Quasars, along with supernovae and γ-ray bursts, are the most energetic sources of electromagnetic radiation in the Universe. About two billion light-years away, the nearest bright quasar, 3C 273 in the Virgo constellation, emits a powerful radio jet. Markos Georganopoulos et al. propose to use X-ray emission from the jet to investigate its origin (Astrophys. J., in the press).

Although most astronomers believe that a quasar is powered by a supermassive black hole, it's not clear how that black-hole engine lights up a quasar. Information on the energy transport mechanism would reveal how black holes were formed in the early Universe. Similarly, it might explain why there are no such quasars in active galaxies nearby.

There are two main theories for the X-ray emission: inverse external Compton (EC) scattering from relativistic electrons that scatter cosmic microwave background (CMB) photons; and synchrotron emission from TeV electrons — the same mechanism as for radio emission from the jet, but from another population of electrons.

To complicate matters, synchrotron radiation would also scatter CMB photons, so would look similar to the EC model. But as the two candidate processes involve electron energies differing by two orders of magnitude, the two energy scales should lead to different γ-ray dynamics. Georganopoulos et al. have come up with a set of diagnostics to distinguish the two models using existing and future γ-ray detectors.

If no GeV or TeV emission is detected, or only low-level GeV, there would be no additional constraint for the synchrotron model but the EC model would lose support. Detection of high-level GeV or TeV emission would confirm the synchrotron model. Although the authors believe that the latter is the most likely outcome, they acknowledge that future observations could refute both hypotheses.