Quasars with Periodic Variability

Quasars with Periodic Variability: Capabilities and Limitations of Bayesian Searches for Supermassive Black Hole Binaries in Time-Domain Surveys

Supermassive black hole binaries (SMBHBs) lurk in the centers of galaxies, and they’re pretty difficult to find. But luckily, supermassive black holes aren’t totally invisible! As these hungry monsters accrete the gas and dust around them, they emit some light. As an SMBHB orbits, the emitted light may regularly appear brighter or dimmer due to the black holes’ motion (formally, this is known as periodic Doppler boosting). By tracking the brightness of many galaxies over multiple years as a light curve (see the example animation above), astronomers can find some excellent candidates that could contain binary systems.

However, this can be tricky. The galaxies containing active supermassive black holes (known as active galactic nuclei (AGN) or quasars) tend to vary in brightness all on their own! This type of variability, named a damped random walk, can even look really similar to the effect of a binary orbit. This means we need to think carefully about how to tell these two effects apart.

In this paper, we constructed a method to choose between two models: one which contained only normal variability, and one which had normal variability plus a binary. By simulating thousands of data sets, with different levels of variability and types of binary signals, we tested just how well this model selection could do for multiple real astronomical surveys. First, we tested the Catalina Real-time Transient Survey, which is currently ongoing. Next, we simulated data from the Legacy Survey of Space and Time (LSST), which will soon begin at the Vera C. Rubin Observatory.

We found that our method works great for both surveys, and due to the quality of the data that LSST will produce, we estimated that it will be a great survey for binary searches. Additionally, we found some clear differences in the populations of light curves with and without a periodic binary signal, which should become apparent when LSST begins. Finally, we expect that we can be much more confident in any binary candidates found in LSST that are analyzed with our method, since it rarely falsely identifies a binary with high-quality data. This is great news, since these SMBHBs may emit gravitational waves that could be detectable by pulsar timing arrays, opening another avenue towards multi-messenger science!

Want to learn more? Check out the paper in the Astrophysical Journal!