The Low End of the Electron Spectrum in Relativistic Jets

There are two problems related to the low end of the relativistic electron spectrum in extragalactic jets that can be addressed with low radio frequency observations with arcsec resolution. The first is a test of the beamed inverse Compton model (IC/CMB) for the X-rayemission from quasar jets. Synchrotron emitting electrons with Lorentz factors of gamma in the range of 50-400 are those responsible for IC/CMB X-ray emission so it is generally necessary to make a blind extrapolation to very low radio frequencies from the arcsec resolution data predominantly obtained at cm wavelengths. The second issue relates to predictions of shock acceleration models that suggest there could be a low energy break in the electron distribution around gamma ~ 2000, i.e. at an energy where the electron momentum is comparable to that of a proton. Intensities and spectral shapes near 100 MHz will significantly reduce the required extrapolation and provide evidence for or against the conflicting expectations: a single power law extending down to gamma ~ 100 versus a break near gamma=2000. In order to interpret the observations that will be forthcoming fromLOFAR and eventually with SKA, it is necessary to understand the uncertainties affecting the determination of the magnetic field strength in the emitting region, the key factor in associating a frequency with gamma. In this contribution we examine the factors involved in computing the equipartition field and argue that invoking parameters that minimize B(eq) will provide an upper limit to gamma corresponding to an observed frequency. We briefly describe a LOFAR observation scheduled for May 2014 which will represent a first step in constraining the low end of the electron spectrum. This work was partially supported by NASA grant GO3-14106X.