Surprise Discovery of an X-Ray Jet

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Accreting, supermassive black holes that reside at galactic centers can power enormous jets, bright enough to be observed from vast distances away. The recent discovery of such a jet in X-ray wavelengths, without an apparent radio counterpart, has interesting implications for our understanding of how these distant behemoths shine.

An Excess of X-Rays

Quasar B3 0727+409 was serendipitously discovered to host an X-ray jet when a group of scientists, led by Aurora Simionescu (Institute of Space and Astronautical Sciences of the Japan Aerospace Exploration Agency), was examining Chandra observations of another object.

The Chandra data reveal bright, compact, extended emission from the core of quasar B3 0727+409, with a projected length of ~100 kpc. There also appears to be further X-ray emission at a distance of ~280 kpc, which Simionescu and collaborators speculate may be the terminal hotspot of the jet.

The quasar is located at a redshift of z=2.5 — which makes this jet one of only a few high-redshift X-ray jets known to date. But what makes it especially intriguing is that, though the authors searched through both recent and archival radio observations of the quasar, the only radio counterpart they could find was a small feature close to the quasar core (which may be a knot in the jet). Unlike what is typical of quasar jets, there was no significant additional radio emission coinciding with the rest of the X-ray jet.

Making Jets Shine

Inverse-Compton CMB models

X-ray-to-radio flux ratio vs. redshift, for X-ray quasar jets detected with Chandra. B3 0727+409 is shown in red (with and without the radio knot). The curves represent inverse-Compton scattering models with different magnetic field strengths. [Simionescu et al. 2016]

What does this mean? To answer this, we must consider one of the outstanding questions about quasar jets: what radiation processes dominate their emission? One process possibly contributing to the X-ray emission is inverse-Compton scattering of low-energy cosmic microwave background (CMB) photons off of the electrons in the jet; these photons can scatter up to X-ray energies.

Interestingly, there’s a testable prediction associated with this mechanism. If this process dominates the X-ray emission of quasar jets, then the X-ray-to-radio flux ratio of the jet would increase with redshift as (1+z)4, due to the increased density of CMB photons at higher redshift.

Thus far, our limited detections of high-redshift X-ray quasars have made it difficult to test this prediction, but quasar B3 0727+409 provides an extremely useful data point. When the authors model the radio-to-X-ray flux ratio for the jet, they find that it’s entirely consistent with the inverse-Compton scenario.

This discovery suggests that the inverse-Compton mechanism may indeed be what dominates the X-ray radiation from jets like this one. And since our current observing strategies focus on Chandra follow-up of known bright radio jets, this could mean that there is an entire population of similar systems — with bright X-ray and faint radio emission — that we have missed!

Citation

A. Simionescu et al 2016 ApJ 816 L15. doi:10.3847/2041-8205/816/1/L15

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