Intra-night optical variability of γ −ray detected narrow-line Seyfert1 galaxies

Abstract

We report the first attempt to systematically characterise intra-night optical variability (INOV) of the rare and enigmatic subset of Narrow-Line Seyfert1 galaxies (NLSy1s), which is marked by detection in the γ-ray band and is therefore endowed with Doppler boosted relativistic jets, like blazars. However, the central engines in these two types of AGN are thought to operate in different regimes of accretion rate. Our INOV search in a fairly large and unbiased sample of 15 γ-ray NLSy1s was conducted in 36 monitoring sessions, each lasting ≥ 3 hrs. In our analysis, special care has been taken to address the possible effect on the differential light curves, of any variation in the seeing disc during the session, since that might lead to spurious claims of INOV from such AGN due to the possibility of a significant contribution from the host galaxy to the total optical emission. From our observations, a duty cycle (DC) of INOV detection in the γ-ray NLSy1s is estimated to be around 25% - 30%, which is comparable to that known for blazars. This estimate of DC will probably need an upward revision, once it becomes possible to correct for the dilution of the AGN’s nonthermal optical emission by the (much steadier) optical emission contributed not only by the host galaxy but also the nuclear accretion disc in these high Eddington rate accretors. Finally, we also draw attention to the possibility that sharp optical flux changes on sub-hour time scale are less rare for γ-ray NLSy1s, in comparison to blazars.

Evidence of jet-induced optical microvariability in radio-loud narrow-line Seyfert 1 galaxies

“…The chance of a false INOV detection signal becomes lower for a smaller value of α. Therefore, in the present work, similar to our previous work, two critical significance levels, α = 0.01 and α = 0.05 are set by us (e.g., see Ojha et al 2021). Following Ojha et al (2021), a NLSy1 is designated as variable for a given session according to this test if the computed value of F η is found to be greater than its F c (0.99).…”

“…5. Nonetheless, bearing the above in mind, we have chosen an aperture radius equal to 2×FWHM for our final analysis, as already elaborated in Ojha et al (2021).…”

“…Table 3 lists coordinates together with some other parameters of the steady comparison stars used for all the sessions. It has been shown in Ojha et al (2021) that the color differences of such orders can be safely discounted while analyzing the variability of the DLCs. (Abolfathi et al 2018) has been used for getting optical positions and apparent magnitudes of the sources and their comparison stars.…”

“…To guarantee the reliability of detection for microvariability events, multi-testing has mostly been used in recent years (e.g., Joshi et al 2011;Goyal et al 2012;de Diego 2014;Ojha et al 2021). Therefore, for unambiguous detection of INOV in a DLC, we have used in the current work two different versions of F-test, which are the standard F-test (hereafter F η -test) and the power-enhanced F-test (hereafter F enh -test).…”

“…This alternative tool is also used to indirectly verify the presence or absence of jets in other sub-classes of AGN because of the well established observational fact that for radio-loud jet dominated sources such as blazars, both INOV amplitude (ψ) and the duty cycle (DC) are found to be distinctively high in comparison to non-blazars, including weakly polarised flat-radio-spectrum (i.e., radio-beamed) quasars (Goyal et al 2013b;Gopal-Krishna & Wiita 2018). Interestingly, such an indirect tool has been used for a decade in searching for the Doppler boosted optical jets in low luminous AGNs such as NLSy1s and weak emission line QSOs (e.g., see Liu et al 2010;Paliya et al 2013a;Kumar et al 2015;Parveen et al 2016;Kumar et al 2017;Ojha et al 2018Ojha et al , 2021. However, this indirect evidence is based upon the observed high amplitude, and duty cycle of INOV as seen in blazars consisting of strongly Doppler boosted jets.…”