Variability and the size-luminosity relation of the intermediate mass AGN in NGC 4395

Abstract

We present the variability study of the lowest-luminosity Seyfert 1 galaxy NGC 4395 based on the photometric monitoring campaigns in 2017 and 2018. Using 22 ground-based and space telescopes, we monitored NGC 4395 with a $\sim$5 minute cadence during a period of 10 days and obtained light curves in the UV, V, J, H, and K/Ks bands as well as the H$\hat{I}\pm$ narrow-band. The RMS variability is $\sim$0.13 mag on \emph{Swift}-UVM2 and V filter light curves, decreasing down to $\sim$0.01 mag on K filter. After correcting for continuum contribution to the H$\hat{I}\pm$ narrow-band, we measured the time lag of the H$\hat{I}\pm$ emission line with respect to the V-band continuum as ${55}_{-31}^{+27}$ to ${122}_{-67}^{+33}$ min. in 2017 and ${49}_{-14}^{+15}$ to ${83}_{-14}^{+13}$ min. in 2018, depending on the assumption on the continuum variability amplitude in the H$\hat{I}\pm$ narrow-band. We obtained no reliable measurements for the continuum-to-continuum lag between UV and V bands and among near-IR bands, due to the large flux uncertainty of UV observations and the limited time baseline. We determined the AGN monochromatic luminosity at 5100à $\hat{I}»L_{\hat{I}}»=\left(5.75\pm 0.40\right)\times 10^{39}\mathrm{erg}{\mathrm{s}}^{-1}$, after subtracting the contribution of the nuclear star cluster. While the optical luminosity of NGC 4395 is two orders of magnitude lower than that of other reverberation-mapped AGNs, NGC 4395 follows the size-luminosity relation, albeit with an offset of 0.48 dex ($\ge$2.5$\ddot{I}ƒ$) from the previous best-fit relation of Bentz et al. (2013).

Accepted for publication in ApJ (Feb. 23rd, 2020). 18 pages, 10 figures