SIMBAD references

We report subarcsecond resolution X-ray imaging spectroscopy of the low-luminosity active galactic nucleus of NGC 4258 and its immediate surroundings with the Chandra X-Ray Observatory. NGC 4258 was observed four times, with the first two observations separated by 1 month, followed over a year later by two consecutive observations. The spectrum of the nucleus is well described by a heavily absorbed (N_H≃7x10²²/cm2, which did not change), hard X-ray power law of variable luminosity, plus a constant, thermal soft X-ray component. We do not detect an iron Kα emission line with the upper limit to the equivalent width of a narrow, neutral iron line ranging between 94 and 887 eV (90% confidence) for the different observations. During the second observation, on 2000 April 17, two narrow absorption features are seen with greater than 99.5% confidence at ≃6.4 and ≃6.9 keV, which we identify as resonant absorption lines of Fe XVIII-Fe XIX Kα and Fe XXVI Kα, respectively. In addition, the 6.9 keV absorption line is probably variable on a timescale of ∼6000 s. The absorption lines are analyzed through a curve of growth analysis, which allows the relationship between ionic column and kinematic temperature or velocity dispersion to be obtained for the observed equivalent widths. We discuss the properties of the absorbing gas for both photo- and collisionally ionized models. Given that the maser disk is viewed at an inclination i=82°, the gas responsible for the 6.9 keV absorption line may be in an inner disk or a disk-wind boundary layer, or may be thermal gas entrained at the base of the jet. The gas that gives rise to the photoelectric absorption may be the same as that which causes the 6.4 keV Fe Kα absorption provided that the gas has a bulk velocity dispersion of a few thousand km/s. This is the first detection of iron X-ray absorption lines in an extragalactic source with a nearly edge-on accretion disk, and this phenomenon is likely to be related to similar X-ray absorption lines in Galactic X-ray binaries with nearly edge-on accretion disks.