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

摘要: We report monitoring of the 0.3-10 keV spectrum NGC 4258 with XMM-Newton Observatory at five epochs over 1.5 yr. also reprocessing an overlapping four-epoch series archival Chandra observations (0.5-10 keV). By including earlier ASCA and BeppoSAX observations, we present a new, 9 year time models fitted to X-ray 4258. model data self-consistently partially absorbed, hard power-law, soft thermal plasma, power-law components. Over years, photoelectric absorbing column (~1023 cm-2) did not vary detectably, except for ~40% drop between two separated by 3 years (in 1993 1996) ~60% rise just 5 months 2001 2002). In contrast, factor 2-3 changes are seen in absorbed flux on timescale years. These uncorrelated indicative central engine variability. The most rapid change luminosity (5-10 keV) that detect (with Chandra) is ~30% 19 days. warped disk, known source H2O maser emission 4258, believed cross line sight engine. propose variations arise from inhomogeneities sweeping across rotating disk radius which crosses sight. estimate ~1015 cm size crossing 0.29 pc, slightly smaller than expected scale height disk. This result thus provides strong evidence accretion absorber this (and possibly other) active galactic nuclei (AGNs). first direct confirmation obscuration type 2 AGNs may, some cases, thin, disks rather geometrically thick tori. Some previous studies detection weak Fe Kα do any our spectra 90% upper limit equivalent width ~49 eV one observation. Weak, time-variable absorption has been reported study. observe lines or reprocessed data. absence consistent being optically thin photons, as well subtending small solid angle because shallowness warp.