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

¹ INAF-Ossservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: kazushi.iwasawa@oabo.inaf.it
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany e-mail: ytanaka@mpe.mpg.de
³ Department of Astronomy and Physics, Saint Mary's University, Halifax, NS B3H 3C3, Canada e-mail: lgallo@ap.smu.ca

Received: 6 May 2009
Accepted: 21 December 2009

Abstract

We present a Suzaku observation of the dwarf Seyfert galaxy NGC 4395 with an estimated black hole mass of ~10⁵ . 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 × 10²² 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.