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Register a new userBAT AGN Spectroscopic Survey-XXIII. A New Mid-Infrared Diagnostic for Absorption in Active Galactic Nuclei
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In this study, we use the SWIFT/BAT AGN sample, which has received extensive multiwavelength follow-up analysis as a result of the BAT AGN Spectroscopic Survey (BASS), to develop a diagnostic for nuclear obscuration by examining the relationship between the line-of-sight column densities ($N_{rm{H}}$), the 2-10 keV-to-$12,rm{mu m}$ luminosity ratio, and WISE mid-infrared colors. We demonstrate that heavily obscured AGNs tend to exhibit both preferentially redder mid-infrared colors and lower values of $L_{rm{X,,Obs.}}$/$L_{12,rm{mu m}}$ than less obscured AGNs, and we derive expressions relating $N_{rm{H}}$ to the $L_{rm{X,,Obs.}}$/$L_{12,rm{mu m}}$ and $L_{22,rm{mu m}}$/$L_{4.6,rm{mu m}}$ luminosity ratios as well as develop diagnostic criteria using these ratios. Our diagnostic regions yield samples that are $gtrsim80$% complete and $gtrsim60$% pure for AGNs with log($N_{rm{H}})geq24$, as well as $gtrsim85$% pure for AGNs with $rm{log}(N_{rm{H}})gtrsim23.5$. We find that these diagnostics cannot be used to differentiate between optically star forming galaxies and active galaxies. Further, mid-IR contributions from host galaxies that dominate the observed $12~rm{mu m}$ emission can lead to larger apparent X-ray deficits and redder mid-IR colors than the AGNs would intrinsically exhibit, though this effect helps to better separate less obscured and more obscured AGNs. Finally, we test our diagnostics on two catalogs of AGNs and infrared galaxies, including the XMM-Newton XXL-N field, and we identify several known Compton-thick AGNs as well as a handful of candidate heavily obscured AGNs based upon our proposed obscuration diagnostics.
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Accreting supermassive black holes (SMBHs), also known as active galactic nuclei (AGN), are generally surrounded by large amounts of gas and dust. This surrounding material reprocesses the primary X-ray emission produced close to the SMBH and gives rise to several components in the broadband X-ray spectra of AGN, including a power-law possibly associated with Thomson-scattered radiation. In this work, we study the properties of this scattered component for a sample of 386 hard-X-ray-selected, nearby ($z sim 0.03$) obscured AGN from the 70-month Swift/BAT catalog. We investigate how the fraction of Thomson-scattered radiation correlates with different physical properties of AGN, such as line-of-sight column density, X-ray luminosity, black hole mass, and Eddington ratio. We find a significant negative correlation between the scattering fraction and the column density. Based on a large number of spectral simulations, we exclude the possibility that this anti-correlation is due to degeneracies between the parameters. The negative correlation also persists when considering different ranges of luminosity, black hole mass, and Eddington ratio. We discuss how this correlation might be either due to the angle dependence of the Thomson cross-section or to more obscured sources having a higher covering factor of the torus. We also find a positive correlation between the scattering fraction and the ratio of [OIII] $lambda$5007 to X-ray luminosity. This result is consistent with previous studies and suggests that the Thomson-scattered component is associated with the narrow-line region.
The bulk of the X-ray emission in Active Galactic Nuclei (AGN) is produced very close to the accreting supermassive black hole (SMBH), in a corona of hot electrons which up scatters optical and ultraviolet photons from the accretion flow. The cutoff energy ($E_{rm C}$) of the primary X-ray continuum emission carries important information on the physical characteristics of the X-ray emitting plasma, but little is currently known about its potential relation with the properties of accreting SMBHs. Using the largest broad-band (0.3-150 keV) X-ray spectroscopic study available to date, we investigate how the corona is related to the AGN luminosity, black hole mass and Eddington ratio ($lambda_{rm Edd}$). Assuming a slab corona the median values of the temperature and optical depth of the Comptonizing plasma are $kT_{rm e}=105 pm 18$ keV and $tau=0.25pm0.06$, respectively. When we properly account for the large number of $E_{rm C}$ lower limits, we find a statistically significant dependence of the cutoff energy on the Eddington ratio. In particular, objects with $ lambda_{rm Edd}>0.1$ have a significantly lower median cutoff energy ($E_{rm C}=160pm41$ keV) than those with $lambda_{rm Edd}leq 0.1$ ($E_{rm C}=370pm51$ keV). This is consistent with the idea that radiatively compact coronae are also cooler, because they tend to avoid the region in the temperature-compactness parameter space where runaway pair production would dominate. We show that this behaviour could also straightforwardly explain the suggested positive correlation between the photon index ($Gamma$) and the Eddington ratio, being able to reproduce the observed slope of the $Gamma-lambda_{rm Edd}$ trend.
The silicate cross section peak near 10um produces emission and absorption features in the spectra of dusty galactic nuclei observed with the Spitzer Space Telescope. Especially in ultraluminous infrared galaxies, the observed absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A foreground screen of obscuration cannot reproduce this observed feature, even at large optical depth. Instead, the deep absorption requires a nuclear source to be deeply embedded in a smooth distribution of material that is both geometrically and optically thick. In contrast, a clumpy medium can produce only shallow absorption or emission, which are characteristic of optically-identified active galactic nuclei. In general, the geometry of the dusty region and the total optical depth, rather than the grain composition or heating spectrum, determine the silicate features observable properties. The apparent optical depth calculated from the ratio of line to continuum emission generally fails to accurately measure the true optical depth. The obscuring geometry, not the nature of the embedded source, also determines the far-IR spectral shape.
We present an atlas of mid-infrared (mid-IR) ~7.5-13micron spectra of 45 local active galactic nuclei (AGN) obtained with CanariCam on the 10.4m Gran Telescopio CANARIAS (GTC) as part of an ESO/GTC large program. The sample includes Seyferts and other low luminosity AGN (LLAGN) at a median distance of 35Mpc and luminous AGN, namely PG quasars, (U)LIRGs, and radio galaxies (RG) at a median distance of 254Mpc. To date, this is the largest mid-IR spectroscopic catalog of local AGN at sub-arcsecond resolution (median 0.3arcsec). The goal of this work is to give an overview of the spectroscopic properties of the sample. The nuclear 12micron luminosities of the AGN span more than four orders of magnitude, nu*Lnu(12micron)~ 3e41-1e46erg/s. In a simple mid-IR spectral index vs. strength of the 9.7micron silicate feature diagram most LLAGN, Seyfert nuclei, PG quasars, and RGs lie in the region occupied by clumpy torus model tracks. However, the mid-IR spectra of some might include contributions from other mechanisms. Most (U)LIRG nuclei in our sample have deeper silicate features and flatter spectral indices than predicted by these models suggesting deeply embedded dust heating sources and/or contribution from star formation. The 11.3micron PAH feature is clearly detected in approximately half of the Seyfert nuclei, LLAGN, and (U)LIRGs. While the RG, PG quasars, and (U)LIRGs in our sample have similar nuclear 12micron luminosities, we do not detect nuclear PAH emission in the RGs and PG quasars.
In this paper I summarize the science motivations, as well as a few mid-infrared spectroscopic methods used to identify the principal mechanisms of energy production in dust enshrouded galactic nuclei. The development of the various techniques is briefly discussed. Emphasis is given to the use of the data which are becoming available with the infrared spectrograph (IRS) on Spitzer, as well as the results which have been obtained by IRS over the past two years.
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