Zoom / On the left, the complete Hubble image. On the right, different images of the object with a gravitational lens.
NASA, ESA, STScI, Wenley Chen, Patrick Kelly
In recent decades, we have gotten much better at observing supernovae as they happen. Orbiting telescopes can now capture the emitted high-energy photons and learn their source, allowing other telescopes to make quick observations. Some automated scanning telescopes have imaged the same parts of the sky night after night, allowing image analysis programs to identify new light sources.
But sometimes, luck still plays a role. Such is the case with 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 were captured different times after the star exploded, allowing the researchers to reconstruct the time course after the supernova, even though it was observed more than a decade ago. .
The new work is based on searching Hubble’s archives for old images that capture fleeting events: something that’s in some photos at one site but not others. In this case, the researchers were specifically looking for events that were modified by gravity. This occurs when a massive frontal object distorts space in such a way as to create a lensing effect, bending the path of light originating behind the lens from Earth’s perspective.
Because gravitational lenses don’t adjust as well as the ones we make, they often create strange distortions of background objects or, in many cases, magnify them in multiple locations. This appears to be what 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; Analysis of the light from that galaxy indicates a red shift indicating that we are seeing it as it was more than 11 billion years ago.
Given the relative brightness, sudden appearance, and location within the galaxy, this event is likely to be a supernova. At this distance, many of the high-energy photons produced in a supernova were redshifted into the visible region of the spectrum, allowing Hubble to pick them up.
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 this cluster allowed them to estimate the properties of the lens that created it. The resulting lens model indicated that there were already four images of the galaxy, but no image was enlarged 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 affected the timing of the light’s arrival. Gravitational lenses force light to take paths between the source and the observer of variable length. And since light moves at a constant speed, these different lengths mean that it takes a different amount of time for the light to get here. Under the conditions with which we are familiar, this is an imperceptibly small difference. But on cosmic scales, it makes a big difference.
Once again, using a lens model, the researchers estimated the potential delays. Compared to the previous image, the first and second images were delayed by 2.4 days and the third by 7.7 days, with an uncertainty of approximately 1 day in all estimates. In other words, a single image of the area produced what was essentially a timeline of a few days.
What was that
When comparing the Hubble data to the different kinds of supernovae we have imaged in the modern universe, it is likely that they are caused by the explosion of a red or blue giant star. The detailed features of the event were best suited to a red giant, 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. The first image indicates that it was approximately 100,000 K, indicating that we were seeing it only six hours after it exploded. The last lens image shows that the debris has already cooled to 10,000 K during the eight days between the two different images.
It is clear that there are more recent and nearby supernovae that we can study in more detail if we want to understand the processes that lead to the explosion of a massive star. If we can find more such supernovae in the distant past, we will be able to infer things about the number of stars that existed earlier in the history of the universe. But for now, this is only the second time we’ve encountered it. The authors of the article they describe endeavor to draw some conclusions, but it is clear that such conclusions would involve a high degree of uncertainty.
So in many ways this doesn’t help us make much progress in understanding the universe. But as an example of the strange consequences of the forces that govern the behavior of the universe, it is impressive.
temper nature2022. DOI: 10.1038/s41586-022-05252-5 (About DOIs).