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Register a new userAGN Black hole mass estimates using polarization in broad emission Lines
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Added by
{\\DJ}or{\\dj}e Savi\\'c
Publication date
2018
fields
Physics
and research's language is
English
Authors
{DJ}. Savic
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The innermost regions in active galactic nuclei (AGNs) were not being spatially resolved so far but spectropolarimetry can provide us insight about their hidden physics and the geometry. From spectropolarimetric observations in broad emission lines and assuming equatorial scattering as a dominant polarization mechanism, it is possible to estimate the mass of supermassive black holes (SMBHs). We explore the possibilities and limits and to put constraints on the usage of the method for determining SMBH masses using polarization in broad emission lines by providing more in-depth theoretical modeling. Methods. We use the Monte Carlo radiative transfer code STOKES for exploring polarization of Type 1 AGNs. We model equatorial scattering using flared-disk geometry for a set of different SMBH masses assuming Thomson scattering. In addition to the Keplerian motion in the BLR, we also consider cases of additional radial inflows and vertical outflows. We model the profiles of polarization plane position angle, degree of polarization and total unpolarized line for different BLR geometries and different SMBH masses. Our modeling confirms that the method can be widely used for Type-1 AGNs when viewing inclinations are between 25 and 45 degrees. We show that the distance between the BLR and scattering region (SR) has a significant impact on the mass estimates and the best mass estimates are when the SR is situated at the distance 1.5-2.5 times larger than the outer BLR radius. Our models show that if Keplerian motion can be traced through the polarized line profile, then the direct estimation of the mass of the SMBH can be performed. When radial inflows or vertical outflows are present in the BLR, this method can be applied if velocities of the inflow/outflow are less than 500 km/s. We find that models for NGC4051, NGC4151, 3C273 and PG0844+349 are in good agreements with observations.
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We present a new method for supermassive black hole (SMBH) mass measurements in Type 1 active galactic nuclei (AGN) using polarization angle across broad lines. This method gives measured masses which are in a good agreement with reverberation estimates. Additionally, we explore the possibilities and limits of this method using the STOKES radiative transfer code taking a dominant Keplerian motion in the broad line region (BLR). We found that this method can be used for the direct SMBH mass estimation in the cases when in addition to the Kepler motion, radial inflows or vertical outflows are present in the BLR. Some advantages of the method are discussed.
The inter-line comparison between high- and low-ionization emission lines has yielded a wealth of information on the quasar broad line region (BLR) structure and dynamics, including perhaps the earliest unambiguous evidence in favor of a disk + wind structure in radio-quiet quasars. We carried out an analysis of the CIV 1549 and Hbeta line profiles of 28 Hamburg-ESO high luminosity quasars and of 48 low-z, low luminosity sources in order to test whether the high-ionization line CIV 1549 width could be correlated with Hbeta and be used as a virial broadening estimator. We analyze intermediate- to high-S/N, moderate resolution optical and NIR spectra covering the redshifted CIV and H$beta$ over a broad range of luminosity log L ~ 44 - 48.5 [erg/s] and redshift (0 - 3), following an approach based on the quasar main sequence. The present analysis indicates that the line width of CIV 1549 is not immediately offering a virial broadening estimator equivalent to H$beta$. At the same time a virialized part of the BLR appears to be preserved even at the highest luminosities. We suggest a correction to FWHM(CIV) for Eddington ratio (using the CIV blueshift as a proxy) and luminosity effects that can be applied over more than four dex in luminosity. Great care should be used in estimating high-L black hole masses from CIV 1549 line width. However, once corrected FWHM(CIV) values are used, a CIV-based scaling law can yield unbiased MBH values with respect to the ones based on H$beta$ with sample standard deviation ~ 0.3 dex.
The EUV provides most of the ionization that creates the high equivalent width (EW) broad and narrow emission lines (BELs, NELs) of quasars. Spectra of Hypermassive Schwarzschild black holes (HMBHs, $M_{BH} geq 10^{10} M_{odot}$) with $alpha$-discs, decline rapidly in the EUV suggesting much lower EWs. Model spectra for black holes of mass $10^{6}-10^{12} M_{odot}$ and accretion rates $0.03 leq L_{bol}/L_{edd} leq 1.0$ were input to the CLOUDY photoionization code. BELs become $sim$100 times weaker in EW from $M_{BH} sim 10^8 M_{odot}$ to $M_{BH} sim 10^{10} M_{odot}$. The high ionization BELs (O VI 1034 $overset{circ}{mathrm {A}}$, C IV 1549 $overset{circ}{mathrm {A}}$, He II 1640 $overset{circ}{mathrm {A}}$) decline in EW from ($M_{BH} geq 10^6 M_{odot}$, reproducing the Baldwin effect, but regain EW for $M_{BH} geq 10^{10} M_{odot}$). The low ionization lines (MgII 2798 $overset{circ}{mathrm {A}}$, H$beta$ 4861 $overset{circ}{mathrm {A}}$ and H$alpha$ 6563 $overset{circ}{mathrm {A}}$) remain weak. Lines for maximally spinning HMBHs behave similarly. Line ratio diagrams for the BELs show that high OVI/H$beta$ and low CIV/H$alpha$ may pick out HMBH, although OVI is often hard to observe. In NEL BPT diagrams HMBHs lie among star-forming regions, except for highly spinning, high accretion rate HMBHs. In summary, the BELs expected from HMBHs would be hard to detect using the current optical facilities. From 100 to $10^{12} M_{odot}$, the emission lines used to detect AGN only have high EW in the $10^6 - 10^9 M_{odot}$ window, where most AGN are found. This selection effect may be distorting reported distributions of $M_{BH}$.
In this contribution we briefly review the reverberation mapping technique and its results for low and intermidiate luminosity AGNs. Then we present a monitoring campaign of high-luminosity high-redshift quasars which will extend these results by two orders of magnitude, probing the broad-line region size and black hole (BH) mass of luminous AGN at redshift ~2-3.
We present the results of high signal-to-noise ratio VLT spectropolarimetry of a representative sample of 25 bright type 1 AGN at z<0.37, of which nine are radio-loud. The sample covers uniformly the 5100 A optical luminosity at $L_{5100}sim 10^{44}-10^{46}$ erg s$^{-1}$, and H$alpha$ width at FWHM$sim 1000-10,000$~ km/s. We derive the continuum and the H$alpha$ polarization amplitude, polarization angle, and angle swing across the line, together with the radio properties. We find the following: 1. The broad line region (BLR) and continuum polarization are both produced by a single scattering medium. 2. The scattering medium is equatorial, and at right angle to the system axis. 3. The scattering medium is located at or just outside the BLR. The continuum polarization and the H$alpha$ polarization angle swing, can both serve as an inclination indicator. The observed line width is found to be affected by inclination, which can lead to an underestimate of the black hole mass by a factor of $sim 5$ for a close-to face-on view. The line width measured in the polarized flux overcomes the inclination bias, and provides a close-to equatorial view of the BLR in all AGN, which allows to reduce the inclination bias in the BLR based black hole mass estimates.
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