In recent decades, we have gotten much better at observing supernovae as they happen. Orbiting telescopes can now catch the emitted high-energy photons and discover their source, allowing other telescopes to make quick observations. And some automated scanning telescopes have taken images of the same parts of the sky night after night, allowing image analysis software to recognize new light sources.
But sometimes, luck still plays a role. The same goes for a Hubble image from 2010, where the image also captured a supernova. But, due to gravitational lensing, the single event appeared at three different places within Hubble’s field of view. Thanks to quirks in how this lens works, the three locations captured different times after the star’s explosion, allowing the researchers to reconstruct the time course that followed the supernova, even though it had been observed more than a decade earlier.
I would need it in triplicate
The new work is based on a search of Hubble’s archives for old images that capture transient events: something that is present in some images of one location but not others. In this case, the researchers were specifically looking for events that had been captured by gravitational lensing. These occur when a massive foreground object distorts space in a way that creates a lensing effect, bending the path of light originating behind the lens from Earth’s perspective.
Because gravitational lenses aren’t as carefully structured as the ones we make, they often create weird distortions of background objects or, in many cases, magnify them in multiple locations. That is what appears to have happened here, as there are three separate images of a transient event within Hubble’s field of view. other images of that region indicate that the site coincides with a galaxy; an analysis of the light from that galaxy suggests a redshift indicating that we are seeing it as it was more than 11 billion years ago.
Given its relative brightness, sudden appearance, and location within a galaxy, this event is most likely a supernova. And, at that distance, many of the high-energy photons produced in a supernova have been redshifted into the visible area of the spectrum, allowing them to be imaged by Hubble.
To understand more about the background supernova, the team discovered how the lens works. It was created by a galaxy cluster called Abell 370, and mapping the mass of that cluster allowed them to estimate the properties of the lens it created. 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 factors of four, six, and eight.
But the model further indicated that the lens also influenced the timing of the light’s arrival. Gravitational force lenses of light take paths between the source and the observer with different lengths. And, since light moves at a fixed speed, those different lengths mean it takes a different amount of time for the light to get here. Under circumstances with which we are familiar, this is an imperceptibly small difference. But on cosmic scales, it makes a dramatic difference.
Once again, using the lens model, the researchers estimated the possible delays. Compared to the oldest image, the second oldest was 2.4 days late and the third was 7.7 days late, with an uncertainty of around one day in all estimates. In other words, a single image of the region produced what was essentially a time course of a few days.
What was that?
Comparing the Hubble data with the different kinds of supernovae we have imaged in the modern Universe, it is likely to be caused by the explosion of a red or blue supergiant star. And the detailed properties of the event were a much better fit for a red supergiant, one that was about 500 times the size of the Sun at the time of its explosion.
The intensity of light at different wavelengths provides an indication of the temperature of the explosion. And the oldest image indicates that it was about 100,000 Kelvin, suggesting that we were seeing it just six hours after it exploded. The last lensed image shows that the debris had already cooled to 10,000 K during the eight days between the two different images.
Obviously, there are more recent and nearby supernovae that we can study in much more detail if we want to understand the processes that drive the explosion of a massive star. However, if we are able to find more of these lensed supernovae in the distant past, we may be able to infer things about the population of stars that were present much earlier in the history of the Universe. At the moment, though, this is only the second we’ve found. The authors of the article describing it make an effort to draw some inferences, but it is clear that they 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 consequences of the forces that govern the behavior of the Universe, it’s pretty impressive.
Nature2022. DOI: 10.1038/s41586-022-05252-5 (About DOIs).
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