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 NGC 4258 with the the XMM-Newton Observatory at five epochs over 1.5 years. We also report reprocessing of an overlapping four-epoch series of archival Chandra observations (0.5– 10 keV). By including earlier ASCA and Beppo-SAX observations, we present a new, nine-year time-series of models fit to the X-ray spectrum of NGC 4258. We model the Chandra and XMM-Newton data self-consistently with partially absorbed, hard power-law, soft thermal plasma, and soft power-law components. Over the nine years, the photoelectric absorbing column (∼ 10 cm) did not vary detectably, except for a ∼ 40% drop between two ASCA epochs separated by 3 years (in 1993 and 1996) and a ∼ 60% rise between two XMM-Newton epochs separated by just 5 months (in 2001 and 2002). 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 (with XMM-Newton and Chandra) is ∼30% over 19 days. The warped disk, a known source of H2O maser emission in NGC 4258, 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 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; [email protected]; [email protected]; [email protected]; [email protected] Visiting physicist, Kavli Institute for Particle Astrophysics and Cosmology, Stanford Linear Accelerator Center, M.S. 29, 2575 Sand Hill Rd, Menlo Park, CA 94025 Department of Astronomy, University of California, Berkeley, CA 94720-3411; [email protected] Renaissance Technologies, Inc., 600 Rt. 25A, E. Setauket, NY 11733; [email protected]