Searching for Patterns from Feeding Supermassive Black Holes


Our universe is filled with distant supermassive black holes that feed on surrounding gas and dust, emitting bright gamma-ray radiation. A new study explores how many of these show periodic patterns in the variations of their high-energy light.

Putting on a Show

At the center of every galaxy lies a supermassive black hole of millions to billions of solar masses. Many of these — like the Milky Way’s own Sagittarius A* — are quiet, largely invisible lurkers. But some are considerably more attention-seeking, actively accreting material, spewing out winds and jets, and emitting radiation that spans the electromagnetic spectrum.

These active galactic nuclei (AGNs) show variability in their light curves on many different timescales — and a few have been caught flickering in a regular pattern.

Why the Patterns?

SMBH binary

Artist’s impression of supermassive black holes that have formed a binary as they’ve sunk to the center of their merged galaxies. [NAOJ]

There are lots of potential explanations for periodic variability in an AGN’s light curve. The black hole may host powerful jets that precess about an axis, driving a lighthouse effect in their emission. The flow of accreting material onto the black hole might wax and wane periodically. And in some cases, the emission we observe might even be coming from a pair of black holes: a supermassive black hole binary whose orbital motion causes periodic patterns.

To better understand the nature of these distant, dramatic sources, we need to study the AGN that show regular variability. But, though a number of studies have explored individual variable AGN, we haven’t yet conducted a large, systematic search for periodic patterns in high-energy AGN light curves. That is, until now.

A Sample of Regular Feeders

A team of scientists led by Pablo Peñil (Complutense University of Madrid, Spain) has leveraged a powerful tool: the Fermi Large Area Telescope (LAT), a space-based gamma-ray detector with a wide field of view that has been scanning the sky consistently for more than a decade. In that time, Fermi LAT has tracked the gamma-ray emission from thousands of AGN, providing Peñil and collaborators with a wealth of data they use to systematically hunt for patterns.

periodic AGN sky map

Sky map showing the locations of the 11 sources with periodic emission (filled symbols) and 13 sources with lower significance (open symbols). Click to enlarge. [Peñil et al. 2020]

By applying ten different algorithms that identify periodicity, the authors discover 11 AGN that robustly show patterns in their variability that repeat on roughly 1- to 5-year timescales, as well as another 13 AGN that show strong hints of periodicity but can’t be confirmed yet with the data we have. The authors’ 11 robust detections include two AGN with previously detected variability and nine new identifications.

Peñil and collaborators’ non-detections are also interesting. There were a number of AGN that, in previous individual studies, were reported to show periodic variability — yet they did not show statistically significant periodicity in the authors’ systematic analysis.

What’s Ahead

So now that we’ve identified periodically variable AGN, what can we do with them? The authors’ consistently identified sample can now be studied to better understand what causes the variability of these sources. We can also use the predictable patterns of these AGNs’ high-energy emission to efficiently schedule follow-up observations with telescopes that have more limited fields of view and observing duty cycles.

And this is just the start! Another few years of observation with Fermi LAT should greatly expand the sample of AGN identified with periodic variability, providing even more insight into the feeding behavior of these distant monsters.


“Systematic Search for γ-Ray Periodicity in Active Galactic Nuclei Detected by the Fermi Large Area Telescope,” P. Peñil et al 2020 ApJ 896 134. doi:10.3847/1538-4357/ab910d