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A star torn apart by a black hole let us watch a mysterious hot corona ignite.
In 2024, a tidal disruption event showed for the first time how a supermassive black hole tears a star apart, like a dry twig tossed into a campfire. The debris swirled like a fiery whirlwind, forming an accretion disk — a glowing ring of remnants. Within just 40 days, powerful radiation flared up in infrared light, invisible to our eyes. Its breakdown into colors pointed not to cosmic dust but to hot plasma — gas where atoms have lost their electrons.
Half a year later, the plasma transformed into a 'corona'—a zone where light particles, colliding with matter, gain enormous energy, like a campfire suddenly erupting into a blazing flame. This flare was caught in X-rays thanks to the Arthur Compton effect — ordinary light became thousands of times more energetic.
The same behavior has been observed in tiny black holes in binary systems: they, too, 'ignite' coronas. This universality, confirmed by precise light measurements and the theories of Rashid Sunyaev, links cosmic giants with their unremarkable cousins. Distances measured via the redshifts of Edwin Hubble helped us understand that corona birth is a common scenario for all active galactic nuclei.
For a black hole to rip apart a star, as in the event 2024aepd, the star had to pass at a distance comparable to the radius of Mercury's orbit. Had it flown just a little farther, the tidal forces wouldn't have been enough to tear it apart.
\alpha_{\rm IR} = \frac{4s-6}{3s-2}
The spectral index of free-free emission is directly tied to the steepness of the plasma envelope's density profile: the shallower the profile, the flatter the infrared spectrum appears.
The classic dust sublimation radius lets us estimate where re-radiating dust might lie if the infrared excess were a dust echo, but observations point to a different mechanism.
