2005ApJ...625..656G

We propose a method for estimating the composition, i.e., the relative amounts of leptons and protons, of extragalactic jets that exhibit X-ray-bright knots in their kiloparsec-scale jets. The method relies on measuring, or setting upper limits on, the component of the cosmic microwave background (CMB) radiation that is bulk Comptonized (BC) by cold electrons in the relativistically flowing jet. These measurements, along with modeling of the broadband knot emission that constrains the bulk Lorentz factor Γ of the jets, can yield estimates of the jet power carried by protons and leptons. We provide an explicit calculation of the spectrum of the BC CMB component and apply these results to PKS 0637-752 and 3C 273, two superluminal quasars with Chandra-detected large-scale jets. What makes these sources particularly suited for such a procedure is the absence of significant nonthermal jet emission in the ``bridge'', the region between the core and the first bright jet knot, which guarantees that most of the electrons there are cold, leaving the BC-scattered CMB radiation as the only significant source of photons in this region. At λ=3.6-8.0 µm, the most likely band to observe the BC-scattered CMB emission, the Spitzer angular resolution (∼1"-3") is considerably smaller than the bridges of these jets (∼10"), making it possible to both measure and resolve this emission.