Dust inflated accretion disc as the origin of the Broad Line Region in Active Galactic Nuclei

The Broad Line Region (BLR) in AGN is composed of dense gas ($\sim 10^{11}$ cm$^{-3}$) on sub-pc scale, which absorbs about 30 per cent of the ionising continuum. The outer size of the BLR is likely set by dust sublimation, and its density by the incident radiation pressure compression (RPC). But, what is the origin of this gas, and what sets its covering factor (CF)? Czerny & Hryniewicz (2011) suggested that the BLR is a failed dusty wind from the outer accretion disc. We explore the expected dust properties, and the implied BLR structure. We find that graphite grains sublimate only at $T\simeq 2000$ K at the predicted density of $\sim 10^{11}$ cm$^{-3}$, and therefore large graphite grains ($\ge 0.3$ $\mu$m) survive down to the observed size of the BLR, $R_{\rm BLR}$. The dust opacity in the accretion disc atmosphere is $\sim 50$ times larger than previously assumed, and leads to an inflated torus-like structure, with a predicted peak height at $R_{\rm BLR}$. The illuminated surface of this torus-like structure is a natural place for the BLR. The BLR CF is mostly set by the gas metallicity, the radiative accretion efficiency, a dynamic configuration, and ablation by the incident optical-UV continuum. This model predicts that the BLR should extend inwards of $R_{\rm BLR}$ to the disc radius where the surface temperature is $\simeq 2000$ K, which occurs at $R_{\rm in}\simeq 0.18 R_{\rm BLR}$. The value of $R_{\rm in}$ can be tested by reverberation mapping of the higher ionisation lines, predicted by RPC to peak well inside $R_{\rm BLR}$. The dust inflated disc scenario can also be tested based on the predicted response of $R_{\rm BLR}$ and the CF to changes in the AGN luminosity and accretion rate.Comment: 28 pages, 15 figures; accepted for publication in MNRA

Structure and Kinematics of the Broad-Line Region and Torus of Active Galactic Nuclei

Energetics considerations imply that the broad-line region (BLR) has a high covering factor. The absence of absorption from the BLR means that the BLR has to have a flattened distribution and be seen through a polar hole. The BLR is the inward extension of the torus and they have similar geometries and covering factors. Reconciling velocity-resolved reverberation mapping, spectropolarimetry, and the increasing blueshifting of BLR lines with decreasing distance from the centre, implies that the BLR has a significant inflow component. This inflow provides the mass inflow rate needed to power the AGN. We suggest that the mechanism producing the outward transport of angular momentum necessary for the net inflow of the BLR is the magneto-rotational instability, and that the BLR and outer accretion disc are one and the same.Comment: Talk given at "The Central Kiloparsec: Active Galactic Nuclei and Their Hosts", Ierapetra, Crete, 4-6 June, 2008. To appear in Volume 79 of the Memorie della Societa Astronomica Italiana. 6 pages, 3 figure

Gamma-Gamma Absorption in the Broad Line Region Radiation Fields of Gamma-Ray Blazars

The expected level of gamma-gamma absorption in the Broad Line Region (BLR) radiation field of gamma-ray loud Flat Spectrum Radio Quasars (FSRQs)is evaluated as a function of the location of the gamma-ray emission region. This is done self-consistently with parameters inferred from the shape of the spectral energy distribution (SED) in a single-zone leptonic EC-BLR model scenario. We take into account all geometrical effects both in the calculation of the gamma-gamma opacity and the normalization of the BLR radiation energy density. As specific examples, we study the FSRQs 3C279 and PKS 1510-089, keeping the BLR radiation energy density at the location of the emission region fixed at the values inferred from the SED. We confirm previous findings that the optical depth due to $\gamma\gamma$ absorption in the BLR radiation field exceeds unity for both 3C279 and PKS 1510-089 for locations of the gamma-ray emission region inside the inner boundary of the BLR. It decreases monotonically, with distance from the central engine and drops below unity for locations within the BLR. For locations outside the BLR, the BLR radiation energy density required for the production of GeV gamma-rays rapidly increases beyond observational constraints, thus making the EC-BLR mechanism implausible. Therefore, in order to avoid significant gamma-gamma absorption by the BLR radiation field, the gamma-ray emission region must therefore be located near the outer boundary of the BLR.Comment: Accepted for publication in The Astrophysical Journa

