by Laurențiu Caramete, PhD
23.12.2024
In recent years, observatories such as the Zwicky Transient Facility (ZTF) have begun continuously scanning the sky. This means astronomers can collect “light curves”—records of how an object’s brightness changes over time—for thousands of cosmic objects on an almost daily basis.
Among the studied objects is AT 2021hdr, which initially seemed quite ordinary. Its brightness in the optical band resembled that of a classic Seyfert 1 galaxy—a well-known active galactic nucleus (AGN) recognized for its bright central region powered by a supermassive black hole. Observations at both visible (optical) wavelengths and higher-energy X-ray bands appeared to confirm this usual classification.
However, at the end of 2021, astronomers observed that AT 2021hdr began to exhibit sudden episodes of increased brightness—peaks that appeared and disappeared in a predictable pattern. Instead of a single outburst, these short bursts recurred every 60 to 90 days, each time producing an increase of about 0.2 magnitudes in the optical g and r bands. Similar oscillations were also seen in ultraviolet and X-ray data from NASA’s Swift observatory.
Several explanations were proposed—standard tidal disruption events (TDEs), binary supermassive black hole (BSMBH) systems, and “changing-look” AGNs—but none convincingly account for the details observed in AT 2021hdr. For example, a typical TDE happens when a star comes too close to a single supermassive black hole and is torn apart, generating a bright flare that eventually fades. In contrast, AT 2021hdr shows repeated outbursts, not a single episode of brightening followed by dimming.
It also does not match the signatures of previously observed binary supermassive black holes or the well-documented phenomenon of “changing-look” AGNs, in which a galaxy’s core abruptly shifts from one spectral state to another. Radio observations from the Very Long Baseline Array (VLBA) did not provide a signal on the milliarcsecond scale, which argues against jet-related scenarios. The pattern of eruptions and their amplitude also appear inconsistent with typical disk or jet instabilities.
In a new study, researchers propose that a binary supermassive black hole system—two black holes orbiting one another in the heart of the same galaxy—could be shredding a cloud of gas, rather than a single star. The gravitational dance of the black holes in this configuration could lead to repeated episodes of increased brightness, as the displaced gas interacts with the system.
Based on the observed outbursts, the study estimates that the separation between the two black holes is about 0.83 milliparsecs (mpc). On cosmic timescales, this pair would merge in a relatively short period of about 70,000 years, although for humans that is still extraordinarily long.
As if the scenario were not already complex enough, the galaxy hosting AT 2021hdr lies only 9 kiloparsecs from a companion galaxy—a distance that suggests the two galaxies might also merge. This two-galaxy system was reported recently, but the authors emphasize that the companion galaxy is unlikely to be responsible for the eruptive behavior of AT 2021hdr.
If confirmed, the tidal destruction of a gas cloud by a binary black hole would offer a rare opportunity to study the final stages of black hole mergers.
