MeerKAT monitoring of the flaring CSO PMN J1603-4904
ATel #17239; G. Bruni (INAF-IAPS), M. Brienza, G. Migliori, M. Giroletti, E. Liuzzo (INAF-IRA), E. Bronzini (U. Bologna & INAF-OAS), M. Kadler (U. Wuerzburg), L. Hernandez-Garcia (U. Valparaiso), F. Panessa, J. Rodi (INAF-IAPS), E. Torresi, P. Da Vela (INAF-OAS), M. Cerruti (U. Paris Cite), P. Goswami (U. Heidelberg), C. Reynolds (CSIRO)
on 20 Jun 2025; 08:12 UT
Credential Certification: Gabriele Bruni (gabriele.bruni@inaf.it)
Subjects: Radio, VHE, AGN
The radio source PMN J1603-4904 has long defied classification within the AGN framework. While its GeV emission, detected with Fermi/LAT (1FHL, Ackermann et al. 2013, ApJS, 209, 34), suggests a jet oriented towards the observer - i.e., a blazar - multiwavelength studies, including VLBI imaging (Mueller et al. 2014, A&A, 562, A4), instead point to a young, misaligned radio galaxy, specifically a compact symmetric object (CSO). Its recent detection in the VHE regime by H.E.S.S. (Wagner et al. 2025, ATel #17205) opens new avenues for understanding high-energy processes in compact radio galaxies, a phenomenon previously observed in only one other CSO with LHAASO (NGC 4278, Cao et al. 2024, ApJS, 271, 25).
Following the H.E.S.S. collaboration call for multiwavelength follow-up by the broader community, we initiated a MeerKAT monitoring campaign to investigate a possible radio brightening of PMN J1603-4904 (DDT-20250604-GB-01, P.I. Bruni). Observations were conducted on 14 June 2025 at 3 GHz (S4 band), with 15 minutes on-source. Phase referencing was applied, and data were reduced using the SARAO Science Data Processor (SDP) continuum pipeline. The resulting image has an angular resolution of 4.66 x 3.82 arcsec, P.A. 53.2 deg, and an RMS noise level of 31 uJy/beam. As expected, the source appears unresolved, with a measured flux density of 1.39 +/- 0.07 Jy, taken 21 days after the H.E.S.S. detection.
PMN J1603-4904 serves as a radio calibrator and is included in the ATCA calibrator database. It was observed in September 2024 at 2.1 and 5.5 GHz, showing flux densities of 1.537 +/- 0.019 Jy and 1.088 +/- 0.016 Jy, respectively (https://www.narrabri.atnf.csiro.au/calibrators/calibrator_database_viewcal.html?source=1600-48). These values yield a spectral index of -0.36 +/- 0.02 between 2.1 and 5.5 GHz, which interpolates to an expected flux density of 1.36 +/- 0.02 Jy at 3 GHz. Therefore, the flux density measured with MeerKAT is consistent with the pre-flare baseline.
A second MeerKAT epoch is scheduled for the end of June, when the VHE observing window - closed during the first epoch due to lunar constraints - will once again be open.
