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Abstract The broad emission lines (BELs) emitted by active galactic nuclei respond to variations in the ionizing continuum emission from the accretion disk surrounding the central supermassive black hole (SMBH). This reverberation response provides insights into the structure and dynamics of the broad-line region (BLR). In 2024, we introduced a new forward-modeling tool, the Broad Emission Line Mapping Code (BELMAC), which simulates the velocity-resolved reverberation response of the BLR to an input light curve. In this work, we describe a new version of BELMAC, which uses photoionization models to calculate the cloud luminosities for selected BELs. We investigated the reverberation responses of Hα, Hβ, MgIIλ2800, and CIVλ1550 for models representing a disk-like BLR with Keplerian rotation, radiatively driven outflows, and inflows. The line responses generally provide a good indication of the respective luminosity-weighted radii. However, there are situations where the BLR exhibits a negative response to the driving continuum, causing overestimates of the luminosity-weighted radius. The virial mass derived from the models can differ dramatically from the actual SMBH mass, depending mainly on the disk inclination and velocity field. In single-zone models, the BELs exhibit similar responses and profile shapes; two-zone models, such as a Keplerian disk and a biconical outflow, can reproduce observed differences between high- and low-ionization lines. Radial flows produce asymmetric line profile shapes due to both anisotropic cloud emission and electron scattering in an intercloud medium. These competing attenuation effects complicate the interpretation of profile asymmetries.
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A New Diagnostic Method for Distinguishing Binary from Single Supermassive Black Holes Using Broad Emission Lines
During cosmic timescales, supermassive binary black holes (SMBBHs) form by galaxy merg- ers, where each galaxy hosts a supermassive black hole (SMBH) at its center. By studying SMBBHs, we can gain insights into galaxy evolution and black hole growth. However, the typical separation between black holes in SMBBHs is usually below 1 pc, making them dif- ficult to resolve using direct imaging or photometry. To be able to distinguish binary black holes (BBHs) from typical AGN powered by single black holes (SBHs), we conducted this research project to develop a new diagnostic method to identify a unique feature of SMBBHs, which can be used to distinguish AGN powered by BBHs from those powered by SBHs. The basic idea of this method is that BBHs have different configurations compared with those of SBHs, such as the circumbinary disk enveloping the whole binary system, the mini- disks around each black hole, and the streams between the circumbinary disk and minidisks. It is these different configurations of accretion disks of black holes that lead to the difference in the spectral energy distributions (SEDs), which in turn will differently photoionize the broad line region (BLR) and produce different strengths of broad emission lines. Thus, it is these differences in strengths of broad emission lines that we expect to see between two different configurations of black holes, either binary or single black holes. By identifying these differences in line strengths, we can distinguish between binary and single black holes. In order to achieve this goal of distinguishing BBHs from SBHs, we organized our project in the steps below: (1) obtained BBH SEDs from Gutiérrez et al., 2022, (2) generated SBH SEDs using XSPEC modeling code OPTXAGNF, (3) produced single-cloud models, which represent the BLR, input BBH and SBH SEDs into the models, and simulated the single0cloud response with a photoionization code Cloudy, (4) built cloud-ensemble models, which represent a more realistic BLR, input BBH and SBH SEDs into the models, and simulated the emission-line response within those clouds using a broad emission line mapping code BELMAC. In step (3), we have explored the differences in line ratios between BBH and SBH for a representative single-cloud photoionization model. The emission lines we used here are: Si IV λ1400Å, C III] λ1909Å, C IV λ1549Å, Mg II λ2798Å, and Lyα λ1216Å. It turned i out that differences do exist, but they are too small to be identified in observational data. Furthermore, we have investigated the line equivalent widths predicted by SBH and BBH models respectively. By doing so, we found some apparent differences between BBHs and SBHs in some specific emission lines: Lyα, CIV, and Hα. However, these differences vanish at the highest mass of black holes (109M⊙). In step (4), we continued the investigation of the equivalent width between SBH and BBH BLR cloud-ensemble models and found that some emission lines show the difference between BBHs and SBHs, such as CIV in the case of BH mass 107M⊙, U ∝ r−2, and log n/1 cm−3 = 10.5, 11.0. For the highest mass in the case of U ∝ r−2, the results are similar to the one in single-cloud models: no difference is shown between BBHs and SBHs across all emission lines. Most importantly, in this step, we found that for CIV λ1549 Å in the case of a black hole mass of 107M⊙, s = −2, and log n/1 cm−3 = 10.5, only the SBH EW falls inside the range of the observed range in SDSS DR7 Quasar Catalog while the BBH EW falls outside the range and becomes an outlier. This is what we want to find to distinguish BBHs from SBHs in the observational data.
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- Award ID(s):
- 2009508
- PAR ID:
- 10644402
- Publisher / Repository:
- Rochester Institute of Technology
- Date Published:
- Format(s):
- Medium: X
- Institution:
- Rochester Institute of Technology
- Sponsoring Org:
- National Science Foundation
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