The Density and Location of the X‐Ray–absorbing Gas in NGC 3516

A new Chandra observation and archival observations by ASCA are used toinvestigate spectral variations in the Sy1 galaxy NGC 3516 over a period of 7years. A large change in flux (factor ~50 at 1keV) is observed between an ASCAobservation in '94 and the Chandra observation in 2000, with the source closeto the all-time maximum and minimum X-ray flux states, respectively. We findthe variations in the observed flux and spectra at these epochs to beconsistent with a constant column density of line-of-sight material reacting tochanges in the ionizing continuum. The data from the two epochs are consistentwith a simple decrease (by a factor 8-10) in the luminosity of a constant0.5-50 keV slope source and a line-of-sight absorber with an equivalenthydrogen column density of 10^21.9 cm^-2. Intermediate luminosities, sampledduring other ASCA observations, are all fitted by the same model with a verysmall change in spectral index (well below \Delta\Gamma=0.2). In addition,analysis of the long (360 ks) ASCA observation in 1998 shows clear "color"variations that are entirely consistent with this model and are interpreted asdue to changes in the opacity of the absorbing gas. The data allow us to put aconservative upper limit of 60 ks on the recombination time which translates toa lower limit of about 2.4 times 10^6 cm^-3 on the density of the recombininggas and an upper limit of about 6 times 10^17 h_{75}^-2 cm on its distance fromthe central source. These are the best limits obtained so far on the densityand location of the X-ray absorbing gas in a type-1 AGN. They indicate that theabsorbing gas is different, in terms of its density and location, from theionized gas commonly observed in type-II AGN. [Abridged]Comment: 8 pages, 5 color figures included, LaTeX emulateapj5.sty, accepted for publication in The Astrophysical Journa