Revealing the Dusty Warm Absorber in MCG −6-30-15 with the [ITAL]Chandra[/ITAL] High-Energy Transmission Grating

Abstract

We present detailed evidence for a warm absorber in the Seyfert 1 galaxy MCG -6-30-15 and dispute earlier claims for relativistic O line emission. The High-Energy Transmission Grating spectra show numerous narrow, unresolved (FWHM ≲ 200 km s -1) absorption lines from a wide range of ionization states of N, O, Mg, Ne, Si, S, Ar, and Fe. The O VII edge and the 1s2-1snp resonance line series to n = 9 are clearly detected at rest in the active galactic nucleus frame. We attribute previous reports of an apparently highly redshifted O VII edge to the 1s2-1snp (n > 5) O VII resonance lines and a neutral Fe L absorption complex. The shape of the Fe L feature is nearly identical to that seen in the spectra of several X-ray binaries and in laboratory data. The implied dust column density agrees with that obtained from reddening studies and gives the first direct X-ray evidence for dust embedded in a warm absorber. The O VII resonance lines and the weak edge are also detected, and the spectral rollover below ∼2 keV is explained by the superposition of numerous absorption lines and edges. We identify, for the first time, a KLL resonance in the O VII photoabsorption cross section, giving a measure of the O VII column density. The O VII (f) emission detected at the systemic velocity implies a covering fraction of ∼5% (depending on the observed vs. time-averaged ionizing flux). Our observations show that a dusty warm absorber model is not only adequate to explain all the spectral features ≳0.48 keV (≲ S26 Å) but that the data require it. This contradicts the interpretation of Branduardi-Raymont and coworkers that this spectral region is dominated by highly relativistic line emission from the vicinity of the black hole.