July 21, 2026 report Astronomers catch a recurring black hole eruption in real time Shreejaya Karantha Author Lisa Lock Scientific Editor Robert Egan Senior Editor For the third time in 35 years, the black hole at the center of the galaxy IC 3599 has erupted, and this time, astronomers were watching it closely. IC 3599, a galaxy roughly 280 million light-years away, has flared this dramatically twice before, in 1990 and 2010, but both eruptions were only pieced together after they had faded. This is the first time researchers have watched one of these outbursts unfold in real time.

The paper outlining its multiwavelength analysis was posted to the arXiv preprint server on July 10. Since the eruptions are recurring, it is natural to wonder what is causing them. These giant flares are debated as either an instability in the accretion disk itself or tidal disruption events involving a star being ripped apart by the black hole.

'Swift' monitoring After the eruptions in 1990 and 2010, astronomers conducted monthly monitoring of this black hole using the Neil Gehrels Swift Observatory (Swift) starting in 2013. In October 2025, the data showed that IC 3599 was 50 times brighter than its usual "low" state. To investigate further, the team led by D.

Grupe of Northern Kentucky University conducted follow-up observations using Swift, XMM-Newton and ground-based telescopes at Lick Observatory, Xinglong Observatory and the Caucasian Mountain Observatory (CMO). Using the host-galaxy mass relation, they estimated the black hole's mass to be roughly 2 million solar masses. The luminosity was found to be consistent with what was expected if the black hole was accreting at the maximum theoretical limit.

A dedicated 120,000-second observation with the XMM-Newton telescope revealed the X-ray brightness oscillating up and down in a repeating pattern, with a possible period of around 7.4 hours. Telescopes on the ground (Lick, Xinglong, CMO) took spectra showing many new or dramatically brightened emission lines—especially "coronal lines" from highly ionized iron that were not visible when the galaxy was quiet. This may indicate the outburst is ionizing gas well beyond the black hole's immediate surroundings.

Instability in the disk Considering several clues from the analysis, researchers favor a radiation-pressure instability in the accretion disk as the cause of IC 3599's repeated giant outbursts. In the black hole's inner disk, light itself pushes so hard on the gas that the region becomes unstable. Gas builds up, reaches a limit and triggers a sudden collapse inward, causing an eruption.

The disk then becomes empty, cools down and slowly refills—repeating the cycle, but not on a perfectly fixed timescale. The gaps between the three known outbursts are different (about 19.5 years, then about 15.7 years). A truly periodic mechanism occurring at regular intervals—like a star or compact object on a fixed orbit repeatedly disturbing the disk—would be much less likely, the team says.

Researchers explain in their paper that some alternative theories still cannot be completely ruled out. It is possible that a star is being repeatedly stripped by the black hole, losing a little energy each time, which could gradually shrink its orbit or make it more elliptical and wobbly.