Peering deep into the heart of a blazar, astronomers have learned how particles are being accelerated to close to the speed of light, to race away in a jet emanating from near the blazar's monstrous supermassive black hole.
Blazars are quasars seen head-on; a quasar is an extremely active galactic nucleus (AGN), which is powered by a black hole accreting vast amounts of matter. The matter circles around the black hole in an accretion disk, where conditions are so extreme that the disk shines at millions of degrees Fahrenheit or Celsius. Tightly entwined magnetic fields wrapped up in the disk are able to funnel away some of the material in tightly collimated jets shooting away from the center of the accretion disk in either direction. The charged particles in these jets spiral around the magnetic field lines, emitting something called synchrotron radiation. It's this radiation that produces most of the light that we see shining from quasars and from blazars, in which one of the jets points toward Earth.
Now, astronomers have used NASA's Imaging X-ray Polarimetry Explorer (IXPE) satellite, which launched in December 2021, to observe the blazar Markarian 501, which is located about 456 million light-years away from Earth. IXPE is particularly talented at observing the polarization of light, which refers to the orientation in which light waves are seen to preferentially oscillate. In a blazar, the polarization is influenced by the strength and structure of the blazar jet's magnetic field, so IXPE's observations can shed light on the blazar's magnetic environment, which in turn can provide clues as to what's accelerating the particles in the jets.
If turbulence or instabilities in the jet were giving energy to the particles, scientists would expect the polarization to be weak and at random angles, indicating a relatively disorganized magnetic field. But previous measurements of the polarization at optical and radio wavelengths have only been sensitive to the parts of the jet farther away from the black hole, seeing the particles days or even weeks after they have been accelerated — too late to be conclusive. However, X-rays are produced closer to the source of acceleration, and their polarization is indicative of whatever mechanism is accelerating the particles in the jet.
"How close the X-rays that we see with IXPE are depends on the spectrum of the source, but in any case they are very close," Ioannis Liodakis, an astronomer at the University of Turku in Finland and lead author on the new research, told Space.com.
The Markarian 501 observations, made in March 2022, measured the level of X-ray polarization to be 10%, which is about twice as much as seen in optical light farther up the jet, away from the black hole. The X-ray polarization angle was also shown to be consistently parallel to the jet close to the jet's source.
Scientists had predicted this; the level and turbulence of polarization seems to correspond to the wavelength of light emitted, with shorter wavelengths closer to the source having higher, more linear polarization than the longer wavelengths farther out in the jet.
This observations seemingly rule out turbulence or plasma instabilities as mechanisms for accelerating the particles because these processes would not produce the highly structured magnetic fields needed for the observed level of X-ray polarization. As a result, the researchers think that a shock wave in the jet is the most likely mechanism for accelerating the particles to their eye-watering speeds.
That leads to another puzzle, what caused the shock. "There are a few ways to make a shock in the jet," Liodakis told Space.com. "Based on our limited understanding, [we think] two ways are common. The first has to do with environmental reasons — changes in pressure and density of the external medium can lead to the creation of shocks. The second way is to have plasma moving at different velocities, and a shock can be created when a slow region collides with a faster one."
If the shock theory is correct, scientists predict that at X-ray wavelengths, the polarization angle will rotate; future observations with IXPE may be able to detect these rotations and support Liodakis' team's conclusions.
The new X-ray results, in combination with the previous optical and radio polarization measurements of Markarian 501's jet, show how important multi-wavelength observations are for getting a complete picture of what is going on. Similarly, earlier this year, the Event Horizon Telescope measured the radio polarization of another blazar jet, finding it to be corkscrew-shaped.
Now, with IXPE's X-ray capabilities, astronomers have the tools to scrutinize blazar and quasar jets at all scales to better understand how the universe's ultimate particle accelerators function.
The paper was published Wednesday (Nov. 23) in the journal Nature.