Intra-night optical variability characteristics of different classes of narrow-line seyfert 1 galaxies

Abstract

In a first systematic effort to characterize the intra-night optical variability (INOV) of different classes of narrow-line Seyfert 1 (NLSy1) Galaxies, we have carried out observations on a sample of radio-loud (RL) and radio-quiet (RQ) NLSy1 galaxies. The RL-NLSy1 galaxies are further divided into γ-ray loud (GL) and γ-ray quiet (GQ) NLSy1 galaxies. Our sample consists of four sets, each set consisting of a RQ-NLSy1, a GQ-NLSy1 and a GL-NLSy1 galaxy, closely matched in redshift and optical luminosity. Our observations on both RQ- and GQ-NLSy1 galaxies consist of a total of 19 nights, whereas the data for GL-NLSy1 galaxies (18 nights) were taken from the literature published earlier by us. This enabled us to do a comparison of the duty cycle (DC) of different classes of NLSy1 galaxies. Using power-enhanced F-test, with a variability threshold of 1 per cent, we find DCs of about 55 per cent, 39 per cent and 0 per cent for GL-, GQ- and RQ-NLSy1 galaxies, respectively. The high DC and large amplitude of INOV (24.0 ± 13.7 per cent) shown by GL-NLSy1 galaxies relative to the other two classes might be due to their inner aligned relativistic jets having large bulk Lorentz factors. The null DC of RQ-NLSy1 galaxies could mean the presence of low power and/or largely misaligned jets in them. However, dividing RL-NLSy1 galaxies into low and high optical polarization sources, we find that sources with large polarization show somewhat higher DCs (69 per cent) and amplitudes (29 per cent) compared to those with low polarization. This points to a possible link between INOV and optical polarization.