A NASA mission has observed a supermassive black hole that is directing its high-energy jet at Earth. Don’t panic yet. As frightening as this cosmic event is, it is located at a very safe distance of about 400 million light-years.
Actively feeding supermassive black holes, including the ones at hand, are surrounded by swirling disks of matter called accretion disks that gradually fuel them over time. Some of the material they do not ingest is then directed towards their poles, where it is subsequently detonated at near-light or relativistic speed. This creates very energetic and bright electromagnetic radiation. In some cases, like with NASA’s latest muse, that plane is pointed directly at Earth. These events are known as blazars.
This blazar, named Markarian 421 and located in the constellation Ursa Major, was observed using NASA’s X-ray Imaging Explorer (IXPE), launched in December 2021. IXPE observes a property of magnetic fields called polarization, which indicates the direction of the fields. The polarization of the jet released by Markarian 421 surprised astronomers, showing that the part of the jet where particles are accelerated is also home to a magnetic field that has a spiral structure.
Blazar Jets can stretch across space millions of light-years away, but the mechanisms that trigger them are not yet well understood. However, these new discoveries surrounding Markarian 421 could shed some light on this extreme cosmic phenomenon.
Related: An X-ray view shows how supermassive black holes accelerate particles in jets
“Markarian 421 is an old friend of high-energy astronomers,” the principal investigator behind the discovery and Italian Space Agency astrophysicist Laura Di Gesso, he said in a statement. “We were sure Blazar would be a useful target for IXPE, but its discoveries were beyond our best expectations, successfully showing how X-ray polarimetry enriches our ability to probe the complex magnetic field geometry and particle acceleration in different regions of the relativistic jets.”
IXPE delves deeper into the twisted hull of Blazar aircraft
The main reason for the brightness of the jets feeding supermassive black holes is that particles approaching the speed of light give off huge amounts of energy and behave according to the physics of Einstein’s theory of special relativity.
Blazar jets also get an extra boost to such brightness because their direction toward us causes wavelengths of light attached to their jets to “pool,” increasing their frequencies and energies. This is similar to how the sound waves from an approaching ambulance siren “rush” in order to create an increase in frequency that makes the sound sharper.
As a result of these two effects, blazars can often outshine the combined light of every star in the galaxies that harbor them. Now, IXPE has used that light to paint a picture of the physics going on at the heart of Markarian 421’s jet and even pinpoint the point of origin of the glowing beam.
Previously, models of blazar jets had hinted that they were accompanied by helical magnetic fields, almost like DNA in living cells, except single-stranded rather than double-stranded. What was not expected, however, was the fact that the magnetic spiral would host regions where particles would be accelerated.
“We expected that the direction of polarization might change, but we thought large cycles would be rare, based on previous optical observations of many blazars,” said co-investigator and MIT physicist Hermann Marshall.
More importantly, analysis of the IXPE data showed that the plane’s polarization dropped to 0% between its first and second observations. This showed the team that the magnetic field was spinning like a corkscrew.
“We realized that the polarization was actually about the same, but its orientation is literally pulling a turn, turning almost 180 degrees in two days,” Marshall said. “Then we were surprised again during the third observation, which began a day later, to note a trend of polarization continuing to rotate at the same rate.”
During these maneuvers, measurements of electromagnetic radiation in the form of optical, infrared, or radio light showed no effect on the stability and structure of the aircraft itself, even when the X-ray emissions changed. This means a shock wave traveling along the twisted magnetic field from Markarian 421.
Hints of such a phenomenon have been seen in another blazar jet witnessed by IXPE, Markarian 501, but the team’s new findings represent even clearer evidence that the spiral magnetic field does indeed contribute to a traveling shock wave that accelerates the jet particles to relativistic velocities.
The team behind the work intends to continue studying Markarian 421 as well as identifying other blazars to find some with similar qualities in the quest to uncover a mechanism that powers the intense, bright outflows that characterize these phenomena.
“Thanks to IXPE, it’s an exciting time for astrophysical jet studies,” de Jessu concluded.
The team’s research is published Monday (July 17) in the journal Nature Astronomy.