Investigation of intra-day variability of the radio galaxy 3C 84 (Perseus A) flux density in centimeter range on the RT-32 VIRAC (Latvia) and RT-32 NSFCTC (Ukraine) radio telescopes

Abstract

Extragalactic radio source 3C 84 (also known as Perseus A) is the active nucleus of the giant galaxy NGC 1275, the largest in the Perseus galaxy cluster (Abell 426). It is one of the brightest X-ray galaxy clusters. The nucleus of galaxy NGC 1275 (Seyfert II type) is a powerful radio source in the Northern Sky (flux density at frequency 8 GHz is about 30 Jy). 3C 84 has a long-term variability in the radio band, with a “wave-like” change in the flux density on a time scale of several decades. The purpose of our work was to search for and study the properties and features of the appearance of fast variability of the 3C 84 radio source in the centimeter radio range. For this, as part of the international Ukrainian–Latvian cooperation between the Institute of Radio Astronomy NAS of Ukraine and the Ventspils International Radio Astronomy Centre, many observation sessions were performed (October 2020 – September 2021) at the 32-m radio telescopes in Ukraine (Zolochiv) and Latvia (Irbene), with additional observations at the 16-m radio telescope (Irbene). As a result, the following main results were obtained: the 3C 84 radio source exhibits manifestations of episodic intraday variability, with characteristic times, usually in the range 3–8 hours, as well as “calm phases”, when there is no significant variability, and the source flux density does not change over several observation sessions. The most likely cause of this variability could be interstellar scintillations, but some quasi-simultaneous observations of 3C 84 carried out at the 32-m radio telescopes in Ukraine and Latvia showed the presence of a correlation in cross-Fourier spectra, with a characteristic time about 6 hours. In addition, the appearance of 3C 84 flux variability with characteristic times from 1–3 hours to several tens of minutes were recorded, which were not associated with radiointerference or hardware effects. They were detected in observations both in Ukraine and in Latvia, in different seasons of the year.