Speaker:	Ashley Hai Tung Tan, Neelesh Amrutha and Zachary Steyn (Australian National Univ.)
Title:	Three collaborative talks (Please see each title in the abstract)
Date (JST):	Tue, May 12, 2026, 11:00 - 12:00
Place:	Seminar Room A
Abstract:	Ashley Hai Tung Tan Probing Accretion and AGN Diversity with Optical Variability AGN variability provides a key probe of their internal structure and accretion processes as the accretion disc cannot be directly resolved. We characterise the optical variability using the structure function in a sample of ~250 low-luminosity AGN at z<0.1 with 8-year ATLAS light curves. We find that their variability deviates from damped random walk models commonly applied to quasars, with no evidence for a characteristic timescale associated with orbital or thermal processes in standard accretion disc models. These results suggest that variability in low-luminosity AGN is not driven by the same accretion disc processes inferred from high-luminosity quasars. Using colour variability as a diagnostic, we show that the observed behaviour is inconsistent with dust extinction and instead traces changes in accretion activity. Combining these results with X-ray constraints from eROSITA and emission-line diagnostics for a complete z<0.1 AGN sample, we find that broad-line AGN subtypes are primarily driven by differences in activity level rather than orientation. Neelesh Amrutha The Impact of Long-Term Optical Variability on Single-Epoch Black Hole Mass Estimates Almost all masses of supermassive black holes in AGN are estimated using the single-epoch virial method, which assumes that the broad-line region size scales with luminosity via the radius-luminosity relation. I show that the intrinsic scatter in this relation can be reduced by accounting for long-term AGN variability. Using a complete, volume-limited sample of AGN at z<0.1 with two optical spectra taken 20 years apart, we find that large changes in optical luminosity are not accompanied by the expected anti-correlated changes in the width of the broad H-beta line. As a result, single-epoch virial mass estimates can vary by up to 0.5 dex. By using luminosities from regions that are kiloparsecs away from the nucleus, such as the narrow [O III] emission, we obtain more repeatable mass estimates, as light-travel time across these regions averages over short-term variability. Changing-look AGN are extreme examples where virial mass estimates fail. I will also introduce an upcoming optical AGN spectroscopic atlas of ~1000 WiFeS IFU cubes from the ANU 2.3 m telescope, covering a complete southern-sky sample of broad-line AGN at z<0.1, which will be monitored by LSST. Zachary Steyn Continuum Reverberation in Bright Quasars The variable emission from AGN can be used to probe the structure of accretion disks via continuum reverberation mapping. In low-luminosity AGN, the inferred disk sizes from inter-band delays are 3x times larger than expected from standard disk theory, although this size discrepancy may decrease in more luminous AGN. Using NASA/ATLAS as a precursor to LSST, we analysed light curves of 10,000 bright quasars at redshift 0.3–2.5 in the largest reverberation study to date. We test various large-scale pipeline procedures on real data, finding a stacked approached greatly improves delay measurements. Here, we find the size discrepancy persists in our luminous sample, with a complex dependence on rest-frame wavelength that suggests contamination of variable continuum emission by diffuse broad line region processes is widespread. We test delay behaviour against a variety of quasar properties finding longer lags in quasars with: higher Eddington ratios, redder colours, stronger optical Fe II EWs, higher iron ratios (both UV Fe II/Mg II and optical Fe II /H β), C IV broad absorption troughs, and lower C IV blueshift.

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