Determining the Composition of Relativistic Jets from Polarization Maps

Abstract

Abstract We present a stationary, axisymmetric, self-similar, semianalytic model of magnetically dominated jet plasma based on force-free regions of a relativistic magnetohydrodynamic simulation. We use this model to illustrate how the composition of relativistic jet plasma can be determined, with special attention to the example of M87. In particular, we compute synthetic Stokes maps in e − e + p plasmas with various positron-to-proton ratios using synchrotron emission models to scale the partial pressure of electrons and positrons that emit at the observed frequency to the magnetic pressure, taking into account Faraday rotation and conversion. The lepton-dominated models produce bilaterally asymmetric radio intensity profiles with strong linear polarization and Stokes Q and U maps that are bilaterally asymmetric (but strongly correlated across the jet axis) and antisymmetric (and sometimes anticorrelated), respectively. The hadronic models produce more centrally brightened intensity and polarization maps. Circular polarization provides the cleanest observational tool for distinguishing the plasmas, as it increases outward from the jet core and central axis for highly ionic plasma, and is suppressed for pair-dominated plasma. We find a measurable degree of circular polarization V/I of for subequipartition hadronic jet plasmas averaged over milliarcsecond scales. Our stationary model predicts that the intensity-normalized autocorrelation functions of Q and U increase and decrease with frequency, respectively, for higher plasma betas in our parameter survey. On the other hand, the autocorrelation of V is sensitive to the frequency at lower betas. Multiband polarimetric observations could therefore be used as a novel probe of the composition of jet plasma.