NASA, ESA, STScI, Wenlei Chen, Patrick Kelly In recent decades, we’ve gotten much better at observing supernovae as they happen. Orbiting telescopes can now pick up the high-energy photons being emitted and understand 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 identify new light sources. NASA, ESA, STScI, Wenlei Chen, Patrick Kelly But sometimes, luck still plays a role. So is a Hubble image from 2010, where the image also happened to capture a supernova. But due to gravitational lensing, the single event appeared in three different locations within Hubble’s field of view. Thanks to the quirks of how this lens works, all three locations were captured at different times after the star exploded, allowing researchers to piece together the time course after the supernova, even though it was observed more than a decade earlier.

I’m going to need it in triplicate

The new project is based on a search of the Hubble archives for old images that happen to record transient events: something present in some images of a location but not in others. In this case, the researchers were specifically looking for events that had gravitational lensing. 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. Advertising
Because gravitational lenses are not as carefully constructed as the ones we make, they often create strange distortions of background objects, or in many cases will magnify it to multiple positions. That seems to have happened here, as there are three distinct images of a transient event within Hubble’s field of view. Other images of this region show that the location coincides with a galaxy. an analysis of the light from this galaxy suggests a redshift that shows we are seeing it as it was more than 11 billion years ago. Given its relative brightness, its sudden appearance, and its location within a galaxy, it is highly likely that this event is a supernova. And, at this distance, many of the high-energy photons produced in a supernova have been red-shifted down into the visible region of the spectrum, allowing them to be imaged by Hubble. To understand more about the background supernova, the team looked at how the lens worked. It was formed by a galaxy cluster 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 showed 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 showed that the lens also affected the arrival time of the light. Gravitational lenses work by forcing light to take paths between the source and the observer that have different lengths. And, since light travels at a constant speed, these different lengths mean that the light takes different times to get here. Under conditions we know, this ends up being a imperceptibly small difference. But on a cosmic scale, it makes a dramatic difference. Advertising
Again, using the lens model, the researchers estimated potential delays. Compared to the oldest image, the second oldest was delayed by 2.4 days and the third delayed by 7.7 days, with an uncertainty of about 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?

Checking this Hubble data against different classes of supernovae we have imaged in the modern Universe, they were likely produced by the explosion of either a red or a blue supergiant star. And the detailed properties of the event were much better suited to a red supergiant, which 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 earliest image shows it was about 100,000 Kelvin, suggesting we were looking at it just six hours after it exploded. The most recent 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 closer 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 can find more of these lensed supernovae in the distant past, we will be able to infer things about the population of stars that existed much earlier in the history of the Universe. For now, however, this is only the second one we’ve found. The authors of the paper describing it make an attempt to draw some conclusions, but it is clear that these will have more uncertainty. So, in many ways, this does not help us make significant advances in understanding the Universe. But as an example of the strange consequences of the forces governing the behavior of the Universe, it is quite impressive. Nature, 2022. DOI: 10.1038/s41586-022-05252-5 (About DOIs). Go to the discussion…