Broadband radio flaring of the FSRQ PKS 1725+123 from 2 to 22 GHz

We report significant cm-band radio brightening of the flat-spectrum radio quasar (FSRQ) PKS 1725+123 observed with the RATAN-600 and RT-32 radio telescopes. This follows the recent enhanced activity in &gamma-rays (ATel #17316 and #17370), optical (ATel #17345), mm-band radio (ATel #17356), and, for the first time, in the VHE band with MAGIC (ATel #17344) during July-August 2025. PKS 1725+123 is a low-synchrotron-peaked blazar with a one-sided milli-arcsecond-scale jet, now detected at VHE, exhibiting prominent multiband variability and considered a candidate neutrino emitter since IceCube event IC201021A.

On 2025 August 25, we measured flux densities of PKS 1725+123 at five frequencies using the RATAN-600 telescope, obtaining the following values (in Jy):

• 1.18 ± 0.25 at 22.3 GHz
• 0.67 ± 0.06 at 11.2 GHz
• 0.62 ± 0.06 at 8.2 GHz
• 0.48 ± 0.04 at 4.7 GHz
• 0.35 ± 0.02 at 2.3 GHz

Additional measurements were carried out with the radio telescopes RT-32 in Zelenchukskaya and Badary observatories (Institute of Applied Astronomy, Russian Academy of Sciences, https://iaaras.ru/en/quasar/) during the sessions on 2025 August 23, 24 and 30. The obtained flux density at 5.05 GHz is 0.55 ± 0.03 Jy on August 23, and 0.53 ± 0.04 Jy on August 24. At 8.63 GHz we obtained the flux density of 0.81 ± 0.03 Jy on August 23, and 0.77 ± 0.04 Jy on August 30.

Figures illustrate the long-term radio light curves in 2017-2025 and broadband spectra of PKS 1725+123 including the recent RATAN-600 and RT-32 measurements and archival data, highlighting the current flaring episode. The spectral shape remains inverted with the median spectral index value of +0.30 (0.05) at 2.3-22.3 GHz (S ~ &nu^&alpha).