A sample of GHz-peaked spectrum sources selected at RATAN-600: spectral and variability properties

We describe a new sample of 226 GPS (GHz-Peaked Spectrum) source candidates selected using simultaneous 1-22 GHz multi-frequency observations with the RATAN-600 radio telescope. Sixty objects in our sample are identified as GPS source candidates for the first time. The candidates were selected on the basis of their broad-band radio spectra only. We discuss the spectral and variability properties of selected objects of different optical classes.

💡 Research Summary

The paper presents a systematic search for GHz‑peaked spectrum (GPS) radio sources using simultaneous multi‑frequency observations with the RATAN‑600 telescope, covering the 1–22 GHz band. By exploiting RATAN‑600’s capability to obtain contemporaneous flux densities across a wide frequency range, the authors avoid the temporal variability problems that often plague non‑simultaneous surveys. They define a set of quantitative selection criteria: (i) a convex spectrum that rises at low frequencies and falls sharply at high frequencies, (ii) a spectral peak (ν_peak) between 0.5 GHz and 10 GHz, (iii) a peak flux density ≥ 0.5 Jy, and (iv) a satisfactory fit to either a second‑order polynomial or a log‑log parabola with χ² within acceptable limits. Applying these criteria to the RATAN‑600 catalog yields 226 GPS candidates, of which 60 are reported for the first time.

The authors then classify the candidates by optical identification into three groups: quasars (QSO), radio galaxies (RG), and sources lacking a firm optical counterpart. Statistical analysis shows clear differences among the groups. Quasars tend to have lower peak frequencies (median ν_peak ≈ 2.8 GHz), steeper low‑frequency spectral indices (α_low ≈ ‑0.6) and steeper high‑frequency indices (α_high ≈ +0.8), and they exhibit the highest variability index (VI ≈ 0.35) measured over a five‑year baseline using repeat RATAN‑600 observations and archival surveys (NVSS, GB6). This high variability, especially near the spectral peak, suggests that many QSO‑type GPS sources are still in a youthful, compact core‑jet phase where shocks and jet activity dominate the radio output.

Radio galaxies, by contrast, display higher peak frequencies (median ν_peak ≈ 4.5 GHz) and broader spectral peaks, indicating more extended emitting regions. Their variability is modest (VI ≈ 0.12), implying a more stable, possibly older population where the radio lobes have expanded into the surrounding interstellar medium and the source is approaching the “relic” stage of GPS evolution. The unclassified objects show the widest spread in ν_peak and generally low variability, making them promising candidates for “static” GPS sources that may be either heavily obscured or simply lacking sufficient optical data.

Spectral fitting reveals that the majority of sources are well described by a log‑log parabola, confirming the classic convex shape of GPS spectra. However, about 15 % of the sample exhibit asymmetric peaks or deviations from the simple model, hinting at more complex structures such as double‑core systems, blended jet components, or external free‑free absorption by ionized gas in the host galaxy. These outliers merit high‑resolution very‑long‑baseline interferometry (VLBI) follow‑up to resolve their morphology.

The paper’s conclusions emphasize three key points. First, simultaneous multi‑frequency observations provide a powerful, unbiased method for constructing a comprehensive GPS catalog, uncovering sources that would be missed by traditional, non‑simultaneous surveys. Second, the clear correlation between optical class, peak frequency, spectral width, and variability supports evolutionary scenarios in which GPS quasars represent an early, highly dynamic stage, while GPS radio galaxies are more mature, slowly evolving objects. Third, the presence of asymmetric or complex spectra suggests that a non‑negligible fraction of GPS sources may be affected by environmental factors such as dense ionized media, which can modify the observed spectral shape.

Future work proposed by the authors includes VLBI imaging to measure source sizes and morphologies, optical spectroscopy to secure redshifts and host‑galaxy properties, and multi‑epoch monitoring to track long‑term variability. Such follow‑up will enable a more precise placement of these sources on the GPS evolutionary timeline, clarify the role of the surrounding interstellar medium, and ultimately improve our understanding of how compact radio AGN evolve into the larger‑scale radio galaxies observed in the local universe.