Five days up, forty-six days down
The source was found in the nucleus of a dwarf galaxy at a redshift of 0.2729 by the Zwicky Transient Facility (ZTF), and the light curve this telescope survey produced is unusual in a specific way. It reached a peak absolute magnitude of minus 21.05 in 5.26 days, then took 46.3 days to fall to half that brightness. Through both phases the fitted blackbody temperature stayed between roughly 14,500 and 16,000 kelvin, so the source did not redden the way an ordinary explosion does as it cools.[1]
Fast rise, slow decay and a colour that will not cool is the combination defining the class of luminous fast blue optical transients against which the authors measure this object. That the temperature is a fit to a blackbody rather than a direct measurement matters: it constrains the emitting surface only as far as the blackbody assumption holds. A colour temperature does not establish that the source has a surface; it describes how the light is assembled.[1]
The number that does not fit
The radio emission peaked 641 days after the optical discovery, at 2.0 times 10 to the power of 31 erg per second per hertz. In this peer-reviewed study the authors state this is more than an order of magnitude above any known luminous fast blue optical transient or superluminous supernova, and the delay is as informative as the brightness. A peak arriving nearly two years late means the emitting shock only reached dense enough surroundings at that point, or was still being fed.[1]
Their preferred reading is a jet launched as a star came apart on an intermediate-mass black hole of roughly 1.3 times 10 to the power of 5 solar masses, seen about 39 degrees off its axis. The isotropic-equivalent jet energy of 6.9 times 10 to the power of 54 erg should be read as bookkeeping rather than a physical energy: it is what the source would need if it radiated equally in every direction, which an off-axis jet by construction does not. The paper states plainly that a jetted magnetar cannot be fully ruled out, so what we hold is two explanations rather than one.[1]
What the next observation has to separate
The authors note that their model departs from the later S-band data and attribute the gap to their own simplifications, a top-hat jet profile and a uniform interstellar density. That is precisely where the two pictures can be pressed, because both simplifications are assumptions about geometry and environment, and continued multi-frequency radio observation constrains them directly.[1]
A black hole of 10 to the power of 5 solar masses sits in the mass range that neither stellar collapse nor the well-studied population of giant black holes accounts for comfortably, which is why an uncertain identification is still worth something. One object with two live explanations does not settle what AT2019ijn was; it narrows where the next survey should point and what a follow-up would have to measure in order to choose. Whether the radio excess belongs to the jet or to what the jet ran into is the question the light has left us.[1]