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

Abstract

We present a deep X-ray observation of the low-luminosity active galactic nucleus in NGC4258 (M106) using ASCA. The soft X-ray spectrum <2keV is dominated by thermal emission from optically-thin plasma with kT~0.5keV. The hard X-ray emission is clearly due to a power-law component with photon index Gamma=1.8 absorbed by a column density of N_H=8x10^22/cm^2. The power-law is readily identified with primary X-ray emission from the AGN central engine. We also clearly detect a narrow iron K-alpha emission line at 6.4keV. 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 which excites this line emission (which is typically identified with a disk corona) must be at least 100GM/c^2 in extent. This is in stark contrast to many higher-luminosity Seyfert galaxies which display a broad iron line indicating a small 10 GM/c^2 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. 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.