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

We report monitoring of the 0.3-10 keV spectrum of NGC4258 with XMM over 1.5years.We als o report reprocessing of an overlapping series of archival Chandraobservations. 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 thenine years, the photoelectric absorbing column (~10^23 cm^-2) did not varydetectably, except for a ~40% drop between two ASCA epochs separated by 3 yearsand 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 ofcentral engine variability. The most rapid change in luminosity (5-10 keV) thatwe detect is ~30% over 19 days. The warped disk, a known source of H2O maseremission in NGC4258, is believed to cross the line of sight to the centralengine. We propose that the variations in absorbing column arise frominhomogeneities sweeping across the line of sight in the rotating disk at theradius where the disk crosses the line of sight. We estimate that theinhomogeneities 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 thusprovides strong evidence that the warped accretion disk is the absorber. Thisis the first direct confirmation that obscuration in type-2 AGN may, in somecases, arise in thin, warped accretion disks, rather than in geometricallythick 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 reprocessedChandra data.Comment: 36 pages,14 figures. Accepted for publication in Ap