Over the last few decades, we’ve gotten much better at observing supernovae as they occur. Orbiting telescopes can now collect the emitted high-energy photons and locate their source, allowing other telescopes to make quick observations. And some automated scanning telescopes have imaged the same parts of the sky each night, allowing image analysis software to identify new light sources.
But sometimes luck still plays a role. This is with a Hubble image from 2010, where the image also captured a supernova. However, due to gravitational lensing, the single event occurred in three different places within Hubble’s field of view. All three positions are captured differently, thanks to the quirks of the way this lensing works. times Although it was observed more than a decade ago, after the star exploded, it allowed researchers to piece together the time course that followed the supernova.
I will need this in triplicate
The new work builds on a search in the Hubble archives for old images that capture transients: something that is present in some images of a location but not in others. In this case, the researchers were specifically investigating gravitationally lensed phenomena. These occur when a massive object in the foreground distorts the field to create a lens effect, bending the path of light coming out from behind the lens from Earth’s perspective.
Because gravitational lenses are not as carefully constructed as the ones we produce, they often create strange distortions in background objects or, in many cases, magnify them in more than one place. This is what’s happening here, as there are three different images of a transient event within Hubble’s field of view. Other images of that region show the site overlapping a galaxy; An analysis of the light from that galaxy shows a redshift that shows we’re looking at it as it was 11 billion years ago.
Given its relative brightness, sudden appearance, and position in a galaxy, this event is likely to be a supernova. And at this distance, most 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.
The team investigated how the lens works to learn more about the background supernova. It was created by a galaxy cluster called Abell 370, and mapping that cluster’s mass allowed them to predict the properties of the lens it formed. The resulting lens model showed that there were actually four images of the galaxy, but one was not magnified enough to be seen; The three that appear were magnified by the factors of four, six, and eight.
But the model also showed that the lens also affects the timing of light’s arrival. Gravitational lenses force light to travel different lengths between the source and the observer. And since light moves at a constant speed, these different lengths mean it takes a different time for the light to get here. Under the familiar conditions, this is an imperceptible difference. But it makes a dramatic difference on cosmic scales.
Again, using the lensing model, the researchers estimated possible delays. Compared to the oldest image, the second earliest had a delay of 2.4 days and the third had a delay of 7.7 days, with an uncertainty of about one day in all estimates. In other words, a single image of the region essentially produced a time span of several days.
What was that?
By checking this Hubble data against the different classes of supernovae we view in the modern Universe, it was likely to have been produced by the explosion of a red or blue supergiant star. And the detailed features of the event were much more appropriate for a red supergiant that was roughly 500 times the Sun at the time of the eruption.
The intensity of the light at different wavelengths provides an indication of the temperature of the explosion. And the oldest image shows it’s roughly 100,000 Kelvin, which means we’re looking at it just six hours after the eruption. The latest lensed image shows that during the eight days between two different images, the debris has already cooled to 10,000 K.
Obviously, there are newer and more recent supernovae that we can study in much greater detail if we are to understand the processes that cause a massive star to explode. However, if we can find more of these lensed supernovae in the distant past, we should be able to make inferences about the population of stars that existed long before in the history of the Universe. But right now, this is only the second we’ve found. The authors of the paper describing this make an effort to make some inferences, but it is clear that these will have a higher degree of uncertainty.
So in many ways this does not help us make great 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 a pretty impressive example.
Nature2022. DOI: 10.1038/s41586-022-05252-5 (About DOIs).
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