Over the past few decades, we’ve gotten much better at observing supernovae as they happen. Orbiting telescopes can now capture the high-energy photons emitted and know 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 happened to capture a supernova. But due to the gravitational lensing, the single event appeared at three different locations within Hubble’s field of view. Thanks to quirks about how this lens works, all three locations were captured differently times After the star exploded, allowing researchers to piece together the time course after the supernova, although it was observed more than a decade ago.
I’ll need it in three copies
The new work is based on searching Hubble’s archives for old images that happen to capture fleeting events: something that is in some photos of a site but not in others. In this case, the researchers were looking specifically for events that were modified by gravity. This occurs when a massive frontal object distorts space in such a way that it creates a lens effect, bending the path of light that originates behind the lens from Earth’s perspective.
Because gravitational lenses are nowhere near as fine-fitting as the ones we make, they often create strange distortions of background objects, or in many cases, magnify them at multiple locations. This appears to be what happened here, as there are three distinct 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 redshift indicating that we are looking at it as it was more than 11 billion years ago.
Given the relative brightness, sudden appearance, and location within the galaxy, it is likely that this event is a supernova. At this distance, many of the high-energy photons produced in a supernova were red shifted to the visible region of the spectrum, allowing Hubble to image them.
To understand more about the background supernova, the team worked out how the lens works. It was created by a cluster of galaxies called Abell 370, and assigning 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 not one image was 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 affected the timing of the light’s arrival. Gravitational lenses force light to take paths between the source and the observer of varying lengths. And since light moves at a constant speed, these different lengths mean that the light takes a different time to get here. Under the conditions we are familiar with, this is an imperceptibly small difference. But on cosmic scales, it makes a big difference.
Again, using a lens model, the researchers estimated potential delays. Compared to the older image, the first and second image was delayed by 2.4 days, and the third by 7.7 days, with an uncertainty of about 1 day across all estimates. In other words, a single image of the area produced what was essentially a time track of a few days.
what was that
By checking the Hubble data against the different classes of supernovae we’ve imaged in the modern universe, they are likely caused by the explosion of a red or blue giant star. The detailed characteristics 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 looking at it just six hours after it exploded. The latest 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 closer 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 found it. The authors of the paper they describe make an effort to draw some conclusions, but it is clear that those 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’s impressive.