Galaxy Mrk 590 behaves like a radio receiver that sometimes picks up a signal and sometimes fades into static. Its supermassive black hole has been brightening and dimming for decades, and astronomers caught every such 'switch-on.' In 2017, the receiver came alive again, but the VLA radio telescope heard not the signal they expected.

From 2015 to 2017, the VLA pointed at Mrk 590 four times and listened at four frequencies, as if tuning into different stations. The radio spectrum changed: at first it looked like a hump peaking around 15.6 GHz, and two years later it rose steeply with frequency. The flux at high frequencies doubled. Such a signature does not match the awakening of the black hole itself.

Astronomers tested four models of how gas might absorb radio waves and compared them with observations. The best fit was a model where the shock wave ionizes gas of varying density. This means the radio flare was born not right at the black hole but about 6 parsecs away, like the echo of a distant explosion rather than the voice of the explosion itself.

Now it is clear why the optical and X-ray 'awakening' of Mrk 590 in 2017 did not coincide with the radio flare: they are different events. In some active nuclei, magnetic fields near the event horizon change; in others, they do not. Mrk 590 turned out to be one of the latter. Its radio receiver picks up not the black hole itself but the aftermath of its jet's impact on gas, and that changes the tuning for future observations.