A multiwavelength study of multiple spectral component jets in AGN: testing the IC/CMB model for the large-scale-jet X-ray emission

Abstract

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.

Electron acceleration and advection model for the high energy emission from the large scale jet of AGNs

“…The X-ray emission feature, located a few arcsecs away from the quasar core, was previously interpreted as IC/CMB radiation. This source appeared in the Fermi/LAT 4FGL 8-year point source catalog and this data was used to estimate the flux upper limits in gamma rays [19]. The study finds that IC/CMB model is ruled out for interpreting the X-ray emission considering the gamma-ray flux upper limit estimates.…”

“…The resolved X-ray jet of PKS J1421-0643 (at 𝑧 = 3.69) and PSO J030947.49+271757.31 (𝑧 = 6.1) were modelled with the IC/CMB interpretation [17,18]. Recently, Breiding et al [19] examined the IC/CMB interpretation for 45 extra-galactic X-ray jets. They found that the IC/CMB model is questioned in 27 Multi-Spectral Component (MSC) X-ray jets due to over-prediction for the observed MeV-to-GeV 𝛾-ray flux.…”

Theory of Gamma-Ray loud AGNs

“…[75,76] for review. Within more recent years, however, IC-CMB models have been disfavoured in an increasing number of sources as they tend to, e.g., over-predict Fermi-LAT gamma-ray flux limits [77,78] or are not supported by detailed X-ray spectral information [79]. An electron synchrotron origin, however, requires to maintain electrons with Lorentz factors γ e ∼ 10 8 all along the jet.…”

Extragalactic Jets from Radio to Gamma-rays

“…However, through the use of a recombined light-curve analysis, Meyer and Georganopoulos (2014) first showed that the IC/CMB model could be strongly ruled out through gamma-ray upper limits for the X-ray jet of 3C 273, before doing the same in PKS 0637-752 (Meyer et al 2015(Meyer et al , 2017. Peter Breiding then widely applied this test to essentially all the 'multiple spectral component' or MSC jets for his PhD thesis work (Breiding et al 2017(Breiding et al , 2023, finding that 24/45 sources tested would require IC/CMB-predicted GeV emission significantly in violation of the gamma-ray limits (see example SEDs for two of the sources from Breiding et al (2023) in Figure 2).…”