Among active galactic nuclei, there is a rare subclass known as changing-look AGN: their brightness in optical, ultraviolet, or X-ray light changes by factors of a few over months or years. Mrk 590 is one of the most studied such systems: it faded slowly since the 1980s and flared again in 2017. The radio band has been barely explored for these objects, even though it could reveal whether changes in accretion are linked to outflows of matter. This work stands apart: neighboring studies focus on fast radio bursts and pulsars, not on the long-term evolution of a specific nucleus.
From June 23, 2015, to January 10, 2017, astronomers conducted four observing sessions of Mrk 590 with the VLA in its most detailed A configuration. They obtained images at frequencies of 1.52, 5.75, 9.30, and 16.0 GHz, then constructed spectra (brightness versus frequency) for each epoch. To identify the absorption mechanism at work, they used Bayesian inference: they compared four models, namely synchrotron self-absorption and three variants of free-free absorption (homogeneous, internal, and inhomogeneous). Model selection relied on the Bayesian evidence computed via nested sampling.
The radio source in Mrk 590 turned out to be compact: its size does not exceed 64 parsecs. The spectrum in 2015 was convex, peaking around 15.6 GHz, and by 2017 it had become steeply rising with frequency. The flux at 15–17 GHz doubled by October 2016, and by January 2017 all frequencies above 2 GHz had changed by at least a factor of 1.3. The Bayesian analysis showed that the 2015 spectrum is best described by the inhomogeneous free-free absorption model: a shock wave from the jet ionizes gas with varying density. This suggests that the radio flare is caused by a shock wave, not by changes in the accretion disk.
This work shows that the radio variability in Mrk 590 does not correlate with the optical and X-ray "awakening" of 2017. Unlike neighboring studies where radio flares were linked to accretion events (for example, in 1ES 1927+654), here the shock wave is located far from the black hole, at a distance of about 6 parsecs. This highlights the diversity of mechanisms in changing-look AGN: in some objects, magnetic fields near the event horizon change, while in others they do not. The stability of the soft X-ray excess in Mrk 590 confirms that the magnetic field near the disk did not change.
This topic could advance through long-term monitoring with very long baseline interferometry (VLBI). Such observations would separate the shock wave emission from the compact core and test whether there is any weak connection between accretion and the jet. The authors already plan to analyze multi-year VLBA data in a follow-up paper.
The results will affect our understanding of the disk–jet connection in active galactic nuclei and how we interpret radio variability in changing-look objects.
Conduct multi-year monitoring of Mrk 590 with the VLBA and compare it with other changing-look AGN to assess how common shock waves are as a source of radio flares.
A key open question in active galactic nuclei astrophysics is what exactly triggers transitions between accretion states and how they relate to outflows. This work shows that radio flares can be caused by local interactions between the jet and gas, rather than global changes in the disk, complicating any unified picture.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors carried out four observing sessions of Mrk 590 with the VLA in its A configuration between 2015 and 2017, obtained images at four frequencies, and constructed spectra. They applied Bayesian inference to compare absorption models and found that the 2015 spectrum is best described by inhomogeneous free-free absorption, pointing to a jet-driven shock in the interstellar medium.
Where its strength is
This work demonstrates the power of Bayesian model comparison for diagnosing absorption mechanisms in compact radio sources, and it establishes that the radio variability in Mrk 590 is not directly tied to changes in the accretion disk.
Where it could go next
Worth trying
Why this may matter
Understanding the mechanisms behind radio variability in changing-look AGN is crucial for clarifying how jets interact with their environment and how accretion and outflows are connected in active galactic nuclei.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.56, against a usual 0.64. It looks like few have drilled nearby.
The radio source in Mrk 590 at 5.75 GHz has a faint "tail" to the northeast, aligned with an outflow seen at very high resolution. If this component is moving at the speed of light, it left the core about two thousand years ago, around the time of the Roman Empire.