Intra-day variability observations of S5 0716+714 over 4.5 years at 4.8 GHz

TL;DR

This study presents 4.5 years of monthly radio observations of S5 0716+714, revealing evidence of annual modulation in intra-day variability likely caused by interstellar scintillation, unaffected by source flares.

Contribution

First long-term monitoring of S5 0716+714 at 4.8 GHz showing evidence of annual modulation in IDV due to interstellar scintillation.

Findings

IDV exhibits annual modulation consistent with ISS.

Flares do not significantly alter IDV characteristics.

Inner-jet position angle changes may introduce noise in models.

Abstract

We aim to search for evidence of annual modulation in the time scales of the BL Lac object S5 0716+714. The intra-day variability (IDV) observations were carried out monthly from 2005 to 2009, with the Urumqi 25m radio telescope at 4.8 GHz. The source has shown prominent IDV as well as long-term flux variations. The IDV time scale does show evidence in favor of an annual modulation, suggesting that the IDV of 0716+714 is dominated by interstellar scintillation. The source underwent a strong outburst phase between mid-2008 and mid-2009; a second intense flare was observed in late 2009, but no correlation between the total flux density and the IDV time scale is found, implying that the flaring state of the source does not have serious implications for the general characteristics of its intra-day variability. However, we find that the inner-jet position angle is changing throughout the years, which could result in an annual modulation noise in the anisotropic ISS model fit. There is also an indication that the lowest IDV amplitudes (rms flux density) correspond to the slowest time scales of IDV, which would be consistent with an ISS origin of the IDV of 0716+714.