Radio Monitoring of the Changing-look AGN Mrk 590 I: VLA Observations Reveal A Bow-shock Driven Radio Brightening Event

Active galactic nuclei are the brilliant, energetic hearts of distant galaxies, powered by supermassive black holes that consume surrounding gas and dust. While these cosmic engines are usually steady, a rare subset known as "changing-look" active galactic nuclei can undergo dramatic transformations, suddenly fading from bright, active states to near silence, or reigniting after years of dormancy. These shifts are well-documented in visible light, ultraviolet, and X-rays, where astronomers can directly see the accretion disk—the swirling disk of material feeding the black hole—change its behavior. However, the radio waves emitted by these same objects, which often trace powerful jets of particles shooting out from the black hole, have remained a mystery. Scientists have long wondered if the radio jets respond in sync with the feeding disk, or if they operate on their own separate schedule. Understanding this relationship is crucial because it reveals how energy moves from the immediate vicinity of a black hole out into the vast space of its host galaxy.

In a recent study, researchers turned their attention to Mrk 590, a changing-look active galactic nucleus that famously dimmed over several decades before beginning to brighten again in 2017. To understand what was happening to its radio emissions during this transition, the team used the Karl G. Jansky Very Large Array, a massive radio telescope in New Mexico, to observe the galaxy at four different times between 2015 and 2017. They tuned the telescope to listen across a wide range of radio frequencies, from 1 to 17 gigahertz, creating a detailed picture of the galaxy's radio source. What they found was a compact, bright core that changed significantly over time. In 2015, the radio spectrum—the way the brightness changed across different frequencies—had a distinct hump, suggesting the radio waves were being absorbed by something nearby. By 2016 and 2017, as the galaxy began to reawaken in visible light, the radio source did not just get brighter; its shape changed, becoming a steady, rising slope that indicated a new, powerful source of energy was emerging.

To figure out what was driving this radio brightening, the team built sophisticated computer models to test different physical explanations. They first considered whether the changes were caused by the interstellar medium of our own Milky Way galaxy, which can sometimes act like a lens to magnify distant radio sources. However, the timing of the changes was far too slow for this effect, which usually happens over hours, not months. They also tested the idea that a star had been torn apart by the black hole, an event known as a tidal disruption event. This was ruled out because the radio brightening started at least a year before the visible light brightened, whereas in known tidal disruption events, the radio signal always follows the light. The most likely explanation, supported by their statistical analysis, is that a bow shock is driving the variability. As the galaxy's radio jet pushes outward, it crashes into the dense gas of the host galaxy, creating a shock front similar to the wave of water in front of a speeding boat. This collision compresses magnetic fields and accelerates electrons, generating a fresh burst of radio waves that causes the observed brightening.

The researchers also examined whether this radio activity was directly linked to the black hole's feeding frenzy that caused the galaxy to reawaken in 2017. By calculating the size of the region where the radio changes were happening, they determined it was located much farther away from the central black hole than the accretion disk itself. This spatial separation suggests that the radio brightening and the optical brightening are not part of the same immediate event. While the galaxy's feeding disk was changing state and lighting up in visible light, the radio jet was experiencing a separate, independent event driven by its interaction with the surrounding galaxy gas. This finding highlights the diversity of changing-look active galactic nuclei; unlike some other galaxies where the radio and light emissions rise and fall together, Mrk 590 shows that these different types of cosmic activity can be driven by distinct mechanisms. The study concludes that while the black hole was indeed re-igniting its engine, the radio waves we see are telling a different story, one of a jet pushing through the cosmic landscape and creating its own shockwaves, independent of the black hole's immediate feeding habits.