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Register a new userX-ray absorption and 9.7 um silicate feature as a probe of AGN torus structure
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Jun Xu
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The dusty torus plays a vital role in unifying active galactic nuclei (AGNs). However, the physical structure of the torus remains largely unclear. Here we present a systematical investigation of the torus mid-infrared (MIR) spectroscopic feature, i.e., the 9.7 um silicate line, of $175$ AGNs selected from the Swift/BAT Spectroscopic Survey (BASS). Our sample is constructed to ensure that each of the $175$ AGNs has Spizter/IRS MIR, optical, and X-ray spectroscopic coverage. Therefore, we can simultaneously measure the silicate strength, optical emission lines, and X-ray properties (e.g., the column density and the intrinsic X-ray luminosity). We show that, consistent with previous works, the silicate strength is weakly correlated with the hydrogen column density ($N_mathrm{H}^mathrm{X}$), albeit with large scatters. For X-ray unobscured AGNs, the silicate-strength-derived $V$-band extinction and the broad-H$alpha$-inferred one are both small; however, for X-ray obscured AGNs, the former is much larger than the latter. In addition, we find that the optical type 1 AGNs with strong X-ray absorption on average show significant silicate absorption, indicating that their X-ray absorption might not be caused by dust-free gas in the broad-line region. Our results suggest that the distribution and structure of the obscuring dusty torus are likely to be very complex. We test our results against the smooth and clumpy torus models and find evidence in favor of the clumpy torus model.
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Studying the composition of dust in the interstellar medium (ISM) is crucial in understanding the cycle of dust in our galaxy. The mid-infrared spectral signature of amorphous silicates, the most abundant dust species in the ISM, is studied in different lines-of-sight through the Galactic plane, thus probing different conditions in the ISM. We have analysed 10 spectra from the Spitzer archive, of which 6 lines-of-sight probe diffuse interstellar medium material and 4 probe molecular cloud material. The 9.7 um silicate absorption features in 7 of these spectra were studied in terms of their shape and strength. In addition, the shape of the 18 um silicate absorption features in 4 of the diffuse sightline spectra were analysed. The 9.7 um silicate absorption bands in the diffuse sightlines show a strikingly similar band shape. This is also the case for all but one of the 18 um silicate absorption bands observed in diffuse lines-of-sight. The 9.7 um bands in the 4 molecular sightlines show small variations in shape. These modest variations in the band shape are inconsistent with the interpretation of the large variations in {tau}_9.7/E(J-K) between diffuse and molecular sightlines in terms of silicate grain growth. Instead, we suggest that the large changes in {tau}_9.7 / E(J-K) must be due to changes in E(J-K).
The reflection spectrum of the torus around AGN is characterized by X-ray fluorescent lines, which are most prominent for type II AGN. A clumpy torus allows photons reflected from the back-side of the torus to leak through the front free-of-obscuration regions. Therefore, the observed X-ray fluorescent lines are sensitive to the clumpiness of the torus. We analyse a sample of type II AGN observed with Chandra HETGS, and measure the fluxes for the Si Ka and Fe Ka lines. The measured Fe Ka/Si Ka ratios, spanning a range between $5-60$, are far smaller than the ratios predicted from simulations of smooth tori, indicating that the tori of the studied sources have clumpy distributions rather than smooth ones. Compared with simulation results of clumpy tori with a half-opening angle of 60$^{circ}$, the Circinus galaxy has a Fe Ka/Si Ka ratio of $sim60$, which is close to the simulation results for $N=5$, where $N$ is the average number of clumps along the line of sight. The Fe Ka/Si Ka ratios of the other sources are all below the simulation results for $N=2$. Overall, it shows that the non-Fe fluorescent lines in the soft X-ray band are a potentially powerful probe of the clumpiness of the torus around AGN.
We propose new diagnostics that utilize the [O IV] 25.89 $mu$m and nuclear (subarcsecond scale) 12 $mu$m luminosity ratio for identifying whether an AGN is deeply `buried in their surrounding material. Utilizing a sample of 16 absorbed AGNs at redshifts lower than 0.03 in the Swift/BAT catalog observed with Suzaku, we find that AGNs with small scattering fractions ($<$0.5%) tend to show weaker [O IV]-to-12 $mu$m luminosity ratios than the average of Seyfert 2 galaxies. This suggests that this ratio is a good indicator for identifying buried AGNs. Then, we apply this criterion to 23 local ultra/luminous infrared galaxies (U/LIRGs) in various merger stages hosting AGNs. We find that AGNs in most of mid- to late-stage mergers are buried, while those in earlier stage ones (including non-merger) are not. This result suggests that the fraction of buried AGNs in U/LIRGs increases as the galaxy-galaxy interaction becomes more significant.
We present mid infrared (Mid-IR) spectra of the Compton-thick Seyfert 2 galaxy NGC,3281, obtained with the Thermal-Region Camera Spectrograph (T-ReCS) at the Gemini South telescope. The spectra present a very deep silicate absorption at 9.7,$mu$m, and [S{sc,iv]},10.5,$mu$m and [Ne{sc,ii]},12.7,$mu$m ionic lines, but no evidence of PAH emission. We find that the nuclear optical extinction is in the range 24 $leq$ A$_{V}$ $leq$ 83,mag. A temperature T = 300,K was found for the black-body dust continuum component of the unresolved 65,pc nucleus and at 130,pc SE, while the region at 130,pc reveals a colder temperature (200,K). We describe the nuclear spectrum of NGC,3281 using a clumpy torus model that suggests that the nucleus of this galaxy hosts a dusty toroidal structure. According to this model, the ratio between the inner and outer radius of the torus in NGC,3281 is $R_0/R_d$ = 20, with {bf 14} clouds in the equatorial radius with optical depth of $tau_{V}$ = 40,mag. We would be looking in the direction of the torus equatorial radius ($i$ = {bf 60$^{circ}$}), which has outer radius of R$_{0},sim$ 11,pc. The column density is N$_{H}approx$,{bf 1.2},$times,10^{24},cm^{-2}$ and iron K$alpha$ equivalent width ($approx$ 0.5 - 1.2,keV) are used to check the torus geometry. Our findings indicate that the X-ray absorbing column density, which classifies NGC,3281 as a Compton-thick source, may also be responsible for the absorption at 9.7,$mu$m providing strong evidence that the silicate dust responsible for this absorption can be located in the AGN torus.
Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band flux variations occur on short time scales (hours) as well as on larger times scales. While short time scale variability is often investigated in single source studies, only few works are able to explore flux variation on very long time scales.This work provides a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2-2 keV band, up to $sim 20$ years rest-frame for a sample of 220 sources. Moreover, we study variability for 2,700 quasars up to $sim 8$ years rest-frame in the same (soft) band.We build our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. In order to select only AGN we use the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigate variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations.Our work shows an increase of the structure function up to 20 years. We do not find evidence of a plateau in the structure function on these long time scales.The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long time scales.
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