Long-term Swift and Metsähovi monitoring of SDSS J164100.10+345452.7 reveals multi-wavelength correlated variability

¹ INAF, Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, 23807 Merate (LC), Italy
e-mail: patrizia.romano@inaf.it
² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³ Aalto University Department of Electronics and Nanoengineering, PO Box 15500 00076 Aalto, Finland
⁴ European Southern Observatory (ESO), Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
⁵ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁶ Department of Physics, Institute for Astrophysics and Computational Sciences, The Catholic University of America, Washington, DC 20064, USA
⁷ Dipartimento di Fisica, Università di Trento, Via Sommarive 14, Trento 38123, Italy
⁸ Dipartimento di Fisica e Astronomia, Università di Padova, 35122 Padova, Italy
⁹ European Space Agency, European Space Astronomy Centre, C/ Bajo el Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, 440 W. Brooks St., Norman, OK 73019, USA
¹¹ Università degli Studi di Milano, Bicocca, Piazza delle Scienze 3, 20126 Milano, Italy

Received: 18 January 2023
Accepted: 8 March 2023

Abstract

We report on the first multi-wavelength Swift monitoring campaign performed on SDSS J164100.10+345452.7, a nearby narrow-line Seyfert 1 galaxy that had formerly been considered to be radio-quiet. It has, however, more recently been detected both in the radio (at 37 GHz) and in the γ-ray, a behaviour that hints at the presence of a relativistic jet. During our 20-month Swift campaign, while pursuing the primary goal of assessing the baseline optical/UV and X-ray properties of SDSS J164100.10+345452.7, we observed two radio flaring episodes, namely, one each year. Our strictly simultaneous multi-wavelength data closely match the radio flare and allow us to unambiguously link the jetted radio emission of SDSS J164100.10+345452.7. Indeed, for the X-ray spectra preceding and following the radio flare, a simple absorbed power-law model does not offer an adequate description and, thus, an extra absorption component is required. The average spectrum of SDSS J164100.10+345452.7 can best be described by an absorbed power-law model with a photon index Γ = 1.93 ± 0.12, modified by a partially covering neutral absorber with a covering fraction of f = 0.91_−0.03^+0.02. On the contrary, the X-ray spectrum closest to the radio flare does not require any such extra absorber and it is much harder (Γ_flare ∼ 0.7 ± 0.4), thus implying the emergence of an additional, harder spectral component. We interpret this as the jet emission emerging from a gap in the absorber. The fractional variability we derived in the optical/UV and X-ray bands is found to be lower than the typical values reported in the literature because our observations of SDSS J164100.10+345452.7 are dominated by the source being in a low state, as opposed to the literature, where the observations were generally taken as a follow-up of bright flares in other energy bands. Based on the assumption that the origin of the 37 GHz radio flare is the emergence of a jet from an obscuring screen also observed in the X-ray, the derived total jet power is P_jet^tot = 3.5 × 10⁴² erg s⁻¹. This result is close to the lowest values measured in the literature.