The Spotlight That Turned Away: The Mystery of Transient AT 2019ijn ✛ ✦
The cosmos is full of screams we can't hear. Every time a black hole tears a star apart, a blinding flash is born, and often — a narrow relativistic jet, shooting out thousands of light-years. But if that searchlight is aimed away from Earth, we see only a faint afterglow — a radio echo arriving with a delay. Transient AT 2019ijn turned out to be just such an event, discovered in the archives of the radio survey VLASS as a point that wasn't there before. Follow-up observations revealed an astonishingly bright optical flare in a distant dwarf galaxy at a redshift of 0.273. And a year later, the radio emission came — growing, relentless, betraying the presence of a powerful jet aimed 60 degrees away from us.
To understand the nature of the object, astrophysicists turned to the laws of thermal radiation. The Stefan-Boltzmann formula, derived by Max Planck, linked the optical luminosity (about 8×10⁴³ erg/s) with a temperature of ~10⁴ K and allowed them to calculate the radius of the emitting region — roughly 2×10¹⁵ cm, typical for the swollen photosphere of a tidal disruption event (TDE). The optical light curve surprised astronomers: a rapid rise (7 days in the rest frame) and a very slow decline — something neither supernovae nor classical TDEs exhibit. The TDE scenario fit well: an intermediate-mass black hole (10⁴–10⁶ solar masses) tore apart an unwary star, and the matter spun into an accretion disk. But the most interesting part came later.
The radio emission arrived a year later and lasted over six years. Its spectrum couldn't be explained by a simple expanding bubble — a model of an off-axis jet was needed. Numerical simulations using synchrotron fitting showed that the data are perfectly described by a narrow relativistic outflow with colossal energy: ~2×10⁵² erg. For comparison, that's equivalent to the complete conversion into radiation of a thousand Earth masses. For the Sun to emit that much energy would require a time many times the age of the universe. The viewing angle was about 1 radian — enough to weaken the jet by hundreds of times, but not hide it entirely. Spectroscopy of the host galaxy, pioneered by Edwin Hubble with his redshift law, revealed active star formation and low metallicity — an environment favorable for the birth of massive stars and, perhaps, black holes.
The discovery of AT 2019ijn forces us to rethink our ideas about jet launching by intermediate-mass black holes. Usually, these 'middleweights' were thought incapable of super-Eddington accretion, necessary for forming a collimated outflow. However, the observed facts say otherwise. Perhaps events like this are the missing link between 'ordinary' TDEs and mysterious LFBOTs (fast blue optical transients), whose nature is still debated. Future radio sky surveys like SKA and DSA will put the search for off-axis jets on a conveyor belt, and we'll be able to paint a full picture of these cosmic catastrophes. Ultimately, each caught whisper of a distant searchlight brings us closer to answering a fundamental question: how matter falls into the abyss and why sometimes it shoots back out as blinding beams.
If the jet of AT 2019ijn had been aimed directly at Earth, we would have seen one of the brightest gamma-ray bursts, but fortunately, it was pointed away, and we observe only its 'echo' in the radio range.
In Carl Sagan's 'Contact,' a radio signal from Vega launches a grand project; if we had mistaken AT 2019ijn's emission for artificial, an equally thrilling adventure might await us.