Variability and the Size–Luminosity Relation of the Intermediate-mass AGN in NGC 4395

Abstract

We present a variability study of the lowest-luminosity Seyfert 1 nucleus of the galaxy NGC 4395 based on photometric monitoring campaigns in 2017 and 2018. Using 22 ground-based and space telescopes, we monitored NGC 4395 with a ∼5-minute cadence during a period of 10 days and obtained light curves in the ultraviolet (UV), V, J, H, and K/Ks bands, as well as narrowband Hα. The rms variability is ∼0.13 mag in the Swift UVM2 and V filter light curves, decreasing down to ∼0.01 mag in the K filter. After correcting for the continuum contribution to the Hα narrow band, we measured the time lag of the Hα emission line with respect to the V-band continuum as - + 55 31 27– - + 122 67 33 minutes in 2017 and - + 49 14 15– - + 83 14 13 minutes in 2018, depending on assumptions about the continuum variability amplitude in the Hα narrow band. We obtained no reliable measurements for the continuum-tocontinuum lag between UV and V bands and among near-IR bands, owing to the large flux uncertainty of UV observations and the limited time baseline. We determined the active galactic nucleus (AGN) monochromatic luminosity at 5100 Å, l l =´ ( ) - L 5.75 0.40 10 erg s 39 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 (>2.5σ) from the previous best-fit relation of Bentz et al