A Cosmic Lightsaber

Black Hole Jet

Active black holes have many quirks, but none so dramatic than the ‘relativistic jet’. Black holes are well known as the devours of the Universe, but, for the short time that they do this, they don’t just suck particles in; they emit a small number of them in relativistic jets: huge streams of particles travelling at almost the speed of light, which can be millions of light years across and literally punch holes in neighbouring galaxies. In fact, they’re so energetic that they’re thought to be the sources of the fastest particles in the Universe: cosmic rays [1][2].

Discovered in 1912, cosmic rays are made up of protons, electrons and atomic nuclei as heavy as uranium (number 92 on the periodic table) created by relativistic jets, but also by other high energy objects such as supernovae. How do we know this? You might think you could just trace their trajectory back to an origin, but because they interact so much with magnetic fields, that could be wildly different by the time it gets to Earth.

Instead, scientists use three methods:

  • Spectroscopy, comparing what light is absorbed and emitted by them and comparing that to other places in the Universe.

  • Weighing the isotopes (varying by number of neutrons) of elements in cosmic rays, since if they’re found in only a few places in the Universe, then it can hint at where the rays came from.

  • Measuring the half-lives (the point where half the isotope has decayed) of radioactive cosmic ray nuclei and how much they’ve decayed, therefore finding out how long they’ve been travelling for [3].

Artist conception of showers of cosmic rays passing through Earth's atmosphere

Artist conception of showers of cosmic rays passing through Earth's atmosphere.

Credit: Simon Swordy (U. Chicago), NASA

Quasars and Blazars

However, this phenomenon of cosmic rays doesn’t just have to be seen in our atmosphere: it can also be observed by looking at its source. Quasars are the view we on Earth have of a black hole’s relativistic jets, seen as strong x-ray (high power) and radio wave (low power) sources (along with many other wavelengths of light), whilst blazars are the face on view of a relativistic jet: the view which gives the most amount of radiation, and so the brightest picture, to the observer [4].

But why are they so interesting? Their brightness means they can be seen from long distances (and therefore seen a long time into the past, since light has a speed), but also galaxies between us and the jets can be studied, since the gas in galaxies absorbs some of the light from the jets, leaving ‘absorption lines’ (missing wavelengths of light, characteristic of certain elements). They also exist in galaxies, meaning they’re another way to learn more about the evolution of galaxies. And finally, the origin of their energy source is not yet fully understood [5].

Artist's concept of a blazar using information from the Fermi Gamma-ray Space Telescope

Artist's concept of a blazar using information from the Fermi Gamma-ray Space Telescope

Credit: NASA/JPL-Caltech/GSFC

The Source

In a galaxy far far away resides the wielder of such a lightsaber, only this one isn’t used by something that wants to be a fundamental force (there’s only room for four!): it’s used by M87, or rather M87’s black hole, M87*.

In the constellation of Virgo (one of around 2,000 that reside there), and 54 million light-years from Earth [6], M87* is special because it’s active, spewing out high energy particles a whopping 5000 light-years into the voids of space (that must have been some bad meal!) [7]. But how is it doing that?

A team of researchers called the ‘Event Horizon Telescope Collaboration’ is trying to answer just that question. Made famous by the image taken in 2019 of the plasma surrounding the event horizon of M87* [8], the team have now taken another image: one which will revolutionise our understanding of the source of relativistic jets.

The image is of polarised light coming from the same plasma ring imaged in 2019. But why polarised light? Light, a transverse 'S-shaped' wave, travels in different planes at different angles, so if you filter that light, letting only one plane through, the light becomes polarised, useful in things such as removing the polarised light reflected off rivers, so fishermen can more easily see fish under the water [9]. However, apart from being polarised when reflecting off water, light is polarised when a hot source emits it into strong magnetic fields in space. Therefore, by observing just that polarised light, astronomers are observing just the light which interacts with the black hole’s magnetic field, creating a picture of the magnetic field lines at the edge of a black hole (a bit more high-tech than iron fillings on a bar magnet!) [7].

It took a lot of effort to tease all the polarised light from the immense amount of data they gathered, with between just 10% and 20% of the light being polarised (and some being artificially dimmer due to polarised light travelling in opposite directions and cancelling) [15].

M87* and it's polarised relativistic jet, revealing M87*'s magnetic field