The Suzaku broadband X-ray spectrum of the dwarf Seyfert galaxy NGC 4395

Abstract

We present a Suzaku observation of the dwarf Seyfert galaxy NGC 4395 with an estimated black hole mass of ∼10 5 M . Rapid and strong X-ray variability with an rms amplitude of ∼60 per cent is observed in the 0.4-10 keV band with the XIS cameras. The shape of the light curve appears to depend on energies. The hard X-ray emission is detected up to 35 keV with the HXD-PIN detector at a similar flux level as observed with the INTEGRAL IBIS. The X-ray spectrum below 10 keV is strongly absorbed by partially ionized (ξ ∼ 35 erg s cm −1 ) gas with a mean equivalent hydrogen column density of ∼2 × 10 22 cm −2 , when a simple absorption model is applied. The spectral shape is also strongly variable but not a simple function of the source brightness. The spectral variability appears to be accounted for mainly by continuum slope changes, but variability in the ionized absorber may also play some part. The apparently flat spectral slope of Γ 1.4 below 10 keV, obtained after correcting for absorption, is marginally inconsistent with the Γ ∼ 2 inferred from the 14-35 keV PIN spectrum. If the true spectral slope had been as steep as that measured in the hard X-ray band, there would have been an extra absorption component, which we are unable to detect. Combined with the INTEGRAL measurements, the hard X-ray emission above 10 keV exceeds the optical emission in terms of luminosity and dominates the broadband energy output, unless a large excess of UV disk emission is yet to be detected in the unobservable band. A weak Fe K line is seen at 6.4 keV with the average equivalent width of 110 eV, which does not show significant flux changes over the 3-day observation.

The effects of X-ray absorption variability in NGC 4395

“…Incidentally, it is also worth noting that the intrinsic 0.5–10 keV emission of NGC 4395 implied in this cold absorption scheme is larger than the observed one by just a factor of ∼2 (Table 1). Taking advantage of the Swift /BAT spectral constraints, the resulting 0.5–100 keV luminosity is ∼6–8 × 10 40 erg s −1 , which exceeds the usual estimates of the bolometric luminosity (see also Moran et al 2005; Iwasawa et al 2010). Depending of the exact value of the black hole mass, the Eddington ratio of NGC 4395 could be much closer to ∼0.01 than previously thought.…”

“…2 (see e.g. the X-ray colour HR3 in Iwasawa et al 2010): again, six different periods have been identified. We have adopted the same model defined above.…”

“…In general, however, the number of counts collected during the archival observations is not sufficient to derive solid and independent constraints on our model. We have therefore analysed the only other long, high‐quality monitoring of NGC 4395 that is available to date, obtained by Suzaku and fully discussed by Iwasawa, Tanaka & Gallo (2010). In the latter work, the time‐averaged spectrum is described by means of a power‐law continuum with Γ∼ 1.4, modified by a warm absorber with ionization parameter ξ W ∼ 35 erg cm s −1 and column density N W ≃ 2 × 10 22 cm −2 ; a narrow iron line and neutral absorption in moderate excess of the Galactic amount are also involved.…”

A Distinctive Disk-Jet Coupling in the Lowest-Mass Seyfert, NGC 4395

“…We froze the power-law spectral index at Γ = 1.8 due to the small number of counts, while still varying the normalization. Although some of the literature on NGC 4395 finds a much lower spectral index of Γ 1.5 (e.g., Lira et al 1999;Shih et al 2003;Moran et al 2005;Iwasawa et al 2010), a higher spectral index, Γ ≈ 2.0 ± 0.2, was found by Nardini & Risaliti (2011) when fitting an averaged Swift/BAT spectrum above 10 keV. This suggests that the hard spectral index at low X-ray energies is partly due to absorption and not intrinsic.…”

The Nature of the Broadband X-Ray Variability in the Dwarf Seyfert Galaxy NGC 4395

“…The dwarf Seyfert 1.8 galaxy NGC 4395 (d = 4.2 Mpc Karachentsev & Drozdovsky 1998) is one of the most X-Corresponding author: E. Kammoun ekammoun@umich.edu ray variable non-jetted active galactic nuclei (AGN; e.g., Iwasawa et al 2000;Vaughan et al 2005;Iwasawa et al 2010). The optical and ultraviolet (UV) spectra of this source show high-ionization forbidden lines with broad wings corresponding to gas velocities larger than ∼ 10 3 km s −1 (Filippenko & Sargent 1989), in addition to permitted lines such as C IV, Mg II, O III and Hα (see e.g., Filippenko et al 1993).…”