WISE view of narrow-line Seyfert 1 galaxies: mid-infrared colour and variability

Abstract

We present the colour and flux variability analysis at 3.4 μm (W1 band) and 4.6 μm (W2 band) of 492 narrow-line Seyfert 1 (NLSy1) galaxies using archival data from the Wide-field Infrared Survey Explorer (WISE). In the WISE colour–colour, (W1 − W2) versus (W2 − W3) diagram, ∼58 per cent of the NLSy1 galaxies of our sample lie in the region occupied by the blazar category of active galactic nuclei (AGNs). The mean W1 − W2 colour of candidate variable NLSy1 galaxies is 0.99 ± 0.18 mag. The average amplitude of variability is 0.11 ± 0.07 mag in long term (multiyear) with no difference in variability between W1 and W2 bands. The W1 − W2 colour of NLSy1 galaxies is anticorrelated with the relative strength of [O iii] to Hβ, strongly correlated with continuum luminosity, black hole mass, and Eddington ratio. The long-term amplitude of variability shows weak anticorrelation with the Fe ii strength, continuum luminosity and Eddington ratio. A positive correlation between colour as well as the amplitude of variability with the radio power at 1.4 GHz was found for the radio-detected NLSy1 galaxies. This suggests non-thermal synchrotron contribution to the mid-infrared colour and flux variability in radio-detected NLSy1 galaxies.