In 2017, the IceCube detector caught a neutrino from the distant galaxy TXS 0506+056. It was like hearing a phone ring from across the ocean: the signal came from where the most energetic particles in the Universe are born. At the same time as the neutrino, astronomers spotted a gamma-ray flare, and two to three years later, radio waves. Why was the radio late? One hypothesis: the particle-producing region expands, becomes transparent to radio waves, and only then do we see its glow.
The authors of the new study built a computer model of this expansion and compared it with data from the RATAN-600 radio telescope. The simple scenario did not work: if the radio came from the same region, the light curve would look different. So the scientists added a new detail to the model — re-energization farther down the jet, after the material had already scattered. After that, the model matched the observations.
The conclusion is simple: the delayed radio is not an echo of neutrino production but a separate event in the jet's life. It's like hearing thunder not from the lightning you saw but from another one that flashed later beyond the horizon.
The RATAN-600 radio telescope is a ring of 895 aluminum panels with a diameter of 576 meters. It scans the sky as the Earth rotates and can pick up radio waves from objects billions of light-years away.
In the movie Interstellar, the characters receive messages through gravitational anomalies, but the signal delay due to the expansion of space reminds us how a radio flare from a blazar arrives years after the event.