Fermi non-detections of four X-ray jet sources and implications for the IC/CMB mechanism

TL;DR

This study uses Fermi, HST, and ALMA observations to test the IC/CMB model for X-ray jets in quasars, finding that the model's predictions are inconsistent with observed gamma-ray and optical data, challenging its validity.

Contribution

The paper provides new gamma-ray upper limits for four quasar jets and demonstrates these limits contradict the IC/CMB emission model, offering strong evidence against it.

Findings

Fermi gamma-ray upper limits violate IC/CMB predictions

HST detects optical jet in PKS 2209+080

ALMA data constrains spectral shape, ruling out IC/CMB

Abstract

Since its launch in 1999, the Chandra X-ray observatory has discovered several dozen X-ray jets associated with powerful quasars. In many cases the X-ray spectrum is hard and appears to come from a second spectral component. The most popular explanation for the kpc-scale X-ray emission in these cases has been inverse-Compton (IC) scattering of Cosmic Microwave Background (CMB) photons by relativistic electrons in the jet (the IC/CMB model). Requiring the IC/CMB emission to reproduce the observed X-ray flux density inevitably predicts a high level of gamma-ray emission which should be detectable with the Fermi Large Area Telescope (LAT). In previous work, we found that gamma-ray upper limits from the large scale jets of 3C 273 and PKS 0637-752 violate the predictions of the IC/CMB model. Here we present Fermi/LAT flux density upper limits for the X-ray jets of four additional sources: PKS 1136-135, PKS 1229-021, PKS 1354+195, and PKS 2209+080, and show that these limits violate the IC/CMB predictions at a very high significance level. We also present new Hubble Space Telescope (HST) observations of the quasar PKS 2209+080 showing a newly detected optical jet, and Atacama Large Millimeter/submillimeter Array (ALMA) band 3 and 6 observations of all four sources, which provide key constraints on the spectral shape that enable us to rule out the IC/CMB model.