A Quiet Quasar Jet: X-Ray Portrait of the Inner Jet ✛

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A spotlight beam cuts through the night fog. Part of the light we see comes directly from the lamp, and part from water droplets that scatter that light. The inner jet of quasar PKS 0637-752 works similarly, only instead of fog there is the cosmic microwave background left over from the Big Bang, and instead of a spotlight there is a stream of electrons moving at nearly the speed of light.

For a long time it remained unclear why the faint inner jet, located just 3.4–7 arcseconds from the core, shines in X-rays. For the bright outer knots of this jet, gamma-ray limits effectively ruled out inverse Compton scattering of the cosmic microwave background: if fast electrons were giving energy to background photons, Fermi would have detected too many gamma rays. But the inner part remained without a detailed model. The astrophysicists took archival Chandra observations from 1999 and 2017, subtracted the bright core's contribution, and built the first quantitative model of the emission from this region.

The model resembles a uniform cylinder filled with relativistic electrons. These particles emit synchrotron radiation in the radio band as they spiral in the magnetic field, and Compton radiation in X-rays when they collide with cosmic microwave background photons. The authors tested three scenarios with different maximum electron energies. The first describes radio and X-rays well but fails to explain the millimeter emission. The second reproduces all three bands but predicts too many gamma rays. The third scenario agrees with all data if inverse Compton scattering accounts for only about 22% of the X-ray flux, with the rest coming from synchrotron radiation of a separate electron population.

The derived parameters: median viewing angle 4–6 degrees, Lorentz factor 6–10, magnetic field 9–15 microgauss. This means the jet is pointed almost at us, but not exactly, and moves at tremendous speed. Previously it was unclear whether Compton scattering of the cosmic microwave background could explain the glow of the faint inner jet, since for the bright outer knots this mechanism is already ruled out. Now it is clear: in the inner part, a hybrid mechanism is possible, where different zones emit in different ways.

The spotlight beam in the fog turned out to be double: part of the light reflects off droplets, part originates in the lamp itself. Likewise, the quasar's inner jet: its X-ray glow combines Compton scattering of the background and synchrotron radiation from another electron population. The Bayesian method for determining the viewing angle with Doppler selection can be applied to other quasars, and the derived parameters are useful for models of particle acceleration and calculations of jet kinetic power. There is no direct application today, but understanding how these giant cosmic spotlights work is gradually coming into focus.