XMM and Chandra Observations of NGC 4258

We report monitoring of the 0.3-10 keV spectrum of NGC4258 with the the XMM 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 SAX observations, we present a new, nine year time-series of models fit to the X-ray spectrum of NGC4258. We model the Chandra and XMM data self-consistently with partially absorbed, hard power-law, soft thermal plasma, and soft power-law components. Over the nine years, the photo-electric absorbing column ( ∼ 10²³ cm^-2) 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 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 uncorrel ated 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 and Chandra) is on the order of 30% over 19 days. The warped disk that is 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 in the rotating disk, as they sweep across the line of sight. We estimate that the inhomogeneities are ∼ 10¹⁵ cm in size at radii >0.27 pc, which is consistent with the estimated radius of the disk crossing. Some previous studies report detection of weak Fe Kα emission in NGC4258. We do not detect this line emission in any of our XMM spectra with a 90% upper limit to the equivalent width of ∼ 49 eV for one observation. Weak, time variable Fe line absorption has also been reported for a previous Chandra study. We do not observe evidence of absorption lines in the XMM or reprocessed Chandra data. The absence of Fe line emission is consistent with the disk being optically thin to hard photons as well as it subtending a small solid angle as seen from the central engine. This work has been supported by NASA GO grant NAG5-10194 and NASA science grant NAS8-39073 to the Chandra X-Ray Center.