X-ray Luminosity and Absorption Column Fluctuations in the H2O Maser Galaxy NGC 4258 from Weeks to Years

Abstract

We report monitoring of the 0.3-10 keV spectrum of NGC4258 with XMM over 1.5 years.We als o report reprocessing of an overlapping series of archival Chandra observations. By including earlier ASCA and SAX observations, we present a new, nine-year time series of models fit to the X-ray spectrum of NGC4258. Over the nine years, the photoelectric absorbing column (~10^23 cm^-2) did not vary detectably, except for a ~40% drop between two ASCA epochs separated by 3 years and a ~60% rise between two XMM epochs separated by just 5 months. In contrast, factor of 2-3 changes are seen in absorbed flux on the timescale of years. These are uncorrelated with changes in absorbing column and indicative of central engine variability. The most rapid change in luminosity (5-10 keV) that we detect is ~30% over 19 days. The warped disk, a known source of H2O maser emission in NGC4258, is believed to cross the line of sight to the central engine. We propose that the variations in absorbing column arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius where the disk crosses the line of sight. We estimate that the inhomogeneities are ~10^15 cm in size at the crossing radius of 0.29 pc, slightly smaller than the expected scale height of the disk. This result thus provides strong evidence that the warped accretion disk is the absorber. This is the first direct confirmation that obscuration in type-2 AGN may, in some cases, arise in thin, warped accretion disks, rather than in geometrically thick tori. We do not detect Fe Kalpha line emission in any of our XMM spectra. We do not observe evidence of absorption lines in the XMM or reprocessed Chandra data.