A multi-band radio flux density catalog of ICRF3 sources using the Onsala Twin Telescopes
This paper presents a multi-band radio flux density catalog for 361 ICRF3 sources derived from Onsala Twin Telescopes observations, demonstrating that these time-series measurements significantly improve the prediction of geodetic signal-to-noise ratios and optimize VGOS scheduling by accounting for the prevalent flux variability and spectral characteristics of the sources.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine the universe as a giant, cosmic stage where the most dramatic actors are Active Galactic Nuclei (AGN). These aren't your average stars; they are the supermassive black holes sitting at the centers of galaxies, feasting on gas and spitting out powerful jets of energy that shoot across space at nearly the speed of light. To us, they look like incredibly bright, flickering beacons in the radio part of the spectrum. But here's the twist: these beacons are notoriously unreliable. Their brightness can change wildly, sometimes in just a few months, like a lightbulb that decides to dim or flare up without warning.
Why do we care about these cosmic mood swings? Because they are the lighthouses for a very precise science called geodesy. Geodesy is the art of measuring the Earth itself—figuring out exactly where our continents are, how the planet wobbles as it spins, and how the ground shifts over time. To do this, scientists use a technique called Very Long Baseline Interferometry (VLBI). Think of VLBI as a giant, planet-sized camera made by linking radio telescopes from all over the world. To take a sharp picture of the Earth's shape, these telescopes need to look at the same AGN at the exact same moment. But to get a clear picture, the telescopes need to know exactly how bright the AGN is right now. If they guess wrong, the "camera" gets blurry, and our map of the Earth gets fuzzy. For years, scientists have had a "cheat sheet" of how bright these stars are, but it's been a bit like using an old, static map for a city that's constantly under construction.
This is where a team of astronomers from Sweden steps in with a fresh, dynamic approach. They realized that the next generation of these Earth-measuring telescopes, known as VGOS, is incredibly fast and sensitive, but it needs a real-time update on the brightness of its targets. To get this, they turned to the Onsala Twin Telescopes in Sweden. Instead of using them as a pair of eyes looking at the sky from two different spots (which is their usual job), they used them as a single, super-sensitive radio ear to listen to the "volume" of 361 different cosmic beacons. They didn't just listen once; they monitored these sources over several years, checking their brightness in four different radio "colors" (frequencies) simultaneously: 3.2, 5.5, 6.6, and 10.4 GHz.
What they found was a universe in constant motion. The team discovered that the vast majority of these cosmic beacons are indeed fickle. Most of them change their brightness significantly over time, and many of them have "flat" or "inverted" radio spectra, meaning their brightness doesn't drop off as you go to higher frequencies like we might expect from a simple lightbulb. In fact, only about 6% of the sources they studied had a "steep" spectrum where the signal drops off quickly. They also noticed that for many sources, the "color" of their radio light (the spectral index) changes over time, shifting from flat to inverted and back again.
The most exciting part of their discovery is how this new, live data improves the "Earth camera." When the researchers used their new, up-to-date brightness measurements to predict how well the telescopes would perform, the predictions were much sharper than before. The old cheat sheet often guessed the brightness wrong, leading to poor scheduling of observation time. The new catalog, however, allowed them to predict the signal strength with much greater accuracy, especially for the most unpredictable, variable sources. While there is still some room for improvement—particularly in accounting for how the Earth's atmosphere and the specific shape of the radio sources affect the signal—this work proves that keeping a live watch on these cosmic beacons is essential. It's the difference between trying to navigate with a map from last year versus having a live GPS feed that updates every time the traffic changes. The authors plan to keep this monitoring program going, adding more telescopes and sources, ensuring that both the astronomers studying the black holes and the geodesists mapping our planet have the most accurate, up-to-the-minute data possible.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.