NESMP sources main variability characteristics

The 2GHz and 8GHz light curves observed in the framework of the GBI monitoring program have been inspected in a search for one-year periodic patterns in the data. Variations on timescales below one year have been isolated through a de-trending algorithm and analysed, looking for possible correlations with the Sun's position relative to the sources. Objects at ecliptic latitude below ~20{deg} 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. Their exact nature is not yet known.

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 GHz 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.