[Paper Review] Radio-Variability in Radio-Quiet Quasars and Low-Luminosity AGN

H. Falcke, Joseph Lehár+3 more|arXiv (Cornell University)|Sep 28, 2000

Radio Astronomy Observations and Technology1 references3 citations

TL;DR

This study investigates radio variability in radio-quiet and low-luminosity AGN using VLA observations, finding significant monthly and annual variability (10–20% RMS in quasars, up to 300% peak-to-peak in low-luminosity AGN), confirming that radio emission in these sources is AGN-related and likely linked to jet activity and accretion changes. The results support a symbiotic model of accretion and jet formation across AGN types, even in radio-weak systems.

ABSTRACT

We report on two surveys of radio-weak AGN to look for radio variability. We find significant variability with an RMS of 10-20% on a timescale of months in radio-quiet and radio-intermediate quasars. This exceeds the variability of radio cores in radio-loud quasars (excluding blazars), which vary only on a few percent level. The variability in radio-quiet quasars confirms that the radio emission in these sources is indeed related to the AGN. The most extremely variable source is the radio-intermediate quasar III Zw 2 which was recently found to contain a relativistic jet. In addition we find large amplitude variabilities (up to 300% peak-to-peak) in a sample of nearby low-luminosity AGN, Liners and dwarf-Seyferts, on a timescale of 1.5 years. The variability could be related to the activity of nuclear jets responding to changing accretion rates. Simultaneous radio/optical/X-ray monitoring also for radio-weak AGN, and not just for blazars, is therefore a potentially powerful tool to study the link between jets and accretion flows.

Motivation & Objective

• To determine whether radio-quiet and low-luminosity AGN exhibit detectable radio variability, challenging the assumption that radio emission is unrelated to the AGN.
• To test the hypothesis that radio emission in radio-quiet quasars originates from relativistic jets linked to accretion processes.
• To explore the connection between accretion rate variability and radio variability in compact nuclear cores.
• To assess the feasibility and scientific value of simultaneous multi-wavelength (radio/optical/X-ray) monitoring for radio-weak AGN.
• To evaluate whether black hole mass or accretion mode differences explain the observed variability amplitude differences between quasars and low-luminosity AGN.

Proposed method

• Conducted monthly VLA observations at 8.5 GHz over two years for 30 radio-quiet and radio-intermediate quasars, with additional observations one year later.
• Used a debiased variability index to quantify radio flux changes, correcting for calibration and measurement uncertainties.
• Performed deep, high-resolution 15 GHz VLA A-array observations of 48 low-luminosity AGN (LINERs and dwarf-Seyferts), with three epochs spaced ~1.5 years apart.
• Selected only compact core flux densities to isolate nuclear emission and avoid contamination from extended structures.
• Analyzed light curves and variability indices to identify significant flux variations and assess statistical significance.
• Compared variability amplitudes across different AGN types and correlated with R-parameter (radio-to-optical flux ratio) and black hole mass estimates.

Experimental results

Research questions

• RQ1 Is radio emission in radio-quiet quasars genuinely associated with the AGN, as indicated by detectable variability?
• RQ2 What is the amplitude and timescale of radio variability in low-luminosity AGN, and how does it compare to that in quasars?
• RQ3 Can radio variability in radio-weak AGN be explained by changes in accretion rate, rather than relativistic beaming?
• RQ4 Does the observed variability support a symbiotic model of accretion disk and relativistic jet formation?
• RQ5 Is the higher variability in low-luminosity AGN due to lower black hole mass or a more volatile accretion mode?

Key findings

• Radio-quiet and radio-intermediate quasars exhibit significant monthly-scale variability with an RMS of 10–20%, exceeding the few percent variability seen in radio-loud quasars (excluding blazars).
• The most extreme variability was found in the radio-intermediate quasar III Zw 2, which hosts a relativistic jet, supporting a jet-related origin for the radio emission.
• Low-luminosity AGN show large-amplitude variability of up to 300% peak-to-peak on a 1.5-year timescale, with NGC2787, NGC4143, and NGC4565 showing particularly strong variations.
• The distribution of variability indices in low-luminosity AGN indicates that significant variability is common, with amplitudes ranging from 20–70%.
• The higher variability in low-luminosity AGN compared to quasars may be due to lower black hole masses or a more volatile accretion mode, such as stellar wind feeding.
• The results confirm that radio emission in radio-quiet AGN is AGN-related and support the need for coordinated multi-wavelength monitoring to study accretion-jet coupling.