The Suzaku broadband X-ray spectrum of the dwarf Seyfert galaxy NGC4395

TL;DR

This study presents Suzaku X-ray observations of the dwarf Seyfert galaxy NGC4395, revealing rapid variability, complex absorption, and broadband spectral properties that inform models of low-mass active galactic nuclei.

Contribution

First detailed broadband X-ray spectral analysis of NGC4395, highlighting variability, absorption features, and the dominance of hard X-ray emission in low-mass AGN.

Findings

Rapid X-ray variability with 60% rms amplitude

Strong absorption by ionized gas with N_H~2e22 cm^-2

Hard X-ray emission exceeds optical luminosity

Abstract

We present a Suzaku observation of the dwarf Seyfert galaxy NGC4395 with an estimated black hole mass of ~10^5 Msun. 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 (xi~35) gas with a mean equivalent hydrogen column density of ~2e22 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 with photon index of 1.4 below 10 keV, obtained after correcting for absorption, is marginally inconsistent with the photon index of 2 inferred from the 14-35 keV PIN spectrum. If the true spectral slope was as steep as that measured in the hard X-ray band, there would be 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 clear flux changes over the 3-day observation.