The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

Abstract

We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, m 5 ≥ 0.02) all exhibit strong 15 GHz intrinsic variability (m 15 ≥ 0.1). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at ∼ 100 µas scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects.

The presence of interstellar scintillation in the 15 GHz interday variability of 1158 OVRO-monitored blazars

“…Of the nine interday variables that have line-of-sight Iα < 10 R, six of them, notably J0721+7120 and J1135−0428, exhibit large flares on timescales less than a year ( Figure C1), which may have skewed their 4-day variability amplitudes towards larger values. However, we argue that a component that is compact enough to exhibit intrinsic variability on interday to monthly timescales must also be sufficiently compact enough to scintillate, if the line-ofsight is highly scattered (Koay et al 2018), which is clearly the case for J0502+1338. J0721+7120 (S5 0716+714) is in fact well-known as a highly varable source at radio, optical, X-ray and gammaray wavelengths (e.g., Fuhrmann et al 2008;Gupta et al 2012).…”

“…In these studies, the 15 GHz flux density variations are always assumed to be intrinsic to the blazar jets. Indeed, the source variability amplitudes from the OVRO lightcurves, as quantified by the intrinsic modulation index (Richards et al 2011), broadly show no significant Galactic dependence (Koay et al 2018), confirming that intrinsic variations likely dominate. This is to be expected since this method of variability characterization is biased towards the largest inflections observed at the longest timescales in the lightcurves, most of which are expected to be intrinsic to the blazars.…”

“…The strength of ISS is not only dependent on the lineof-sight scattering strength, but also on the compactness of the source (Narayan 1992;Rickett 1990;Koay et al 2018). The non-scintillating sources seen through strongly scattered sight-lines may simply not be sufficiently compact to exhibit significant ISS.…”

“…If these compact blazars are brightness temperature-limited, either due to the inverse-Compton catastrophe (Kellermann & Pauliny-Toth 1969) or energy equipartition between the magnetic fields and electrons (Readhead 1994;Lähteenmäki et al 1999), their angular sizes are expected to scale as θ ∝ √ S15. The weakest sources are also therefore most likely to be the most compact in angular size and thereby scintillate more strongly (Lovell et al 2008;Koay et al 2018).…”

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron

“…This indicates that the component size might still be slightly underestimated and not yet quite resolved. A size of the emitting region of about 100 µas is expected for sources with strong variability at higher frequencies, while not showing signs of scintillation due to the ISM (Koay et al 2018).…”