Throughout recorded history, humans have looked up at the night sky and witnessed the major astronomical events known as a “supernova.” The name, still used by astronomers, referred to the belief that these bursts of light in the “firmament” signaled the birth of a “new star.” With the birth of telescopes and modern astronomy, we have since learned that supernovae are what occur at the end of a star’s lifecycle. At this point, when a star has exhausted its hydrogen and helium fuel, it experiences gravitational collapse at its center.
This leads to a tremendous explosion that can be seen billions of light-years distant, releasing tremendous amounts of energy and blowing the star’s outer layers off. Thanks to an international team of astronomers led by the University of Southhampton, the most powerful cosmic explosion has been confirmed! The stellar explosion, AT2021lwx, took place about 8 billion light-years away in the constellation Vulpecula and was over ten times brighter than any supernova ever observed and 100 times brighter than all the stars in the Milky Way combined!
The team consisted of astronomers from the University of Southampton’s School of Physics and Astronomy, the CAS Key Laboratory of Optical Astronomy, the CSIS Institute of Space Sciences, the DTU National Space Institute, the European Southern Observatory (ESO), the Instituto de Astrofísica de Canarias (IAC), the Institut d’Astrophysique de Paris (IAP), the Italian National Institute for Astrophysics (INAF), the Institute of Space Studies of Catalonia (IEEC), and multiple universities and institutes. Their findings were shared in a study published in the Monthly Notices of the Royal Astronomical Society.
AT2021lwx was first detected in 2020 by the Zwicky Transient Facility (ZTF) in California. It was subsequently detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii and is still being observed by a global network of telescopes. This makes AT2021lwx a record-setting supernova since most others have only been visible to astronomers for a few months. In addition to outshining other supernovae on record by a factor of 10, it is also three times as bright as any tidal disruption event – when a star is consumed by a supermassive black hole (SMBH).
The record for the brightest explosion is currently held by GRB221009A, an unusually bright and long-lasting gamma-ray burst observed last year that took place 2.4 billion light-years from Earth in the constellation Sagitta. While GRB221009A was brighter than AT2021lwx, it lasted for just a fraction of the time (10 hours), meaning the overall energy released by the latter is far greater. While the supernova has been observed for three years, the scale of the explosion was not known until now. Said lead researcher Dr. Philip Wiseman, a Research Fellow at the University of Southampton:
“We came upon this by chance, as it was flagged by our search algorithm when we were searching for a type of supernova. Most supernovae and tidal disruption events only last for a couple of months before fading away. For something to be bright for two plus years was immediately very unusual.”
As part of their investigation, the team consulted data collected by the ZTF and ATLAS observatories and the Wide-field Infrared Survey Explorer (WISE). Further observations were made with the Neil Gehrels Swift space telescope, the ESO’s New Technology Telescope (NTT) in Chile, and the Gran Telescopio Canarias (GTC) in Spain. This provided data in the optical, X-ray, ultraviolet (UV), and near- and mid-infrared (NIR and MIR) wavelengths. Tomás E. Müller Bravo, an ICE-CSIC and IEEC postdoctoral researcher, was the Principal Investigator of the GTC proposal. As she indicated:
“Given the uniqueness of this event, we observed it at different wavelengths with the aim of understanding its nature. In particular, our successful proposal to Gran Telescopio Canarias (GTC) allowed us to observe the presence and absence of different spectral lines, and compare these to those from other known transients to better understand the physical processes behind this bright source.”
“It is really shocking when you find a transient, check its brightness, and you realize it is an order of magnitude brighter than any other supernova,” added co-author Lluís Galbany, a researcher from the ICE-CSIC and member of the IEEC. “When our collaborators told us the existence of this transient, we rapidly prepared a fast-turnaround proposal to observe the transient with the Gran Telescopio Canarias (GTC).”
There are several theories about what caused it, but the team believes the most likely explanation is that an SMBH violently disrupted an extremely large cloud of gas or dust. As a portion of the cloud was consumed, it sent shockwaves through the rest of the remnants and the accretion disk surrounding the black hole. This theory is based on the fact that the only things as bright as AT2021lwx are the active nuclei of galaxies (quasars), where the SMBH has a constant flow of gas falling onto them at high velocity. It is rare for such events to be witnessed, and nothing on this scale has ever been seen before.
“With a quasar, we see the brightness flickering up and down over time,” explained Professor Mark Sullivan, another co-author from the University of Southampton. “But looking back over a decade, there was no detection of AT2021lwx, then suddenly it appears with the brightness of the brightest things in the universe, which is unprecedented.”
As a next step, the team will collect more data on the explosion and run updated computer simulations to test their theory about what caused it. It is hoped that additional multi-wavelength data will reveal details about the object’s surface and temperature and provide insight into what internal processes are at work. Said co-author Claudia Gutiérrez, a postdoctoral researcher at the ICE-CSIC:
“When you find a transient with properties different from those you are used to monitoring, your first reaction is to try to explain what events can produce them and how. Consequently, multiple alternatives appear, but only very few can describe your observations. This is what happened with this event.”
The team also hopes to take advantage of next-generation facilities that will be coming online in the next few years, like the Vera C. Rubin Observatory. Once operational, the observatory will conduct the Legacy Survey of Space and Time (LSST), a ten-year campaign covering the entire southern sky and providing the deepest views of the Universe ever observed. Among the many things that the LSST will reveal, astronomers hope that more extremely-rare events like AT2021lwx will be detected. Given how energetic they are, these massive supernovae could be a process that helps shape galaxies over time.
Further Reading: Institute of Space Studies, MNRAS