Variability of emission at scales from fractions of a second to tens of years is one of characteristic properties of different celestial objects. Transient events are also the emission variability though not regularly observed. Such events are difficult to detect for that reason. During the analysis of the surveys carried out on RATAN-600, it was found that almost one fifth of the detected sources shows significant radio flux variations. We discovered three radio transients in our search (Fig.1).
    The flux variability detected with the telescopes in different ranges of electromagnetic radiation can be caused by variable processes in the active galactic nucleus (AGN), the variability of relativistic jet velocity or its orientation relative to an observer, absorption variations in the line of sight, and gravitational microlensing. Stars may also be variable. Transients are associated with different types of events and objects. They can be supernovae including radio supernovae, afterglows of gamma-ray bursts (GRB). Slow radio transients in the Galaxy are generated by different stellar populations such as outbursts detected from M dwarfs, cataclysmic variables, and X-ray binaries. The events of interest are also associated with a sudden increase of accretion rate due to tidal explosion of a star approached too close to a supermassive object with a mass of millions and billions of solar masses. This can cause a soft X-ray burst or, probably, radio emission. In the majority of papers dedicated to radio transients they usually were found incidentally in archive data but this information, nevertheless, allows one to estimate the frequency of occurrence of such objects and to attempt to explain their nature.
    We used the data from "Cold" radio surveys carried out with RATAN-600 in 1980-1994 to search for radio sources with significant flux variability. Seventy-three variable radio sources were detected with considerable fluctuations (Fig. 1) which are variable at scales of several years according to different statistical criteria. The long-term variability for fifty-two radio sources was detected for the first time. Host objects of the variable radio sources refer to active galaxies. First half of these objects was determined from the Sloan Digital Sky Survey spectra and the rest - optically fainter - from the SDSS classification or from color indices of the WISE infra-red survey.
    For the three detected transient events (Fig.2), we searched for coordinate matching with the lists of supernovae and gamma-ray burst events, with cataclysmic events in the Galaxy, with Solar system objects, and space missions. Drawing the data from radio and optical surveys and also from Vizier, SIMBAD, and NED astronomical databases, we tried to interpret the nature of transients. Most probably, they refer to three different events. Radio emission of the first event can be caused by outburst activity in radioquiet AGN, the second one is linked with GRB afterglows or radio supernovae. The nature of the third radio transient is debatable, as it is probably localized in the Solar system according to the changes of its coordinates.
Published:
1. Е.К.Майорова, О.П.Желенкова. On possibility of detection of variable sources using the data of Cold surveys carried out on RATAN-600. Astrophysical Bulletin, Vol. 67, Issue 3, pp. 318-339 (2012).
2. E.Majorova, O.Zhelenkova. Detection of Variable Sources Using the Data of "Cold" Surveys. Astrophysical Bulletin 68, pp. 371-395 (2013);
3. Majorova E., Zhelenkova O., Temirova A. Search for variable sources using the data of Cold surveys in the right-ascension interval 2^h≤RA≤6^h. Astrophysical Bulletin, Volume 70, Issue 1, pp.33-44 (2015).
4. O.Zhelenkova, E.Majorova. Search for Radio Transients and New Detection of Radio Sources Using the RATAN-600 Surveys in 1980-1994. (Astrophysical Bulletin, in press).
5. O.Zhelenkova, E.Majorova. Observational Manifestations and Intrinsic Properties of the RCR Sources in the Framework of the Normalized Model. (Astrophysical Bulletin, in press).