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 (∼ 10 4 − a few × 10 5 M ) 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 H ∼ 1.6×10 22 −3.4×10 23 cm −2 , ∼ 0.8−7.8×10 22 cm −2 , and 3.8×10 22 cm −2 , respectively. The source also shows intrinsic variability by a factor of ∼ 3, on short timescales, due to changes in the nuclear flux, assumed to be a power law (Γ = 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, eXTP, and Athena, which will allow us to characterize the different absorbers, study their dynamics, and will help us identify their locations and sizes.

A hard look at local, optically-selected, obscured Seyfert galaxies

“…This effect has been confirmed in Type-2 AGN by Ricci et al (2014), and later confirmed in a large sample of CT sources by Boorman et al (2016). The fact that the slope of the R − L 2−10 correlation for NGC 4395 is consistent with unity is suggestive of a constant reprocessed flux despite the intrinsic variability (see Kammoun et al 2019b). This may indicate that despite the change in the LOS column density in this source, the overall covering fraction of the clouds remains constant.…”

“…Such a configuration is not trivial to implement in the framework of the MYTorus models. A detailed analysis of NGC 4395 is presented in Kammoun et al (2019b) (see also Nardini & Risaliti 2011). As for NGC 3362, UGC 6100, and UGC 8621, the low quality of the data (for UGC 6100) makes the application of the MYTorus fits non-trivial.…”

“…In our case, considering the three moderately-obscured sources in our sample, it is well-known that NGC 4395 exhibits strong variability in its obscuring column. Those variations are associated with obscuring clouds in the BLR (see Nardini & Risaliti 2011;Kammoun et al 2019b). Instead, UM 146 and NGC 5674 are two face-on galaxies.…”

“…Our model consists of a partial covering neutral absorber and two ionized partial covering absorbers. A detailed analysis is presented in Kammoun et al (2019b). The only difference between the current work and Kammoun et al (2019b) is that now we tie the Pexmon normalization to the one of the power law component and we let R be free.…”

“…A detailed analysis is presented in Kammoun et al (2019b). The only difference between the current work and Kammoun et al (2019b) is that now we tie the Pexmon normalization to the one of the power law component and we let R be free. This does not alter any of the previous results.…”