Here’s a story that’s both mind-bending and light-bending. It involves a supernova that, on the one hand, happened 10 billion years ago, and on the other hand, has appeared in our skies not once but three times, with a fourth in the works. In play here is gravitational lensing, in which light from a background galaxy bends around a foreground galactic cluster known as MACS J0138.0-2155. Out of this we get multiple mirror images, and researchers predict another supernova appearance in the year 2037.
Three of the appearances of the supernova, labeled AT 2016jka and nicknamed ‘Requiem,’ are in the image below, a Hubble view from 2016, all three circled for ease of identification. The light of the supernova has been split into different images by the lensing effect. Using archival data, researchers led by Steve Rodney (University of South Carolina) have analyzed differences in brightness and color that reflect different phases of the event as the supernova faded.
“This new discovery is the third example of a multiply imaged supernova for which we can actually measure the delay in arrival times,” says Rodney. “It is the most distant of the three, and the predicted delay is extraordinarily long. We will be able to come back and see the final arrival, which we predict will be in 2037, plus or minus a couple of years.”
Image: Three views of the same supernova appear in the 2016 image on the left, taken by the Hubble Space Telescope. But they’re gone in the 2019 image. The distant supernova, named Requiem, is embedded in the giant galaxy cluster MACS J0138. The cluster is so massive that its powerful gravity bends and magnifies the light from the supernova, located in a galaxy far behind it. Called gravitational lensing, this phenomenon also splits the supernova’s light into multiple mirror images, highlighted by the white circles in the 2016 image. The multiply imaged supernova disappears in the 2019 image of the same cluster, at right. The snapshot, taken in 2019, helped astronomers confirm the object’s pedigree. Supernovae explode and fade away over time. Researchers predict that a rerun of the same supernova will make an appearance in 2037. The predicted location of that fourth image is highlighted by the yellow circle at top left. The images were taken in near-infrared light by Hubble’s Wide Field Camera 3. Image processing credit: Joseph DePasquale (STScI).
Cluster and supernova are at vastly different distances from us, with the light from the lensing cluster MACS J0138.0-2155 taking about four billion years to reach us, while the light from the supernova has traveled an estimated 10 billion years. Computer modeling makes the call on the supernova’s return appearance as researchers untangle the complex path followed by the light.
In fact, says Rodney, the longer delay in the predicted 2037 light is the result of its traveling through the middle of the cluster and thus encountering the densest amount of dark matter. While dark matter remains controversial in many ways, it’s telling that the assumption of dark matter in the cluster explains the current three images and makes the call on the upcoming fourth. There appears to be a likelihood for a fifth appearance some time after the 2037 event, although the prediction is that it will be extremely hard to detect.
The lensing supernova images were discovered by Gabe Brammer (Niels Bohr Institute, University of Copenhagen), who found the three mirrored images while analyzing lensing magnification effects for the REQUIEM (REsolved QUIEscent Magnified Galaxies ) program, which uses Hubble data. Comparing the 2019 data with data from three years earlier showed that what he thought was a single image of a lensed galaxy had disappeared. Says Brammer:
“But then, on further inspection of the 2016 data, I noticed there were actually three magnified objects, two red and a purple. Each of the three objects was paired with a lensed image of a distant massive galaxy. Immediately it suggested to me that it was not a distant galaxy but actually a transient source in this system that had faded from view in the 2019 images like a light bulb that had been flicked off.”
Co-author Johan Richard (University of Lyon) developed a map of the amount of dark matter in the foreground cluster that drew on inferences from the lensing effects found in Brammer’s data. The map fits with the locations of the lensed objects based on Richard’s assumptions. Analyzing the fourth image in 2037, assuming all happens as expected, will allow astronomers to more accurately measure the time delays between the four images, which in turn will yield further data on the distortions to spacetime through which the light transited. Adds Rodney:
“These long time delays are particularly valuable because you can get a good, precise measurement of that time delay if you are just patient and wait years, in this case more than a decade, for the final image to return. It is a completely independent path to calculate the universe’s expansion rate. The real value in the future will be using a larger sample of these to improve the precision.”
In the excerpt from the paper below, MRG-M0138 refers to the background galaxy containing the supernova, which is being lensed by the foreground galaxy cluster MACS J0138.0-2155:
We model the mass distribution in the cluster core as the combination of a cluster-scale and galaxy-scale potentials… From this model we derive estimates for the lensing magnification and time delay of each of the SN images, including two predicted future images… The lens model predicts that the SN should appear in the fourth MRG-M0138 image in the year 2037±2, demagnified with µ = 0.4 ± 0.2. A fifth image will also appear at a still later date, located near the center of the cluster and much more significantly demagnified, so it will not be easily observable. We anticipate that future lens modelling of the cluster will improve on these predictions primarily by exploring a wider range of mass models and incorporating more observational constraints.
The building of the model that makes the 2037 prediction is fascinating, fully explicated in the ‘Methods’ section of the paper, as the researchers use a software program called LENSTOOL to develop five lens models that would yield the lensing effects seen in the data. The best fit model was then used to predict the magnification and time delays for the three images observed by Hubble, as well as the location of the fourth and fifth images, which have yet to appear. An education on lens modeling is available here for those interested in digging into the details.
The name Requiem comes into use for a reason beyond the reference to the REQUIEM program. I like the note at the end of the paper that explains it:
HST observations enabled us to find this SN. We anticipate that HST may be deorbited and make its final plummet to Earth around the time of the reappearance of AT 2016jka, so we coin the name SN Requiem as an ode to the vast new discovery space that HST continues to unveil.