Abstract:
The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope has detected
ƴ -ray emission in about half a dozen narrow-line Seyfert 1 (NLSy1) galaxies. This indicates
the presence of relativistic jets in these sources similar to blazars and radio galaxies. In
an attempt to have an idea of the intranight optical variability (INOV) characteristics of
these ƴ-ray-loud NLSy1 galaxies, we have carried out optical flux monitoring observations
of three NLSy1 galaxies detected by Fermi/LAT: 1H 0323+342, PMN J0948+0022 and
PKS 1502+036. These optical monitoring observations in RC band carried out during 2012
January–May showed the presence of rapid optical flux variations in these sources. The
intranight differential light curves of these sources have revealed flux variations on time-scales
of hours with amplitudes of variability >3 per cent for most of the time. However, for one
source, PMN J0948+0022, we observed amplitude of variability as large as 52 per cent. On
using the F-statistics to classify the variability nature of these sources, we obtained a duty
cycle (DC) of INOV of ∼85 per cent. Alternatively, the more commonly used C-statistics gave
a DC of INOV of ∼57 per cent. Such high DC of INOV is characteristics of the BL Lac class
of active galactic nucleus. The results of our monitoring observations thus indicate that there
is similarity in the INOV nature of γ -ray-loud NLSy1 galaxies and BL Lac objects, arguing
strongly for the presence of relativistic jets aligned closely to the observers line of sight in
ƴ -ray-loud NLSy1s. Moreover, our dense monitoring observations on some of the nights have
led to the clear detection of some miniflares superimposed on the flux variations during the
night over time-scales as short as 12 min. The detection of short time-scale flux variability in
the sources studied here is clearly due to stronger time compression leading to the jets in these
sources having large Doppler factors, similar to that of the inner jets of TeV blazars.
