Q&A WITH A BLACK HOLE DETECTIVE
In 2019, a distant galaxy suddenly and dramatically brightened. Now scientists think it was something never witnessed before – a giant black hole waking up
How did you first become aware something unusual was happening in galaxy SDSS1335+0728?
For two decades, the galaxy showed no changes. However, in December 2019 it started showing significant optical variability, detected by the Zwicky Transient Facility (ZTF) – the galaxy was starting to shine. It was later classified as an active galactic nucleus (AGN), meaning it’s emitting a lot of radiation from its central region, which is believed to be produced by matter falling into the supermassive black hole at its centre.
Since the discovery, the galaxy has continued showing random optical variations. The fact that the galaxy didn’t show any previous activity and now varies like an AGN is very puzzling. Normally transient events related to the galaxy’s central supermassive black hole show very smooth variations and only last for a few dozens or hundreds of days.
What do you think is causing the changes?
The galaxy could either be a low-mass AGN igniting for the first time or it could be a very exotic tidal disruption event (TDE), where a star is disrupted by a supermassive black hole. Time will confirm which of these hypotheses are correct, while future monitoring will be crucial to understanding the nature of this extraordinary event.
How has the galaxy changed in the last few years? To understand the light variations observed in the nucleus we gathered archival data and conducted follow-up observations using various telescopes, including ESO’s Very Large Telescope, SOAR, Keck, Swift and Chandra.
We found that the galaxy is now brighter in the ultraviolet, optical and infrared ranges compared to two decades ago. More surprisingly, we discovered that in February 2024 it began emitting X-rays more than four years after the first alert from ZTF. Moreover, our spectroscopic follow-up campaign revealed the galaxy changed its classification from star-forming galaxy to an AGN. However, we haven’t yet seen the formation of broad emission lines which are normally seen in AGNs and transient events like TDEs. All these observations demonstrate that this is a very peculiar source, which behaves in a way that has not been seen before.
What can we learn from watching this happen in real time?
If the source corresponds to a newborn AGN, it will allow us to observe an AGN while it’s activating, which has never been done before. At the moment, there are no models that properly describe what happens when an AGN fires up and this observation will allow us to develop these models in the future. If it corresponds to a transient event, it will represent a new phenomenon which has never observed before.
Why has it taken until now to see an AGN fire up?
In another paper, led by Patricia Arevalo, we presented another newborn AGN. However, we didn’t see the activation of the black hole in real time. We just know that the galaxy was not active 18 years ago and now it is. In the case of SDSS1335+0728, we observed the source while it was in the middle of becoming active, which can teach us a lot about how this process happens and how these systems evolve.
Interestingly, we discovered these two objects using the same machine-learning classifier, which our collaboration has been using since 2020 to discover objects of interest.
These kinds of discoveries have been possible recently thanks to all-sky surveys, such as the Zwicky Transient Facility and the upcoming Vera Rubin Observatory Legacy Survey of Space and Time, which are facilitating the identification of transient events in the centres of galaxies hosting supermassive black holes.
What will be the next steps of this study?
We’re now monitoring the source with different telescopes and in different wavelength regimes, and we plan to keep monitoring it while it shows activity. This will finally allow us to understand what exactly has happened with this mysterious galaxy.