NGC 4388: A test case for relativistic disk reflection and fe k fluorescence features

Abstract

We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin Seyfert 2 galaxy NGC 4388. The spectrum above ∼2 keV can be described by a remarkably simple and rather mundane model, consisting of a uniform, neutral spherical distribution of matter, with a radial column density of 2.58±0.02×10²³ cm⁻² ⁠, and an Fe abundance of 1.102-₀.₀₂₁⁺⁰.⁰²⁴ relative to solar. The model does not require any phenomenological adjustments to self-consistently account for the low-energy extinction, the Fe Kα and Fe Kβ fluorescent emission lines, the Fe K edge, and the Compton-scattered continuum from the obscuring material. The spherical geometry is not a unique description, however, and the self-consistent, solar abundance MYTORUS model, applied with toroidal and non-toroidal geometries, gives equally good descriptions of the data. In all cases, the key features of the spectrum are so tightly locked together that for a wide range of parameters, a relativistic disk-reflection component contributes no more than ∼2 per cent to the net spectrum in the 2–20 keV band. We show that the commonly invoked explanations for weak X-ray reflection features, namely a truncated and/or very highly ionized disk, do not work for NGC 4388. If relativistically-broadened Fe Kα lines and reflection are ubiquitous in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2 galaxies. The case of NGC 4388 shows the need for similar studies of more Compton-thin AGN to ascertain whether this is true.