Dark matter may be magnifying a black hole's jets — and explaining the source of cosmic neutrinos

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Astronomers have discovered that a jet of plasma erupting from a distant supermassive black hole at near-light-speed is being gravitationally lensed by an unseen clump of dark matter. The discovery could tell us about the source of cosmic "ghost" particles called neutrinos.

The supermassive black hole in question is powering a type of quasar called a blazar. All quasars involve supermassive black holes surrounded by vast quantities of material upon which the black holes feed. Material that isn't consumed by these feasting cosmic titans is channeled to the poles of the black holes, from where it is blasted out as plasma jets. A blazar differs from quasars in general because the jets it blasts out are directed at Earth. Blazars, like the one central to this study, designated PKS 2233-148, have long been proposed to be the cosmic particle accelerators that blast out neutrinos.

Neutrinos get their ghostly nickname because they carry no charge and very little mass, meaning that 100 trillion of them can pass through your body every second without you noticing a thing. This makes them hard to both detect and trace back to a source. This gravitationally lensed blazar could assist in that hunt, finally proving blazars are filling the universe with cosmic ghosts.

"These large-scale jets are cosmic accelerators and might be generating neutrinos," Silke Britzen at the Max Planck Institute for Radio Astronomy in Germany told Phys.org. "We study them to search for any peculiarities which might help us to gain a better understanding of neutrino emission."

Blazar jet receives a cosmic course correction

Astronomers have long been interested in PKS 2233-148, identifying this blazar as one that could solidify the link between neutrinos and the jets of feeding supermassive black holes. That is because one of its polar jets is aligned along our line of sight from Earth.

Britzen and colleagues took a new look at observations of PKS 2233-148 made by the Very Long Baseline Array (VLBA) on Earth, the Fermi space telescope , which viewed the blazar in gamma rays, and the X-ray instrument aboard the Swift space observatory.

An illustration of the Swift spacecraft which observed PKS 2233-148 in X-rays. (Image credit: NASA) This revealed how the motion of the jet of PKS 2233-148 changed over time, uncovering hitherto hidden details, including the fact that this blazar jet had been suddenly shifted from its expected path.

"We are very happy to have discovered as-yet-undetected phenomena in the jet, as well as in the gamma-ray light curve," Britzen said.

What could have caused this cosmic course correction? A phenomenon first predicted by Albert Einstein back in 1915 and an unseen cluster of the universe's strangest stuff, dark matter .

What is gravitational lensing and how can dark matter cause it?

The concept of gravitational lensing emerged from Einstein's theory of gravity, general relativity . General relativity states that objects with matter warp the very fabric of space, much like a bowling ball placed on a stretched rubber sheet. Gravity arises from that curvature.

And just as a cannonball would dent that hypothetical rubber sheet more than a bowling ball, an object with greater mass would cause a more extreme warp in space; its gravitational influence is greater.

Something cool happens when light passes this warped space; its usually straight path is curved. This means that when light from a background source passes a massive foreground object acting as a gravitational lens, it arrives at our telescopes at different times, meaning that the background source is magnified or, in extreme cases, can appear in multiple places in the same image.

In this case, it is the jet of PKS 2233-148 that is being lensed. But there is no object of great mass, like a galaxy or galaxy cluster, that can be seen in the right position to be doing the lensing. That leaves the possibility that the lensing body is something that can't be seen at all. Dark matter fits the bill.

An illustration shows the basic principles behind gravitational lensing (Image credit: NASA, ESA & L. Calçada) Dark matter is effectively invisible because it doesn't directly interact with electromagnetic radiation, or light. In other words, dark matter doesn't emit light, and light doesn't bounce off it. That means even a clump large enough to play the role of a gravitational lens would be undetectable. But that doesn't mean light can't respond to the curvature of space caused by dark matter, just as it does to curvature caused by any body composed of "ordinary" matter like a star or a galaxy.

Like any gravitational lensing set-up, the lensing of the jet of PKS 2233-148 caused by this clump of dark matter requires precise alignment, and that means it is only a temporary thing.

"Because these are short-term phenomena, they are hard to find," Britzen said. "This is the first time that we find a lensing phenomenon which might hint at dark matter substructure."

Britzen and colleagues now hope to find other similar lensing events to further solidify the connection between blazars and neutrino factories.

The team's research was published on July 31 in the journal Monthly Notices of the Royal Astronomical Society.