Bulk Comptonization of the Cosmic Microwave Background by Extragalactic Jets as a Probe of Their Matter Content

We propose a method for estimating the composition, i.e. the relative amountsof leptons and protons, of extragalactic jets which exhibit X-ray bright knotsin their kpc scale jets. The method relies on measuring, or setting upperlimits on, the component of the Cosmic Microwave Background (CMB) radiationthat is bulk-Comptonized by cold electrons in the relativistically flowing jet.These measurements, along with modeling of the broadband knot emission thatconstrain the bulk Lorentz factor of the jets, can yield estimates of the jetpower carried by protons and leptons. We provide an explicit calculation of thespectrum of the bulk-Comptonized (BC) CMB component and apply these results toPKS 0637--752 and 3C 273, two superluminal quasars with Chandra-detected largescale jets. What makes these sources particularly suited for such a procedureis the absence of significant non-thermal jet emission in the `bridge', theregion between the core and the first bright jet knot, which guarantees thatmost of the electrons are cold there, leaving the BC scattered CMB radiation asthe only significant source of photons in this region. At lambda=3.6-8.0microns the most likely band to observe the BC scattered CMB emission, theSpitzer angular resolution (~ 1''-3) is considerably smaller than the `bridges'of these jets (~10''), making it possible to both measure and resolve thisemission.Comment: to appear in the Ap