AAS Nova

Echoes of a Tidal Disruption Event

By on Features
Share:
TDE
Artist’s illustration of a tidal disruption event, in which a black hole tears apart a passing star and lights up as it accretes the stellar material. [NASA, ESA, and D. Player (STScI)]

What happens when a star is ripped apart within the accretion disk of an active black hole? New research probes the infrared echo from one of the first known events of this kind to learn more.

Feeding Habits of Black Holes

The centers of galaxies are hubs of activity, and in many galaxies, it’s mealtime. A galaxy’s central supermassive black hole can feast on a disk of dust and gas that persists for tens to hundreds of thousands of years, creating a luminous and variable active galactic nucleus.

On shorter timescales — more of a galactic snack — there are tidal disruption events, which occur when a star ventures too close to a supermassive black hole and is summarily torn apart by the black hole’s titanic tidal forces.

In the past decade, it’s become clear that these events aren’t mutually exclusive, and a black hole might snack on a star while dining on a disk. What does it look like when a tidal disruption event happens in the accretion disk of an active galactic nucleus?

Dusty Disk and Shredded Star

multi-wavelength light curve of PS16dtm

Multi-wavelength light curves of PS16dtm. The infrared behavior (triangles) is significantly different from the optical and ultraviolet behavior (squares and circles). Click to enlarge. [Jiang et al. 2025]

To probe this question, Ning Jiang (University of Science and Technology of China) and collaborators studied PS16dtm, one of the first tidal disruption events thought to occur in an active galactic nucleus disk. The team amassed archival data from the Neil Gehrels Swift Obsevatory, the Asteroid Terrestrial Last Alert System, the All Sky Automated Survey for SuperNovae, and the Wide-field Infrared Survey Explorer, piecing together a multi-wavelength record of the system’s behavior.

These data show that the ultraviolet and optical emission from the event rocketed to peak brightness in a matter of weeks before declining more gradually over the course of multiple years. The infrared emission behaved differently, rising more slowly and remaining high rather than falling. This persistently bright infrared emission is not a universal feature of tidal disruption events and instead appears to be a unique feature of tidal disruption events in active galactic nucleus disks.

Torus Insights

Why does the infrared emission behave so differently from other wavelengths? The answer lies beyond the accretion disk of the active galactic nucleus, in a vast dusty torus that surrounds the system. As light from the disrupted star reaches the torus, the dusty gas absorbs the light and re-emits it in the infrared, creating an infrared “echo” of the tidal disruption event.

schematic of the accretion disk and torus system

A schematic of the black hole, accretion disk, and dusty torus. The gray area shows the portion of the torus that may have been evaporated due to the tidal disruption event. The black hole and accretion disk are not to scale; if they were to scale, they would be too small to be visible in this diagram. [Jiang et al. 2025]

Using a dust echo model, Jiang’s team showed that in order for the infrared emission to remain bright for so long, the tidal disruption event powering the emission must be extremely luminous — so luminous that the radiation from the star’s destruction may have evaporated some of the torus, pushing its inner radius outward from 3.6 to 5.2 light-years. The fact that so much of the system’s energy emerges in the form of infrared light may offer a solution to the “missing energy” problem, in which tidal disruption events observed only at optical wavelengths appear to emit less energy than expected. In general, tidal disruption events that happen within active galactic nucleus accretion disks may also be more energetic than typical tidal disruption events due to interactions between the disrupted star and the gas of the disk.

For now, observing this dusty torus directly is out of reach, as it is too faint and too small to be captured by existing facilities. Jiang and coauthors speculated that GRAVITY+, an upgrade to the existing GRAVITY instrument on the Very Large Telescope, may be able to observe the torus directly. In the meantime, upcoming surveys like the Vera C. Rubin Observatory Legacy Survey of Space and Time will discover hundreds to thousands more tidal disruption events, giving researchers a glimpse into the intricacies of stellar disruption.

Citation

“The Extraordinary Long-Lasting Infrared Echo of PS16dtm Reveals an Extremely Energetic Nuclear Outburst,” Ning Jiang et al 2025 ApJL 980 L17. doi:10.3847/2041-8213/adaeb9