In 2019, a team of astronomers led by Dr. Samantha Oates of the University of Birmingham discovered one of the most powerful transients ever seen – where astronomical objects change their brightness over a short period. Oates and her colleagues found this object, known as J221951-484240 (or J221951), using the Ultra-Violet and Optical Telescope (UVOT) on NASA’s Neil Gehrels Swift Observatory while searching for the source of a gravitational wave (GW) that was thought to be caused by two massive objects merging in our galaxy.

Multiple follow-up observations were made using the UVOT and Swift’s other instruments – the Burst Alert Telescope (BAT) and X-Ray Telescope (XRT), the Hubble Space Telescope, the South African Large Telescope (SALT), the Wide-field Infrared Survey Explorer (WISE), the ESO’s Very Large Telescope (VLT), the Australia Telescope Compact Array (ATCA), and more. The combined observations and spectra revealed that the source was a supermassive black hole (SMBH) in a distant galaxy that mysteriously “switched on,” becoming one of the most dramatic bursts of brightness ever seen with a black hole.

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. This thin disc of rotating material consists of the leftovers of a Sun-like star which was ripped apart by the tidal forces of the black hole. Shocks in the colliding debris as well as heat generated in accretion led to a burst of light, resembling a supernova explosion. Credit: ESO, ESA/Hubble, M. Kornmesser

Dr. Oates and her colleagues recently presented their findings at the 2023 National Astronomy Meeting (NAM 2023) in Cardiff. The presentation, titled “Swift/UVOT discovery of Swift J221951-484240: A UV luminous ambiguous nuclear transient,” was part of a session on explosive and high energy transients on Tuesday, July 4th. Along with researchers from the University of Birmingham, University College London, Queen’s University Belfast, and the European Southern Observatory (ESO), they published their findings in a paper that appeared in the Monthly Notices of the Royal Astronomical Society.

Artist’s impression of two neutron stars colliding, known as a “kilonova” event. Credits: Elizabeth Wheatley (STScI)

As they described, the team found J221951 while searching for the progenitor of a gravitational wave (GW) event (S190930t) detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo detector on April 1st, 2019. Swift was one of several observatories participating in the search for the sources of GW candidates released by the LIGO-Virgo collaboration. Based on the GW signal, the event was thought to be the result of a kilonova, where two neutron stars merge (or a neutron star and a black hole), releasing a tremendous amount of energy and gravitational waves in the process.

Kilonova events typically appear as bright blue bursts that fade and turn redder over the next few days. But when Dr. Oats and her team viewed J221951 with Swift’s UVOT, they noticed that the transient did not appear blue or change color or fade as rapidly as expected. Follow-up observations with Hubble‘s Cosmic Origins Spectrograph (COS) obtained ultraviolet spectra from J221951, which revealed that it was not associated with the previously-detected GW event.

The spectra further indicated that the source was about 10 billion light-years distant, whereas the GW signal was detected less than 0.5 billion light-years away. “The key discovery was when the ultraviolet spectrum from Hubble ruled out a Galactic origin,” said Dr. N. Paul Kuin, a team member from the Mullard Space Science Laboratory at University College London, in a recent Royal Astronomical Society press release. “This shows how important it is to maintain a space-based UV spectrograph capability for the future.”

The team also consulted data from Hubble’s Advanced Camera for Surveys (ACS), the ALLWISE catalog, the Dark Energy Survey (DES), the Galaxy Evolution Explorer (GALEX) satellite, the Inamori-Magellan Areal Camera and Spectrograph (IMACS) on the Baade Telescope at the Las Campanas Observatory, the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) instrument on the MPG/ESO 2.2-meter telescope at the La Silla Observatory, and the Ultra-Violet Imaging Telescope (UVIT) aboard India’s AstroSat satellite.

The team’s research suggested that J221951 resulted from an SMBH that consumed surrounding material suddenly and rapidly. This was confirmed by optical and infrared data that previously detected a red galaxy in the vicinity of J221951, and the location of the bright burst is consistent with the galaxy’s center. Furthermore, the UV spectra showed absorption features consistent with a huge release of energy, which pushed and was absorbed by gas and dust surrounding the black hole. Combined with its brightness, the data revealed that J221951 is one of the most dramatic events ever seen where a black hole suddenly “switched on.”

A star (in the foreground) experiencing spaghettification as it’s sucked in by an SMBH (in the background) during a “tidal disruption event.” Credit ESO/M. Kornmesser

This discovery is part of a growing body of research that shows how SMBHs play a very active role in a galaxy’s star formation. As these behemoths gobble up material, such as gas, dust, and even stars, they release intense bursts of energy that disrupt star-forming material within the galaxy’s central region and disk. Dr. Matt Nicholl, a member of the team from Queen’s University Belfast, explained:

“Our understanding of the different things that supermassive black holes can do has greatly expanded in recent years, with discoveries of stars being torn apart and accreting black holes with hugely variable luminosities. J221951 is one of the most extreme examples yet of a black hole taking us by surprise. Continued monitoring of J221951 to work out the total energy release might allow us to work out whether this is a tidal disruption of a star by a fast-spinning black hole, or a new kind of AGN switch on”.

The team also identified two possible mechanisms that could explain the sudden and voracious feeding behavior. On the one hand, it is possible that an orbiting star passed close to the SMBH and was pulled apart – known as a tidal disruption event (or more commonly as “spaghettification”). A second possibility is that J221951 is an active galactic nucleus (AGN), known as a “quasar,” that began feeding on its accretion disk. In other words, the SMBH at the center of this galaxy “woke up” from its previously dormant state. In the future, Dr. Oates and her colleagues hope to take advantage of next-generation telescopes and their imaging capabilities to investigate J221951 further. As she added:

“In the future, we will be able to obtain important clues that help distinguish between the tidal disruption event and active galactic nuclei scenarios. For instance, if J221951 is associated with an AGN turning on we may expect it to stop fading and to increase again in brightness, while if J221951 is a tidal disruption event we would expect it to continue to fade. We will need to continue to monitor J221951 over the next few months to years to capture its late-time behavior.”