Recently, a research team led by Professor Zhen-Ya Zheng from the Early Universe and High-Redshift Galaxy Group at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS), in collaboration with researchers from the University of Science and Technology of China, Sun Yat-sen University, and the Yunnan Astronomical Observatory of CAS, has achieved significant progress in the variability study of Quasars. From the million quasars catalog released by Professor Xuebing Wu's team at Peking University^[1], the research team conducted multiband "tracking" observations of four quasars exhibiting high variability in the near-infrared (Ks-band) as detected by the VVV/VVVX time-domain survey of the Visible and Infrared Survey Telescope for Astronomy (VISTA). They successfully obtained the variability information of these quasars from optical to mid-infrared (MIR) wavelengths. This multiband joint analysis provides an opportunity to probe the dust structure in the quasar's central region and holds promise for revealing key properties such as its scale and distribution. Consequently, it offers crucial observational evidence for refining the "unified model" of AGNs. The results have been published in the internationally renowned academic journal The Astrophysical Journal.

Quasars are extremely luminous active galactic nuclei (AGNs) in the universe, characterized by their bright point-like morphology and significant brightness variability. This variability enables scientists to apply reverberation mapping (RM) techniques to trace the propagation of central radiation, thereby revealing the material structure in the vicinity of black holes. Theoretically, combining reverberation mapping observations across optical, near-infrared (NIR), and MIR bands can provide a more comprehensive perspective for such studies.

Building on the prerequisite of NIR observational data from the VVV/VVVX time-domain survey, this study conducted time-delay analyses for three highly variable quasars across optical (Pan-STARRS, ZTF), NIR (VISTA-VVV/VVVX), and MIR (WISE) bands (Figure 1). Based on the measured time delays and the typical sublimation temperatures of dust particles, the researchers further applied theoretical formulas for the sublimation radii of graphite and silicate grains ^{[2, 3]} to estimate that the relative size ratio of graphite to silicate particles in the dust torus is approximately 0.4. This result indicates that within the dust torus, smaller graphite particles dominate the radiation in the NIR, while larger silicate particles primarily contribute to the MIR radiation.

Figure 1. Light curves (left) and ICCF results (right) of the target sources VVV J1834-2925 (top), VVV J1833-2731 (middle), and VVV J1845-2426 (bottom). The CCF analysis results in the top-right corner correspond to optical-NIR and NIR-MIR correlations, respectively. All other CCFs represent optical-NIR and optical-MIR correlations. In the right panels, the orange lines indicate the CCCD, and the CCF results are shown in black.

The research team conducted a detailed analysis of the brightest quasar in the sample, VVV J1845-2426. By combining spectroscopic data obtained from the Lijiang 2.4-meter optical telescope (Figure 2), the researchers measured the quasar's redshift to be approximately z = 0.25 and estimated the mass of its central black hole to be log₁₀(M_BH/M_sun)=8.6±0.1. Through further multi-band spectral energy distribution (SED) fitting (Figure 3), the team also identified indications of the presence of polar dust in this source.

Figure 2: Spectral fitting results for VVV J1845-2426 (Lijiang 2.4m optical telescope). In the top panel, the spectrum (black line) is decomposed into a power-law continuum (orange line), Fe II emission (cyan line), and emission lines (blue line). The emission lines include broad (red line) and narrow (green line) components. The middle panel shows the residuals between the spectrum and the fitted model. The bottom panel presents the fitting results for the H_α 、H_βand H_γ emission lines, respectively.

Figure 3. SED fitting results for VVV J1845-2426. The fitting was performed using AGN templates^[4] (intrinsic template: black solid line; warm-dust-deficient template^[4]: black dotted line; hot-dust-deficient template^[4]: black dashed line) and a polar emission component^[5] (green line). The best-fit model (red line) is a combination of the warm-dust-deficient AGN template and the polar dust component.

Lin Long, the first author of the paper and a PhD candidate, stated: "By analyzing the multi-band joint variability of AGNs, this study successfully captured the 'echo' of dust responding to the central radiation across different bands and estimated the size ratio of the two main components in the dust torus. This result is significant for understanding the distribution and physical properties of dust around AGNs."

Corresponding author Professor Zhen-Ya Zheng further noted: "Despite the rapid development of time-domain astronomy, comprehensive studies covering optical, NIR, and MIR bands together remain relatively scarce, leading to a limited sample size. To overcome this limitation, it is necessary to systematically integrate multiband observational data to identify more AGNs that exhibit strong variability across multiple bands, thereby significantly expanding the sample base for reverberation mapping research. It is encouraging to anticipate that with the Vera C. Rubin Observatory and the SPHEREx space telescope commencing scientific operations, they will bring new observational opportunities for dust reverberation mapping studies of AGNs."

This work is supported by the National Key R&D Program of China, the China-Chile Joint Research Fund, the China Manned Space Program, and other projects.

Paper link: https://doi.org/10.3847/1538-4357/ae1b95

Reference:

Fu, Y., X.-B. Wu, Y. Li, et al., "CatNorth: An Improved Gaia DR3 Quasar Candidate Catalog with Pan-STARRS1 and CatWISE", The Astrophysical Journal Supplement Series, 271, 54 (2024).

Barvainis, R., "Hot Dust and the Near-Infrared Bump in the Continuum Spectra of Quasars and Active Galactic Nuclei", The Astrophysical Journal, 320, 537 (1987).

Netzer, H., "Revisiting the Unified Model of Active Galactic Nuclei", Annual Review of Astronomy and Astrophysics, 53, 365-408 (2015).

Lyu, J., Rieke, G. H., "The Intrinsic Far-infrared Continua of Type-1 Quasars", The Astrophysical Journal, 841, 76 (2017).

Lyu, J., Rieke, G. H., "Polar Dust, Nuclear Obscuration, and IR SED Diversity in Type-1 AGNs", The Astrophysical Journal, 866, 92 (2018).

Scientific Contacts:
Lin Long, SHAO, CAS, longlin@shao.ac.cn

Zhen-Ya Zheng, SHAO, CAS, zhengzy@shao.ac.cn