Over the past few decades, we have gotten much better at observing supernovae as they occur. Orbiting telescopes can now pick up emitted high-energy photons and determine their source, allowing other telescopes to make rapid observations. And some automated telescopes have imaged the same parts of the sky night after night, allowing image analysis software to recognize new light sources.
But sometimes luck always plays a role. The same is true with a Hubble image from 2010, where the image also captured a supernova. But, due to gravitational lensing, the single event happened at three different locations in Hubble’s field of view. Thanks to the quirks of how this lens works, the three locations captured different time after the star exploded, allowing researchers to piece together the time course following the supernova, even though it had been observed more than a decade earlier.
I will need triplicate
The new work is based on a search of the Hubble Archive for old images that capture transient events: something that is present in some images of a location but not others. In this case, the researchers were specifically looking for events that had been gravitationally focused. These occur when a massive object in the foreground distorts space in a way that creates a lens effect, bending the path of light coming from behind the lens from Earth’s perspective.
Because gravitational lenses aren’t nearly as carefully structured as the ones we make, they often create strange distortions of background objects or, in many cases, magnify them in multiple places. That’s what appears to have happened here, as there are three separate images of a transient event in Hubble’s field of view. Other images of this region indicate that the site coincides with a galaxy; an analysis of light from this 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 were red-shifted toward the visible region of the spectrum, allowing them to be imaged by Hubble.
To learn more about the background supernova, the team worked on how the lens works. It was created by a cluster of galaxies called Abell 370, and mapping the mass of this 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 enlarged by factors of four, six and eight.
But the model further indicated that the lens also influenced the timing of light arrival. Gravitational lenses force light to take paths between source and observer with different lengths. And, since light travels at a fixed speed, these different lengths mean that the light takes a different time to get here. Under the circumstances we know, this is an imperceptibly small difference. But on a cosmic scale, it makes a dramatic difference.
Again, using the lens model, the researchers estimated the likely delays. Compared to the first image, the second oldest was 2.4 days late and the third was 7.7 days late, with an uncertainty of about one day on 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?
Checking these Hubble data against the different classes of supernovae we have imaged in the modern Universe, they were likely to be produced by the explosion of a red or blue supergiant star. And the detailed properties of the event corresponded much better to a red supergiant, which was about 500 times the size of the Sun when it exploded.
The intensity of light at different wavelengths provides an indication of the temperature of the explosion. And the first image says it was around 100,000 Kelvin, suggesting we looked at it only six hours after it exploded. The last lens image 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 greater detail if we want to understand the processes that lead to the explosion of a massive star. If we are able to find more of these lensed supernovae in the distant past, however, we can infer things about the population of stars that were present much earlier in the history of the Universe. At the moment, however, it’s only the second one we’ve found. The authors of the article that describes it are trying to draw conclusions, but it is clear that these will have a higher uncertainty.
So in many ways it’s not helping us make major advances in understanding the Universe. But as an example of the strange consequences of the forces that govern the behavior of the Universe, it’s quite impressive.
Nature2022. DOI: 10.1038/s41586-022-05252-5 (About DOIs).
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