2025MNRAS.542.2733K

We continue investigating the observed properties of the quasar 2005$+$403 seen through the highly turbulent plasma in the Cygnus region. Our earlier study revealed a great influence of propagation effects on the observed data, e.g. numerous episodes of multiple imaging formation, angular broadening of the source size, and the detection of an extreme scattering event (ESE) in 2019, making it a good probe for thermal plasma in the interstellar medium (ISM). We report the first detection of multifrequency and multi-epoch ESEs revealed with the RATAN-600 daily light curves of the quasar 2005$+$403. The most prominent ESE flux density modulations are found in 2011, 2015, and 2020 and each lasted about 4-5 months. We fitted the detected ESEs jointly at 4.7/4.8, 7.7/8.2, and 11.2 GHz using the ESE models, which allowed us to constrain the angular and linear size of the scattering lens averaged over the three ESEs $0.3\pm 0.1$ mas and $0.6\pm 0.1$ au, proper motion $8.3\pm 0.7$ mas yr$^{-1}$, and transverse velocity $70.1\pm 5.7$ km s$^{-1}$ assuming that the lens distance is 1.8 kpc, maximum free-electron density on the line of sight $1183\pm 124$ cm$^{-3}$ and lens mass $(0.8\pm 0.4)\times 10^{-15}\, \rm M_\odot$. Using the fitting results, we reconstructed the average intrinsic unscattered angular size of the quasar which is equal to $1.57\pm 0.14$, $0.95\pm 0.08$, and $0.68\pm 0.04$ mas at 4.7/4.8, 7.7/8.2, and 11.2 GHz, respectively. We also report on the first detection of up to six ESEs in a row that occurred in the period of 2015-2016, apparently created by multiple lenses successively crossing the line of sight.