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Quasars are bright and unobscured active galactic nuclei (AGN) thought to be powered by the accretion of matter around supermassive black holes at the centers of galaxies. The temporal variability of a quasar’s brightness contains valuable information about its physical properties. The UV/optical variability is thought to be a stochastic process, often represented as a damped random walk described by a stochastic differential equation (SDE). Upcoming wide-field telescopes such as the Rubin Observatory Legacy Survey of Space and Time (LSST) are expected to observe tens of millions of AGN in multiple filters over a ten year period, so there is a need for efficient and automated modeling techniques that can handle the large volume of data. Latent SDEs are machine learning models well suited for modeling quasar variability, as they can explicitly capture the underlying stochastic dynamics. In this work, we adapt latent SDEs to jointly reconstruct multivariate quasar light curves and infer their physical properties such as the black hole mass, inclination angle, and temperature slope. Our model is trained on realistic simulations of LSST ten year quasar light curves, and we demonstrate its ability to reconstruct quasar light curves even in the presence of long seasonal gaps and irregular sampling across different bands, outperforming a multioutput Gaussian process regression baseline. Our method has the potential to provide a deeper understanding of the physical properties of quasars and is applicable to a wide range of other multivariate time series with missing data and irregular sampling.

The accretion disks of active galactic nuclei (AGNs) are promising locations for the merger of compact objects detected by gravitational wave (GW) observatories. Embedded within a baryon-rich, high-density environment, mergers within AGNs are the only GW channel where an electromagnetic (EM) counterpart must occur (whether detectable or not). Considering AGNs with unusual flaring activity observed by the Zwicky Transient Facility (ZTF), we describe a search for candidate EM counterparts to binary black hole (BBH) mergers detected by LIGO/Virgo in O3. After removing probable false positives, we find nine candidate counterparts to BBH mergers during O3 (seven in O3a, two in O3b) with a p -value of 0.0019. Based on ZTF sky coverage, AGN geometry, and merger geometry, we expect ≈3( N _BBH /83)( f _AGN /0.5) potentially detectable EM counterparts from O3, where N _BBH is the total number of observed BBH mergers and f _AGN is the fraction originating in AGNs. Further modeling of breakout and flaring phenomena in AGN disks is required to reduce our false-positive rate. Two of the events are also associated with mergers with total masses >100 M _⊙ , which is the expected rate for O3 if hierarchical (large-mass) mergers occur in the AGN channel. Candidate EM counterparts in future GW observing runs can be better constrained by coverage of the Southern sky as well as spectral monitoring of unusual AGN flaring events in LIGO/Virgo alert volumes. A future set of reliable AGN EM counterparts to BBH mergers will yield an independent means of measuring cosmic expansion ( H _0 ) as a function of redshift.

The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate toward regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes in an example of a model AGN disk, using an augmented N-body code that includes analytic approximations to migration torques, stochastic gravitational forces exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in our model disk as stellar-mass black holes migrate toward the migration trap. These sBHBs are likely to subsequently merge on short timescales. The process continues, leading to the build-up of a population of over-massive stellar-mass black holes. The formation of sBHBs in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO. [ABSTRACT FROM AUTHOR]

Black hole (BH) mergers detectable with the Laser Interferometer Gravitational-wave Observatory (LIGO) can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum, and the spin spectrum of BHs in AGN disks. The predicted merger rate spans ∼10−3–104 Gpc−1 yr−1, so upper limits from LIGO (<212 Gpc−1 yr−1) already constrain it. The predicted mass spectrum has the form of a broken power law, consisting of a pre-existing BH power-law mass spectrum and a harder power-law mass spectrum resulting from mergers. The predicted spin spectrum is multipeaked with the evolution of retrograde spin BHs in the gas disk playing a key role. We outline the large uncertainties in each of these LIGO observables for this channel and we discuss ways in which they can be constrained in the future. [ABSTRACT FROM AUTHOR]