High-Frequency Radio Flaring of the FSRQ PKS 1725+123

ATel #17356; J. Hessdoerfer, F. Eppel, M. Kadler, W. Schulga (all University of Wurzburg), P. Benke (GFZ Potsdam), A. Gokus (WUSTL)
on 23 Aug 2025; 05:26 UT
Credential Certification: Matthias Kadler (matthias.kadler@astro.uni-wuerzburg.de)

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We report radio flaring activity of the flat-spectrum radio quasar PKS 1725+123 at radio frequencies in the 20mm (14-17GHz), 14mm (19-25GHz) and 7mm (36-44GHz) band, detected through regular monitoring observations within the TELAMON program (see Eppel et al., 2024, A&A, 684, A11). This increase in radio flux density coincides with the first TeV-detection of PKS 1725+123 by MAGIC/LST-1 (ATel#17344) and H.E.S.S (ATel#17346), as well as the optical light curve being close to its historical maximum (ATel#17345) and an ongoing active state in the Fermi gamma-ray band (ATel#17316).

We have observed PKS 1725+123 on 2025/08/20 with the Effelsberg 100-m radio telescope and found the following flux densities, averaged over several sub-frequencies per band:

20mm: (1.094 ± 0.034) Jy

14mm: (1.141 ± 0.055) Jy

7mm: (1.51 ± 0.12) Jy.

This corresponds to an increase of the 7mm flux density by a factor of about four over the last year, reaching unprecedented values for this source since the start of our monitoring in July 2021.

We find an inverted spectral index of α = 0.275 ± 0.080 (S ∝ ν^α) for the most recent observation which is similar to the spectral indices seen in this source before the flare.

Additionally, at 20mm we find a linear polarization degree of (5.27 ± 0.36)% with an EVPA of (-25.2 ± 3.8) deg. There are no indications of significant polarization angle swings throughout our monitoring observations, however the linear polarization degree seems to be on the lower end, with previous measurements in 2022 reaching values as high as (10 ± 1)%.

The full light curve measured within the scope of the TELAMON program can be inspected on our website.

We started observing this source in July 2021 as part of a neutrino follow-up of the IceCube event IC201021A. Nanci et al. (2022, A&A 663, A129) suggest PKS1725+123 to be a promising neutrino candidate as it was in an enhanced activity state at the time of the neutrino detection. In July 2021, 10 months after the neutrino event, the 7mm flux density was still slightly elevated at (0.514 ± 0.027) Jy, before continuously dropping to its low state (0.291 ± 0.024) Jy in May 2022.

The TELAMON program uses the Effelsberg 100-m telescope to monitor the radio spectra of very-high-energy emitting active galactic nuclei (AGN), namely TeV blazars and candidate neutrino-associated AGN, with high-cadence high-frequency observations every ~3 weeks and at multiple frequencies up to 44 GHz. We will continue monitoring the source regularly and encourage further multi-messenger observations.