The HST view of the broad line region in low luminosity AGN

We analyze the properties of the broad line region (BLR) in low luminosity AGN by using HST/STIS spectra. We consider a sample of 24 nearby galaxies in which the presence of a BLR has been reported from their Palomar ground-based spectra. Following a widely used strategy, we used the [SII] doublet to subtract the contribution of the narrow emission lines to the H-alpha+[NII] complex and to isolate the BLR emission. Significant residuals that suggest a BLR, are present. However, the results change substantially when the [OI] doublet is used. Furthermore, the spectra are also reproduced well by just including a wing in the narrow H-alpha and [NII] lines, thus not requiring the presence of a BLR. We conclude that complex structure of the narrow line region (NLR) is not captured with this approach and that it does not lead to general robust constraints on the properties of the BLR in these low luminosity AGN. Nonetheless, the existence of a BLR is firmly established in 5 Seyferts, and 5 LINERs. However, the measured BLR fluxes and widths in the 5 LINERs differ substantially with respect to the ground-based data. The BLR sizes in LINERs, which are estimated by using the virial formula from the line widths and the black hole mass, are about 1 order of magnitude greater than the extrapolation to low luminosities of the relation between the BLR radius and AGN luminosity observed in more powerful active nuclei. We ascribe the larger BLR radius to the lower accretion rate in LINERs when compared to the Seyfert, which causes the formation of an inner region dominated by an advection-dominated accretion flow (ADAF). The estimated BLR sizes in LINERs are comparable to the radius where the transition between the ADAF and the standard thin disk occurs due to disk evaporation.Comment: 16 pages. Accepted for publication in A&

The location of the dust causing internal reddening of active galactic nuclei

We use the Balmer decrements of the broad-line regions (BLRs) and narrow-line regions (NLRs) of active galactic nuclei (AGNs) as reddening indicators to investigate the location of the dust for four samples of AGNs with reliable estimates of the NLR contribution to the Balmer lines. Intercomparison of the NLR and BLR Balmer decrements indicates that the reddening of the NLR sets a lower limit to the reddening of the BLR. Almost no objects have high NLR reddening but low BLR reddening. The reddening of the BLR is often substantially greater than the reddening of the NLR. The BLR reddening is correlated with the equivalent widths of [O III] lines and the intensity of the [O III] lines relative to broad H\beta. We find these relationships to be consistent with the predictions of a simple model where the additional dust reddening the BLR is interior to the NLR. We thus conclude that the dust causing the additional reddening of the accretion disc and BLR is mostly located at a smaller radius than the NLR.Comment: Monthly Notices of the Royal Astronomical Society in press. 6 pages. 6 figure

"Flat" broad line region and gamma-ray absorption in blazars

We study the impact of the geometry of the broad line region (BLR) on the expected absorption, through the \gamma\gamma\rightarrow e^{\pm} process, of gamma rays produced in the relativistic jet of flat spectrum radio quasars (FSRQ). We consider "flat" (or "disky") BLR models, and use BLR spectra calculated with the photoionization code CLOUDY, already used to investigate the emission and the absorption of high-energy photons in FSRQ. We characterize the energy-dependent optical depth of the process, \tau(E), for different accretion disk luminosities, aperture angles of the BLR (\alpha, as measured from the equatorial plane), and initial injection eighths of the high-energy photons, $R_{\rm o}$. We study in particular how the change of these parameters influences the spectral break at GeV energies, predicted if the emission occurs within the BLR. We found a well defined relation between the break energy and the post-break slope, both uniquely determined by \alpha. We finally find that even a rather disk-like BLR (\alpha \sim 25^\circ) corresponds to important absorption (\tau>1) of photons above few tens of GeV produced within the BLR. We therefore conclude that the VHE emission detected from FSRQs occurs beyond the BLR.Comment: 8 pages, submitted to MNRA

