Supermassive black holes have been witnessed for the first time ever ejecting high-energy particles in the form of jets into space, and the results are stunning.
Astronomers report on November 23 in Nature that shock waves travelling along the jet of one such blazar distort magnetic fields, speeding up fleeing particles to nearly the speed of light. The investigation of such high rates of acceleration can provide light on many topics in fundamental physics that would otherwise remain unanswered.
From millions to billions of light-years away, blazars shine brightly due to active black holes ejecting high-energy particles in our direction (SN: 7/14/15). The geometry of the magnetic fields surrounding black holes was suspected by astronomers to be related to the jets' extraordinary velocities and narrow columnar beams, but the details were unclear.
In comes the IXPE, an orbiting telescope for x-ray polarimetry, set for launch in December 2021. To determine the orientation of X-ray light as it passes across space is its primary objective. Polarized X-rays can peer into a blazar's active core (SN: 3/24/21) while polarised radio waves and optical light investigated portions of jets days to years after they had been accelerated.
According to astronomer Yannis Liodakis of Finland's University of Turku, "with X-rays, you're actually looking at the heart of the particle acceleration." You're gazing right at the heart of the action!
IPXE observed a blazar named Markarian 501 that was around 450 million light-years away from Earth in March 2022.
There were two primary hypotheses put forth by Liodakis and his coworkers regarding how magnetic fields could speed up the jet on Markarian 501. Magnetic reconnection, in which magnetic field lines break, rebuild, and join to other neighbouring lines, may provide a boost to particles. Sun plasma also undergoes this acceleration process (SN: 11/14/19). If that was the particle accelerator, then light of every wavelength, from radio to X-ray, would be polarised in the same way as it travelled along the jet.
Alternative explanations involve a shock wave that fires particles down the jet. An abrupt transition from turbulence to order can be observed in the magnetic fields at the shock's epicentre. If you flipped that switch, particles may shoot off like water from a hose. A return to turbulence is expected when particles move away from the shock source. For the acceleration to have been caused by a shock, other telescopes' measurements of polarisation should show that shorter-wavelength X-rays are more polarised than longer-wavelength optical and radio radiation.
Researchers found the same thing, according to Liodakis. In his opinion, the shock wave explanation is supported by "a unambiguous outcome," which he says they obtained.
According to astrophysicist James Webb of Florida International University in Miami, further research is needed to determine the finer points of the particle flow. One problem is that we don't know what would cause the shock to occur. However, he acknowledges that "this is a start in the right direction." When we change our perspective and look at the object from a different angle, it's like opening a new window; we notice details we missed previously. It's a thrilling prospect.
CITATIONS
I. Liodakis et al. Polarized blazar X-rays imply particle acceleration in shocks. Nature. Published online November 23, 2022. doi: 10.1038/s41586-022-05338-0.
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