Revealing the dusty warm absorber in MCG-6-30-15 with the Chandra high-energy transmission grating

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 less than or similar to 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 1s(2)-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 vu edge to the 1s(2)-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 VIII resonance lines and the weak edge are also detected, and the spectral rollover below similar to2 keV is explained by the superposition of numerous absorption lines and edges. We identify, for the first time, a KLL resonance in the O vr photoabsorption cross section, giving a measure of the O VI column density. The O vn (f) emission detected at the systemic velocity implies a covering fraction of similar to5% (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 greater than or similar to0.48 keV (less than or similar to 26 Angstrom) 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.