Detection of Polarized Broad Emission in the Seyfert 2 Galaxy Mrk 573

We report the discovery of the scattered emission from a hidden broad-line region (BLR) in a Seyfert 2 galaxy, Mrk 573, based on our recent spectropolarimetric observation performed at the Subaru Telescope. This object has been regarded as a type 2 AGN without a hidden BLR by the previous observations. However, our high quality spectrum of the polarized flux of Mrk 573 shows prominent broad (~3000 km/s) H_alpha emission, broad weak H_beta emission, and subtle Fe II multiplet emission. Our new detection of these indications for the presence of the hidden BLR in the nucleus of Mrk 573 is thought to be owing to the high signal-to-noise ratio of our data, but the possibility of a time variation of the scattered BLR emission is also mentioned. Some diagnostic quantities such as the IRAS color, the radio power, and the line ratio of the emission from the narrow-line region of Mrk 573 are consistent with the distributions of such quantities of type 2 AGNs with a hidden BLR. Mrk 573 is thought to be an object whose level of the AGN activity is the weakest among the type 2 AGNs with a hidden BLR. In terms of the systematic differences between the type 2 AGNs with and without a hidden BLR, we briefly comment on an interesting Seyfert 2 galaxy, Mrk 266SW, which may possess a hidden BLR but has been treated as a type 2 AGNs without a hidden BLR.Comment: 9 pages including 6 figures, to appear in The Astronomical Journa

The central pc-scale region in blazars: insights from multi-band observations

The empirical relations in the black hole-accretion disk-relativistic jet system and physical processes behind these relations are still poorly understood, partly because they operate close to the black hole within the central light year. Very long baseline array (VLBA) provides unparalleled resolution at 15 GHz with which to observe the jet components at sub-milliarcsecond scales, corresponding to sub-pc-scales for local blazars. We discuss the jet inner structure of blazars, location and radiation mechanisms operating in the innermost parsec-scale region of blazars, and evidence for jet-excited broad-line region (BLR) ouflowing downstream the jet. Outflowing BLR can provide necessary conditions for production of high energy emission along the jet between the base of the jet and the BLR and far beyond the BLR as evidenced by recent observations. Flat spectrum quasars and low synchrotron peaked sources are the most likely objects to host the outfllowing BLR. From the $\gamma$-ray absorption arguments, we propose that the jet-excited region of the outflowing BLR in quasars is small and/or gas filling factor is low, and that the orientation and opening angle of the outflowing BLR can lead to relevant $\gamma$-ray absorption features observed in quasars.Comment: 4 pages, will appear in Conference Proceedings for IAU Symposium 304 "Multi-wavelength AGN Surveys and Studies" held in Yeravan, Armenia, October 1-6, 201

Measuring black hole mass of type I active galactic nuclei by spectropolarimetry

Black hole (BH) mass of Type I active galactic nuclei (AGN) can be measured or estimated through either reverberation mapping (RM) or empirical $R-L$ relation, however, both of them suffer from uncertainties of the virial factor ($f_{\rm BLR}$), thus limiting the measurement accuracy. In this letter, we make an effort to investigate $f_{\rm BLR}$ through polarised spectra of the broad-line regions (BLR) arisen from electrons in the equatorial plane. Given the BLR composed of discrete clouds with Keplerian velocity around the central BH, we simulate a large number of spectra of total and polarised flux with wide ranges of parameters of the BLR model and equatorial scatters. We find that the $f_{\rm BLR}$-distribution of polarised spectra is much narrower than that of total ones. This provides a way of n accurately estimating BH mass from single spectropolarimetric observations of type I AGN whose equatorial scatters are identified.Comment: MNRAS Letters (accepted): 5 pages, 3 figure

External Compton Scattering in Blazar Jets and the Location of the Gamma-Ray Emitting Region

I study the location of the $\gamma$-ray emission in blazar jets by creating a Compton-scattering approximation valid for all anisotropic radiation fields in the Thomson through Klein-Nishina regimes, which is highly accurate and can speed up numerical calculations by up to a factor $\sim10$. I apply this approximation to synchrotron self-Compton, and external Compton-scattering of photons from the accretion disk, broad-line region (BLR), and dust torus. I use a stratified BLR model and include detailed Compton-scattering calculations of a spherical and flattened BLR. I create two dust torus models, one where the torus is an annulus, and one where it is an extended disk. I present detailed calculations of the photoabsorption optical depth using my detailed BLR and dust torus models, including the full angle dependence. I apply these calculations to the emission from a relativistically moving blob traveling through these radiation fields. The ratio of $\gamma$-ray to optical flux produces a predictable pattern that could help locate the $\gamma$-ray emission region. I show that the bright flare from 3C 454.3 in 2010 November detected by the Fermi Large Area Telescope is unlikely to originate from a single blob inside the BLR since it moves outside the BLR in a time shorter than the flare duration, although emission by multiple blobs inside the BLR is possible; and $\gamma$-rays are unlikely to originate from outside the BLR from scattering of photons from an extended dust torus, since then the cooling timescale would be too long to explain the observed short variability.Comment: Accepted by ApJ. 22 pages, 19 Figures, 5 Table