Sun-related variability in the light curves of compact radio sources

Context. Compact radio sources can show remarkable flux density variations at gigahertz frequencies on a wide range of timescales. The origin of the variability is a mix of source-intrinsic mechanisms and propagation effects, the latter being generally identified with scattering from the interstellar medium. Some of the most extreme episodes of variability, however, show characteristics that are not consistent with any of the explanations commonly proposed. Aims. We carried out an in-depth analysis of variability at radio frequencies on light curves from the impressive database of the US Navy’s extragalactic source monitoring program at the Green Bank Interferometer (GBI) – a long-term project mainly aimed at the investigation of extreme scattering events. The GBI data have been complemented for selected sources by light curves from the University of Michigan Radio Astronomy Observatory (UMRAO). The purpose of the present work is to identify events of flux density variations that appear to correlate with the position of the Sun. Methods. We inspected the 2 GHz and 8 GHz light curves observed in the framework of the GBI monitoring program in order to search for one-year periodic patterns in the data. Variations on timescales below one year were isolated through a de-trending algorithm and analysed, with the aim of looking for possible correlations with the Sun’s position relative to the sources. Results. Objects at an ecliptic latitude below ∼20° show one-year periodic drops in flux densities, centred close to the time of minimum solar elongation. Both interplanetary scintillation and instrumental effects may contribute to these events. However, in some cases, the drops extend to much larger angular distances, affecting sources at high ecliptic latitudes and causing variability on timescales of months. Three different kinds of such events have been identified in the data, and their exact nature is not yet known. Conclusions. In the present study we show that the variability of compact radio sources is heavily influenced by effects that correlate with solar angular distance. This unexpected contribution significantly alters the sources’ variability characteristics estimated at gigahertz frequencies. In particular, we found that many extreme scattering events previously identified in the GBI monitoring program are actually the consequence of Sun-related effects; others occur simultaneously in several objects, which excludes interstellar scattering as their possible cause. These discoveries have a severe impact on our understanding of extreme scattering events. Furthermore, Sun-related variability, given its amplitude and timescale, can significantly alter results of variability studies, which are very powerful tools for the investigation of active galactic nuclei. Without a thorough comprehension of the mechanisms that cause these variations, the estimation of some essential information about the emitting regions, such as their size and all the derived quantities, might be seriously compromised.