The Spectral Behaviour and Variability of Narrow-line Seyfert 1 Galaxies with Australia Telescope Compact Array Observations

Abstract

We present multi-frequency radio data for a sample of narrow-line Seyfert 1 galaxies. We first focus on the sub-class of gamma-ray emitting narrow-line Seyfert 1 galaxies, studying the long-term radio variability of five sources and comparing it to their gamma-ray state. We then extend the observations of the southern narrow-line Seyfert 1 galaxy sample of Chen et al. by observing several candidate narrow-line Seyfert 1 sources for the first time, and re-observing several other gamma-ray quiet sources to obtain a first indication of their radio variability. We find that the gamma-ray emitting narrow-line Seyfert 1 galaxies are highly variable radio emitters and that there are instances of contemporaneous flaring activity between the radio and gamma-ray band (PKS 0440$-$00, PMN J0948+0022 and PKS 1244$-$255). However, there are also cases of significant radio outbursts without gamma-ray counterparts (PMN J0948+0022 and PKS 2004$-$447). The five gamma-ray NLS1s favour flat or inverted radio spectra, although the spectral indices vary significantly over time. For the gamma-ray quiet sample, the difference between the previous observations at 5.5 GHz and new ATCA observations indicates that over half of the 14 sources exhibit apparent variability. In contrast to gamma-ray loud sources, gamma-ray quiet objects tend to have steep spectra especially in the lower radio band (887.5$-$1367.5 MHz), with a number of the variable sources having flatter spectra at higher radio frequencies.