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

Abstract

A new Chandra observation and archival observations by ASCA are used to investigate spectral variations in the Seyfert 1 galaxy NGC 3516 over a period of 7 yr. A large change in flux (factor of ~50 at 1 keV) is observed between an ASCA observation in 1994 and the Chandra observation in 2000, with the source close to the all-time maximum and minimum X-ray flux states, respectively. We find the variations in the observed flux and spectra at these epochs to be consistent with a constant column density of line-of-sight material reacting to changes in the ionizing continuum. The data from the two epochs are consistent with a simple decrease (by a factor of 8-10) in the luminosity of a constant 0.5-50 keV slope source and a line-of-sight absorber with an equivalent hydrogen column density of 1021.9 cm-2. Intermediate luminosities, sampled during other ASCA observations, are all fitted by the same model with a very small change in spectral index (well below ΔΓ = 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 as due to changes in the opacity of the absorbing gas. The data allow us to put a conservative upper limit of 60 ks on the recombination time, which translates to a lower limit of about 2.4 × 106 cm-3 on the density of the recombining gas and an upper limit of about 6 × 1017 h cm on its distance from the central source. These are the best limits obtained so far on the density and location of the X-ray-absorbing gas in a type 1 active galactic nucleus (AGN). They indicate that the absorbing gas is different in terms of its density and location from the ionized gas commonly observed in type II AGNs. The Chandra ACIS/LETGS data also reveals a strong (EW = 290 eV), unresolved 6.4 keV iron line, a strong O VII 0.561 keV line, and a marginally detected N VI 0.419 keV line. The former is interpreted as originating in a large column of gas of a lower state of ionization seen in reflection and is consistent with the spectrum at high energies at all epochs. The two other emission lines are probably emitted by the gas also responsible for the line-of-sight absorption.