By looking deep into the heart of a blazar, astronomers have learned how particles are accelerated to near light speeds, to flee in a jet emanating from near the blazar’s monstrous supermassive black hole.
The blazars are quasars seen from the front; a quasar is an extremely active galactic nucleus (AGN), which is powered by a black hole accreting large amounts of matter. Matter orbits the black hole in an accretion disk, where conditions are so extreme that the disk glows millions of degrees Fahrenheit or Celsius. The tightly intertwined magnetic fields enveloped in the disc are able to evacuate some of the material into tightly collimated jets pulling away from the center of the accretion disk in both directions. The charged particles in these jets spin around the magnetic field lines, emitting what is called synchrotron radiation. It is this radiation that produces most of the light we see shining from quasars and blazars, in which one of the jets points toward Earth.
Now astronomers have used NASA data X-ray Imaging Polarimetry Explore (IXPE), launched in December 2021, to observe Markarian blazar 501, located approximately 456 million light-years from Earth. IXPE is particularly good at observing the polarization of light, which refers to the orientation in which light waves preferentially oscillate. In a blazar, the polarization is influenced by the magnetic field strength and structure of the blazar jet, so IXPE observations can shed light on the magnetic environment of the blazar, which in turn can provide clues as to what is accelerating the particles in the jets.
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If turbulence or instabilities in the jet were energizing the particles, scientists would expect the polarization to be weak and at random angles, indicating a relatively disorganized magnetic field. But previous measurements of polarization at optical and radio wavelengths were only sensitive to the parts of the jet farthest from the black hole, seeing the particles days or even weeks after they accelerated – too late to be conclusive. However, X-rays are produced closer to the source of acceleration and their polarization is indicative of the mechanism that accelerates the particles in the jet.
“How close the X-rays we see to IXPE depends on the spectrum of the source, but in all cases they are very close,” said Ioannis Liodakis, an astronomer at the University of Turku in Finland and lead author of the news. research. , told Space.com.
Markarian 501 observations, made in March 2022, measured the level of X-ray polarization at 10%, about twice what is seen in optical light higher up in the jet, away from the black hole. The X-ray polarization angle was also found to be consistently parallel to the jet near the jet source.
Scientists predicted it; the polarization level and turbulence appear to match the wavelength of the emitted light, with shorter wavelengths closer to the source having higher and more linear polarization than longer wavelengths further away in the jet.
These observations apparently rule out plasma turbulence or instabilities as particle acceleration mechanisms, as these processes would not produce the highly structured magnetic fields necessary for the observed level of X-ray polarization. As a result, the researchers believe that a shock wave in the jet is the most likely mechanism for accelerating the particles to their breathtaking speed.
This leads to another riddle, which caused the shock. “There are several ways to cause 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 environment can lead to the creation of shocks. The second method is to have the plasma move at different speeds, and a shock can be created when a slow region collides with a faster region.”
If the shock theory is correct, scientists predict that at x-ray wavelengths the angle of polarization will rotate; future observations with IXPE may be able to detect these rotations and support the findings of Liodakis’ team.
The new X-ray results, in combination with previous optical and radio polarization measurements from the Markarian 501 jet, show how important observations at multiple wavelengths are to get a full picture of what’s going on. Likewise, earlier this year, the Event Horizon Telescope measured the radio polarization of another blazar jet, finding that it was corkscrew shaped.
Now, with IXPE’s X-ray capabilities, astronomers have the tools to peer into jet blazars and quasars at all scales to better understand how the universe’s ultimate particle accelerators work.
The article was published on Wednesday November 23 in the journal Nature (opens in a new tab).
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