using a suite of observations from a diverse group of telescopes, including the Hubble Space Telescope (HST), Chandra X-ray Observatory (CXO), Fermi Gamma-ray Space Telescope and various radio telescope arrays. Our results strongly favor the hypothesis that the X-ray emission is synchrotron radiation from a separate, high-energy electron population. We discuss the observations, results and new questions brought up by these surprising results. We investigate the physical processes and magnetic field structure that may help to accelerate particles to such extreme energies.