The nature of the broadband X-ray variability in the dwarf Seyfert galaxy NGC 4395

Abstract

We present a flux-resolved X-ray analysis of the dwarf Seyfert 1.8 galaxy NGC 4395, based on three archival $XMM-Newton$ and one archival $NuSTAR$ observations. The source is known to harbor a low mass black hole ($\sim 10^4- {\rm a~ few~}\times 10^{5}~\rm M_\odot$) and shows strong variability in the full X-ray range during these observations. We model the flux-resolved spectra of the source assuming three absorbing layers: neutral, mildly ionized, and highly ionized ($N_{\rm H} \sim 1.6\times 10^{22}-3.4 \times 10^{23}~\rm cm^{-2}$, $\sim 0.8-7.8 \times 10^{22}~\rm cm^{-2}$, and $ 3.8 \times 10^{22}~\rm cm^{-2}$, respectively. The source also shows intrinsic variability by a factor of $\sim 3$, on short timescales, due to changes in the nuclear flux, assumed to be a power law ($\Gamma = 1.6-1.67$). Our results show a positive correlation between the intrinsic flux and the absorbers' ionization parameter. The covering fraction of the neutral absorber varies during the first $XMM-Newton$ observation, which could explain the pronounced soft X-ray variability. However, the source remains fully covered by this layer during the other two observations, largely suppressing the soft X-ray variability. This suggests an inhomogeneous and layered structure in the broad line region. We also find a difference in the characteristic timescale of the power spectra between different energy ranges and observations. We finally show simulated spectra with $XRISM$, $Athena$, and $eXTP$, which will allow us to characterize the different absorbers, study their dynamics, and will help us identify their locations and sizes.