A deep X-ray observation of NGC 4258 and its surrounding field

We present a deep X-ray observation of the low-luminosity active galactic nucleus (AGN) in NGC 4258 (M106) using the Advanced Satellite for Cosmology and Astrophysics (ASCA). Confinning previous results, we find that the X-ray spectrum of this source possesses several components. The soft X-ray spectrum (<2 keV) is dominated by thermal emission from optically thin plasma with kT ∼ 0.5 keV. The hard X-ray emission is clearly due to a power-law component with photon index Γ ≈ 1.8 absorbed by a column density of N_H ≈ 8 × 10²² cm_-2. The power law is readily identified with primary X-ray emission from the AGN central engine. Underlying both of these spectral components is: an additional continuum, which is possibly due to thermal emission of a very hot gaseous component in the anomalous arms and/or the integrated hard emission of X-ray binaries in the host galaxy. We also clearly detect a narrow iron Ka emission line at ∼6.4 keV. No broad component is detected. We suggest that the bulk of this narrow line comes from the accretion disk and, furthermore, that the power-law X-ray source that excites this line emission (which is typically identified with a disk corona) must be at least ∼ 100GM/c² in extent. This is in stark contrast to many higher luminosity Seyfert galaxies that display a broad iron line indicating a small ( ∼ 10GM/c₂) X-ray-emitting region. It must be stressed that this study constrains the size of the X-ray-emitting corona rather than the presence/absence of a radiatively efficient accretion disk in the innermost regions. If, instead, a substantial fraction of the observed narrow line originates from material not associated with the accretion disk, limits can be placed on the parameter space of possible allowed relativistically broad iron lines. We include a discussion of various aspects of iron line limb darkening for highly inclined sources, including the effect of gravitational light bending on the apparent limb-darkening law. By comparing our data with previous ASCA observations, we find marginal evidence for a change in absorbing column density through to the central engine and good evidence for a change in the AGN flux. We conclude with a brief discussion of two serendipitous sources in our field of view: QSO Q1218+472 and a putative z ∼ 0.3 cluster of galaxies.