The inventors of the Canadian radio telescope CHIME did not know what they were getting into.

The Canadian Hydrogen Intensity Mapping Experiment (its full name) was designed to observe large parts of the sky and efficiently create a hydrogen map over a significant portion of the universe.

But as it turns out, it is almost perfectly designed to detect unusually fast bursts of radio waves, so-called fast radio bursts.

Fast radio bursts were first discovered in 2007. But during construction on CHIME between 2015 and 2017, astronomers reviewing archived data discovered that these events occurred far more frequently than expected.

CHIME has now observed thousands of such outbreaks – but they remain mysterious.

What is unusual is their diversity. Fast radio bursts occur at different frequencies, last between a few milliseconds and a few seconds, and while some repeat, others do not.

CHIME has observed just one fast radio burst that appears to have come from the Milky Way, but others appear to have originated in galaxies billions of light-years away.

With the rapidly growing catalog of such objects (and all the hydrogen that still needs to be mapped), the CHIME team had its hands full.

But others are currently studying the telescope’s extensive data sets, and a study by a collaboration of astronomers from China and Australia seems to have found something interesting in CHIME’s collection of repeating fast radio bursts.

Gravitational lensing

The team wanted to test an old idea: that fast radio bursts could be influenced by gravitational lensing.

Lensing is a process in which light is bent by gravity. You’ve probably seen it in images of distorted galaxies from the Hubble Space Telescope and the James Webb Space Telescope.

Normally, light from a distant galaxy is deflected when it passes close to a nearby galaxy or galaxy cluster. But smaller, massive objects – passing black holes or free-floating planets – can also cause a lensing effect.

Such microlensing effects lead to a brightening of a background source, but can also lead to a delay in the light reaching the Earth. How long such a delay lasts depends on the situation.

Different paths around the lens take different amounts of time, so instead of a single fast radio pulse, we would receive multiple radio waves of the same pulse, as if we were seeing several different reflections of ourselves in a distorting mirror.

The team used a machine learning technique to look for similarities between different bursts of the same repeating fast radio burst.

They hoped to show that it is not a true repeat – which is difficult to explain when the cause of these events is high-energy and likely destructive – but that lensing turns a single burst into something we see more than once.

They examined 42 fast radio bursts and found one from 2019 that may be a lens.

Their outbursts were so similar that they looked suspicious, although they were not identical – probably due to distortions caused by their passage around the lensing object.

More observations and more radio bursts are needed before anyone is convinced of this result. It may be that the lensing effects are a red herring and the similarity between the bursts is just a coincidence.

Fortunately, CHIME is still scanning the skies, and in a minute there will be another eruption.

Chris Lintott read A candidate for a lens-shaped FRB in the first CHIME/FRB catalogue and its possible implications by Chenming Chang et al. Read it online at: arxiv.org/abs/2406.19654