Single Hubble image captured supernova at three different times

In recent decades, we’ve gotten much better at observing supernovas as they happen. Telescopes orbiting Earth can now capture the emitted high-energy photons and identify their source, allowing other telescopes to make quick observations. And some automated survey telescopes have imaged the same parts of the sky night after night, allowing image analysis software to spot new light sources.

But sometimes luck still plays a role. So it is with a Hubble image from 2010, where the image also happened to capture a supernova. But because of gravitational lensing, the single event occurred at three different locations within Hubble’s field of view. Thanks to the quirks of how this lens works, all three locations are captured differently time after the star’s explosion, allowing researchers to piece together the time course after the supernova, even though it had been observed more than a decade earlier.

I need that in triplicate

The new work is based on a search of the Hubble archives for old images that happen to capture transient events: something that’s present in some images of a location, but not others. In this case, the researchers specifically looked for events that had been gelled by gravity. These occur when a massive object in the foreground distorts space in a way that creates a lens effect, deflecting the path of light coming from behind the lens from the perspective of the Earth.

Because gravitational lenses are not nearly as carefully structured as the lenses we make, they will often cause strange distortions of background objects, or in many cases magnify in multiple locations. That seems to have happened here, as there are three different images of a transient event within Hubble’s field of view. Other images of that region indicate that the location coincides with a galaxy; an analysis of the light from that galaxy suggests a redshift, indicating that we are looking at it as it was more than 11 billion years ago.

Given its relative brightness, sudden appearance and location within a galaxy, this event is highly likely to be a supernova. And at that distance, many of the high-energy photons produced in a supernova are shifted in red to the visible part of the spectrum, allowing them to be imaged by Hubble.

To learn more about the supernova in the background, the team figured out how the lens worked. It was created by a galaxy cluster called Abell 370, and by mapping that cluster’s mass, they were able to estimate the properties of the lens that created it. The resulting lens model indicated that there were actually four images of the galaxy, but one was not magnified enough to be visible; the three that were visible were magnified by a factor of four, six and eight.

But the model went on to say that the lensing also affected the timing of the light’s arrival. Gravity lenses force light to take paths of different lengths between the source and the observer. And because light moves at a fixed speed, those different lengths mean that the light takes a different amount of time to get here. Under conditions that we know of, this is an imperceptibly small difference. But on a cosmic scale, it makes a dramatic difference.

Again, using the lens model, the researchers estimate the likely delays. Compared to the earliest image, the second-oldest had a 2.4-day delay and the third a 7.7-day delay, with an uncertainty of about a day on all estimates. In other words, a single image of the region produced what was essentially a lapse of a few days.

What was that?

Comparing that Hubble data to different classes of supernovae we’ve imaged in the modern universe, it would likely have been produced by the explosion of a red or blue supergiant. And the detailed properties of the event were much better suited to a red supergiant, one that was about 500 times the size of the sun at the time of its explosion.

The intensity of the light at different wavelengths gives an indication of the temperature of the explosion. And the earliest image indicates it was about 100,000 Kelvin, suggesting we were looking at it just six hours after the blast. The latest lens shot shows that the debris had already cooled to 10,000 K in the eight days between the two different images.

Obviously there are more recent and closer supernovae that we can study in much more detail if we want to understand the processes that cause the explosion of a massive star. However, if we can find more of these lensed supernovae in the distant past, we can infer things about the population of stars that were present much earlier in the history of the Universe. At the moment, however, this is only the second one we have found. The authors of the article describing it do their best to draw some conclusions, but it is clear that those will have greater uncertainty.

So in many ways this doesn’t help us make great strides in understanding the universe. But as an example of the strange effects of the forces that govern the behavior of the universe, it’s a pretty impressive one.

Nature2022. DOI: 10.1038/s41586-022-05252-5 (About DOIs).

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