Astronomers saw the Same Supernova Three Times Thanks to Gravitational Lensing. And in Twenty Years They Think They’ll see it one More Time

It is hard for humans to wrap their heads around the fact that there are galaxies so far away that the light coming from them can be warped in a way that they actually experience a type of time delay.  But that is exactly what is happening with extreme forms of gravitational lensing, such as those that give us the beautiful images of Einstein rings.  In fact, the time dilation around some of these galaxies can be so extreme that the light from a single event, such as a supernova, can actually show up on Earth at dramatically different times.  That is exactly what a team led by Dr. Steven Rodney at the University of South Carolina and Dr. Gabriel Brammer of the University of Copenhagen has found. Except three copies of this supernova have already appeared – and the team thinks it will show up again one more time, 20 years from now.

Finding such a supernova is important not just for its mind bending qualities – it also helps to settle an important debate in the cosmological community.  The rate of expansion of the universe has outpaced the rate expected when calculated from the cosmic microwave background radiation. Most commonly, this cosmological conundrum is solved by invoking “dark energy” – a shadowy force that is supposedly responsible for increasing the acceleration rate.  But scientists don’t actually know what dark energy is, and to figure it out they need a better model of the physics of the early universe.

One way to get that better model is to find an event that is actively being distorted through a gravitational lens.  Importantly – the same event must show up at two separate, distinct times in order to provide input to a calculation about the ratio of the distance between the galaxy doing the lensing and the background galaxy that was the source of the event.

That ratio is an important component in calculating some of the variables associated with dark energy.  And the supernova candidate Drs. Rodney and Brammer found is one of the best defined to date.  It is only the third such example of a multiply lensed supernova.  Quasars have also been caught with their own time delays, but the variable nature of quasars themselves make them less than ideal for the kind of angular distance calculations needed by cosmologists.  

The new supernova, known as AT2016jka, was mined from Hubble data collected back in 2016.  Located in “the most spectacular galaxy targeted by REQUIEM [the observational program at Hubble that captured the data]”, it is in the galaxy known as MRG-M0138.

MRG-M0138 is “quadruply lensed” meaning that four copies of the galaxy can be seen dispersed around a galaxy cluster closer to our own galaxy, known as MAC J0138.02155.  So when the team was surveying data in the region in July 2019, they noted the three point sources of light that were present in data from July 2016 were no longer there.  Most likely the data in July 2016 captured a supernova lensed 3 different ways.

However, the expected fourth lensing did not show up in the Hubble data.  Using their lensing model for the system, the team determined that the fourth image should show up sometime around 2037, plus or minus a few years. With such a long baseline time between appearances of the same event, this supernova would provide valuable data to the debate over time dilation in gravitational lensing events.

Unfortunately, that also means that scientists have to wait almost 20 years to get their hands on that data.  It also means that they have to keep a watchful eye on that part of the sky in the 2 year window the calculations predict the fourth image of the supernova would appear in.  It probably wouldn’t be a bad idea to keep half an eye there the rest of the time as well just in case it appears sooner than expected.

If all goes well, that final piece of data as to the exact date of peak brightness of the supernova will be well monitored by a new fleet of cosmological instruments.  Telescopes like the Vera Rubin and Nancy Grace promise to observe hundreds of these lensed supernovae that can provide even more data to further constrain dark energy.  Hopefully they’ll be able to catch the final gasp of the supernova in MRG-M0138 as well, to cap off some great detective work and prove how incredible gravitational time dilation truly is.

Learn More:
arXiv – A Gravitationally Lensed Supernova with an Observable Two-Decade Time Delay
Sci-News – Astronomers Discover Twelve New Quadruply-Imaged Quasars
Forbes – Eight New Quadruple Lenses Aren’t Just Gorgeous, They Reveal Dark Matter’s Temperature

Lead Image:
Image of the MAC J0138.02155 cluster and gravitationally lensed MRG-M0138 galaxy showing the locations of the three observed instances of the supernova (SN1-3) and the expected location of the fourth instance (SN4), estimated to appear around 2037.
Credit – Rodney, Brammer et al.