8.4. The X-Ray Hotspots
The ROSAT HRI image of Cygnus A has also revealed an excess of emission from the vicinity of the radio hotspots in both the northern and southern lobes (Harris et al. 1994a). Harris et al. show that these X-ray hotspots are compact and spatially coincident with the radio hotspots. The Galactic absorption corrected X-ray flux between 0.5 keV and 2 keV is 6.6 x 10-14 ergs cm-2 sec-1 for hotspot A, and 9.8 x 10-14 ergs cm-2 sec-1 for hotspot D.
Harris et al. consider, and reject, both thermal emission and an extrapolation of the radio synchrotron spectrum to very high energies for the X-ray hotspots. They conclude that the X-ray emission is most likely to be synchrotron self-Compton (SSC) radiation, i.e. up-scattering by the relativistic electrons of their own synchrotron radio photons. The combination of SSC emissivity and synchrotron emissivity allows them to derive the total energy density in relativistic particles and the energy density in the magnetic field independent of minimum energy assumptions. They calculate magnetic field strengths of 200 µG for the hotspot regions, close to the minimum energy value. They then argue that the agreement between minimum energy and SSC fields both supports the SSC interpretation for the X-ray hotspots, and suggests that conditions in the hotspots are close to equipartition. These conclusions hold for a small value of k. If we assume that the pressures are dominated by relativistic protons, e.g. k 20 (Böhringer et al. 1993), then the minimum energy fields increase by a factor 2.5, pushing them significantly higher than the SSC value.
Lastly, detecting SSC emission from the hotspots in Cygnus A provides independent evidence for a population of relativistic electrons, thereby confirming the original suggestion that the radio emission is synchrotron radiation (Shklovskii 1963) - a conclusion previously based solely on the high fractional polarizations and non-thermal power-law spectra observed in the radio.