Polarized X-rays are helping astronomers take a closer look at blazars’ “plasma guns,” the particle jets powered by supermassive black holes.

Blazar jet interior
This illustration shows the IXPE spacecraft (at right), observing the blazar Markarian 501 (at left). The inset shows high-energy particles coursing down the blazar's plasma jet (blue). When they hit the shock wave, depicted as a white bar, the particles become energized. They emit photons of decreasing energy (X-rays, then visible light, then infrared and radio) as they move away from the shock. Streamlines show the magnetic field, which becomes more chaotic further downstream.
Pablo Garcia (NASA / MSFC)

At the heart of most large galaxies is a potential plasma gun. When such a galaxy’s central supermassive black hole feeds, it fires energetic particles out along its poles, and as those particles flit around magnetic fields, they emit light across the electromagnetic spectrum. That light becomes especially bright when we’re looking down the barrel of the gun, as in the case of blazars.

Astronomers think magnetic fields must be the gunpowder that fires up these particles, but they’ve long debated how exactly that acceleration happens. Now, a new eye in the sky is helping them get a closer look.

NASA’s Imaging X-ray Polarimetry Explorer (IXPE), which launched late last year, has a unique talent for detecting the polarization of X-rays. All light can be polarized, meaning that the waves vibrate in a preferred direction. And while most cosmic sources produce unpolarized light, some things, such as magnetic fields, can align the waves.

But while we’ve measured the polarization of light from blazars before, we’ve only done so at longer visible-light, infrared, and radio wavelengths. As particles shoot down the barrel, emitting light along the way, they lose energy, so that longer-wavelength light comes from particles shot out days to years ago. More energetic X-rays, on the other hand, are emitted much closer to the acceleration site.

Using IXPE, Ioannis Liodakis (University of Turku, Finland) and colleagues for the first time measured the polarization of X-rays coming from Markarian 501, a blazar in Hercules. While taking the IXPE observations, the team also simultaneously monitored the blazar at visible and radio wavelengths.

The blazar’s X-rays turned out to have twice the polarization of the visible light, which in turn is more polarized than the radio waves. That sequence is exactly what's expected if a stationary shock were acting as this blazar’s gunpowder, accelerating particles down the jet, the team reports in Nature.

The polarization also indicates that the front of the shock is perpendicular to the jet, like a dam in a river. “Particles ‘surf’ along the shock front, crossing it many times as they gyrate about the magnetic field lines,” says team member Alan Marscher (Boston University).

The results rule out other possible means of acceleration, such as a reordering of the magnetic field or instabilities in the jet stream. But the result isn’t completely clear-cut. IXPE took two sets of observations about two weeks apart, and the results remained surprisingly steady. Turbulence in the flow of plasma entering the shock ought to create variations over just a few days, so the lack of variability suggests that tweaks are needed for the researchers’ detailed physical model. The overall picture it provides, however, appears to be bang on.

“Liodakis and colleagues’ multi-wavelength polarimetric data provide clear evidence of the particle-acceleration mechanism in Markarian 501,” writes Lea Marcotulli (Yale University) in an accompanying perspective piece in Nature. “This huge leap forward brings us yet another step closer to understanding these extreme particle accelerators, the nature of which has been the focus of much research since their discovery.”




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