The ionized absorber and nuclear environment of IRAS 13349+2438: multi-wavelength insights from coordinated Chandra HETGS, HST STIS, HET and Spitzer IRS

Abstract

We present results from a multi-wavelength infrared (IR)-to-X-ray campaign of the infrared bright (but highly optical-ultraviolet extincted) quasi-stellar object (QSO) IRAS 13349+2438 obtained with the Chandra High Energy Transmission Grating Spectrometer (HETGS), the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS), the Hobby–Eberly Telescope (HET) 8 m and the Spitzer Infrared Spectrometer (IRS). Based on HET optical spectra of [O III], we refine the redshift of IRAS 13349 to be z = 0.108 53. The weakness of the [O III] in combination with strong Fe II in the HET spectra reveals extreme Eigenvector-1 characteristics in IRAS 13349, but the 2468 km s⁻¹ width of the Hβ line argues against a narrow-line Seyfert 1 classification; on average, IR, optical and optical-ultraviolet (UV) spectra show IRAS 13349 to be a typical QSO. Independent estimates based on the Hβ line width and fits to the IRAS 13349 spectral energy distribution (SED) both give a black hole mass of M_(BH) = 10⁹  M_⊙. The heavily reddened STIS UV spectra reveal for the first time blueshifted absorption from Ly Α, N V and C IV, with components at systemic velocities of −950 km s⁻¹ and −75 km s⁻¹⁠. The higher velocity UV lines are coincident with the lower ionization (ξ ∼ 1.6) WA-1 warm absorber lines seen in the X-rays with the HETGS. In addition, a ξ ∼ 3.4 WA-2 is also required by the data, while a ξ ∼ 3 WA-3 is predicted by theory and seen at less significance; all detected X-ray absorption lines are blueshifted by ∼ 700-900 km s⁻¹ . Theoretical models comparing different ionizing SEDs reveal that including the UV (i.e. the accretion disc) as part of the ionizing continuum has strong implications for the conclusions one would draw about the thermodynamic stability of the warm absorber. Specific to IRAS 13349, we find that an X-ray–UV ionizing SED favours a continuous distribution of ionization states in a smooth flow (this paper) versus discrete clouds in pressure equilibrium (previous work by other authors). Direct detections of dust are seen in both the IR and X-rays. We see weak polycyclic aromatic hydrocarbon (PAH) emission at 7.7 μm and 11.3 μm which may also be blended with forsterite, and 10 μm and 18 μm silicate emission, as well as an Fe L edge at 700 eV indicative of iron-based dust with a dust-to-gas ratio >90 per cent. We develop a geometrical model in which we view the nuclear regions of the QSO along a line of sight that passes through the upper atmosphere of an obscuring torus. This sight line is largely transparent in X-rays since the gas is ionized, but it is completely obscured by dust that blocks a direct view of the UV/optical emission region. In the context of our model, 20 per cent of the intrinsic UV/optical continuum is scattered into our sight line by the far wall of an obscuring torus. An additional 2.4 per cent of the direct light, which likely dominates the UV emission, is Thomson-scattered into our line of sight by another off-plane component of highly ionized gas.

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