Although we know that supermassive black holes (millions of times the mass of our Sun) lurk at the centre of most galaxies, their very nature makes them difficult to spot and study. In contrast to the popular idea of black holes constantly ‘gobbling up’ matter, these gravitational monsters can spend long periods of time in a dormant, inactive phase.

This was true of the black hole at the heart of SDSS1335+0728, a distant and unremarkable galaxy 300 million light-years away in the constellation of Virgo. After being inactive for decades, it suddenly lit up and recently began producing unprecedented flashes of X-ray light.

The first signs of activity appeared in late 2019, when the galaxy unexpectedly began shining brightly, attracting the attention of astronomers. After studying it for several years, they concluded that the unusual changes they saw were probably the result of the black hole suddenly ‘switching on’ – entering an active phase. The bright, compact, central region of the galaxy is now classified as an active galactic nucleus, nicknamed ‘Ansky’.

“When we first saw Ansky light up in optical images, we triggered follow-up observations using NASA’s Swift X-ray space telescope, and we checked archived data from the eROSITA X-ray telescope, but at the time we didn’t see any evidence of X-ray emissions,” says Paula Sánchez Sáez, a researcher at the European Southern Observatory, Germany, and leader of the team that first explored the black hole’s activation.