Serendipitous VLBI detection of rapid, large-amplitude, intraday variability in QSO 1156+295

TL;DR

This paper reports the serendipitous detection of rapid, large-amplitude flux density variations in quasar 1156+295 at 15 GHz, likely caused by interstellar scintillation, revealing extreme intraday variability.

Contribution

First detection of rapid, large-amplitude variability in 1156+295 during VLBA observations, linking it to interstellar scintillation effects.

Findings

Flux density varies up to 40% in 2.7 hours.

Variability consistent with interstellar scintillation by a turbulent scattering screen.

High modulation index suggests an unusually high scattering measure for the source's galactic latitude.

Abstract

We report a serendipitous detection of rapid, large amplitude flux density variations in the highly core-dominated, flat-spectrum radio quasar 1156+295 during an observing session at the Very Long Baseline Array (VLBA). The source was observed as a part of the MOJAVE survey programme with the VLBA at 15 GHz on February 5, 2007. Large amplitude variability in the correlated flux density, unexplainable in terms of the source structure, was first discovered while processing the data, and later confirmed by calibrating the antenna gains using 24 other sources observed in the experiment. The source shows variations in the correlated flux density as high as 40% on a timescale of only 2.7 hours. This places 1156+295 between the classical IDV sources and the so-called intra-hour variables. The observed variability timescale and the modulation index of 13% are consistent with interstellar scintillation by a nearby, highly turbulent scattering screen. The large modulation index at 15 GHz implies a scattering measure that is atypically high for a high galactic latitude source such as 1156+295.