In active galactic nuclei, supermassive black holes are surrounded by a dense ring of dust that acts like a curtain: it hides everything happening at the center. If a star ventures too close to the black hole, tidal forces tear it apart, and the resulting flare is usually visible in optical and X-ray light. But behind the dust curtain, such a flare can go unnoticed unless you look in the infrared, where the dust itself starts to glow from the heating.
The astronomers used ten years of observations from the WISE space telescope, which surveyed the sky in the mid-infrared. They selected nearly one hundred thousand active nucleus candidates and looked not just for changes in brightness, but for changes in color in two infrared bands. The color changes when dust is heated by a flare, so this approach can catch events hidden from optical view.
Both flares were interpreted as tidal disruption events. For the first one, J2244+0816, an optical flare was visible simultaneously with the infrared one, but its decline was too fast for the classical picture. The authors suggest that the star was torn apart not by the supermassive black hole itself, but by a small stellar-mass black hole hidden in its accretion disk. This is the first observational evidence for so-called micro-TDEs, which were predicted theoretically. For the second one, J2150−1132, the broad hydrogen line in the spectrum disappeared after the flare, indicating that the active nucleus changed its state.
The work shows for the first time that the WISE infrared survey can find tidal disruption events in active galaxies, including events completely hidden by dust. This opens the way to a complete census of such events, which will help estimate their frequency and role in galaxy evolution. In the future, more sensitive infrared surveys, such as the Roman Space Telescope, will be able to follow the fast phases of these flares in detail and distinguish between full and partial stellar disruptions.
The flare in J2150−1132 was so hidden by dust that it was completely invisible in the optical, even though it brightened by two magnitudes in the infrared. It is like a powerful explosion happening behind a thick curtain: we only see the curtain heating up, not the explosion itself.
The micro-TDE scenario, where a star is torn apart by a small black hole hiding in the accretion disk of a giant black hole, echoes science fiction ideas about secondary black holes populating the vicinity of supermassive objects, such as in Terry Goodkind's novel 'Pillars of Creation' or in some episodes of 'Star Trek' where characters encounter unexpected sources of gravity in dense stellar clusters.