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Active galactic nuclei do not exhibit strictly sinusoidal brightness variations

Kareem El-Badry, David W. Hogg, Hans-Walter Rix

TL;DR

This study tests whether Gaia DR3-identified strictly sinusoidal AGN variability signals truly indicate close SMBH binaries by extending the time baseline with ZTF and CRTS data. Across 181 Gaia candidates, all 116 with ZTF data fail to maintain the Gaia-predicted sinusoidal pattern, with most extended-baseline light curves displaying stochastic red-noise behavior rather than coherence. Quantitatively, strictly sinusoidal variability on 100–1000 day scales appears exceedingly rare, at most a few in $10^{6}$ AGN, suggesting that most short-period binaries do not produce simple sinusoidal light curves in the Gaia band. The authors estimate that the Gaia sample should host roughly 23–45 binaries with $P_{ m orb} \lesssim 600$ d and $q \gtrsim 0.1$, but the lack of coherent sinusoidal signals implies that identifying binaries will require stochastic or quasi-periodic modeling rather than pure sinusoidal templates. These results underscore the necessity of extended baselines and multi-survey validation in the search for SMBH binaries and motivate moving beyond simple sinusoidal variability toward more nuanced, stochastic approaches.

Abstract

Periodic variability in active galactic nuclei (AGN) light curves has been proposed as a signature of close supermassive black hole (SMBH) binaries. Recently, 181 candidate SMBH binaries were identified in Gaia DR3 based on apparently stable sinusoidal variability in their $\sim$1000-day light curves. By supplementing Gaia photometry with longer-baseline light curves from the Zwicky Transient Facility (ZTF) and the Catalina Real Time Transient Survey (CRTS), we test whether the reported periodic signals persist beyond the Gaia DR3 time window. We find that in all 116 cases with available ZTF data, the Gaia-inferred periodic model fails to predict subsequent variability, which appears stochastic rather than periodic. The periodic candidates thus overwhelmingly appear to be false positives; red noise contamination appears to be the primary source of false detections. We conclude that truly periodic and sinusoidal AGN variability is exceedingly rare, with at most a few in $10^6$ AGN exhibiting it on 100 to 1000 day timescales. Models predict that the Gaia AGN light curve sample should contain dozens of true SMBH binaries with periods within the observational baseline, so the lack of strictly periodic light curves in the sample suggests that most short-period binary AGN do not have light curves dominated by simple sinusoidal periodicity.

Active galactic nuclei do not exhibit strictly sinusoidal brightness variations

TL;DR

This study tests whether Gaia DR3-identified strictly sinusoidal AGN variability signals truly indicate close SMBH binaries by extending the time baseline with ZTF and CRTS data. Across 181 Gaia candidates, all 116 with ZTF data fail to maintain the Gaia-predicted sinusoidal pattern, with most extended-baseline light curves displaying stochastic red-noise behavior rather than coherence. Quantitatively, strictly sinusoidal variability on 100–1000 day scales appears exceedingly rare, at most a few in AGN, suggesting that most short-period binaries do not produce simple sinusoidal light curves in the Gaia band. The authors estimate that the Gaia sample should host roughly 23–45 binaries with d and , but the lack of coherent sinusoidal signals implies that identifying binaries will require stochastic or quasi-periodic modeling rather than pure sinusoidal templates. These results underscore the necessity of extended baselines and multi-survey validation in the search for SMBH binaries and motivate moving beyond simple sinusoidal variability toward more nuanced, stochastic approaches.

Abstract

Periodic variability in active galactic nuclei (AGN) light curves has been proposed as a signature of close supermassive black hole (SMBH) binaries. Recently, 181 candidate SMBH binaries were identified in Gaia DR3 based on apparently stable sinusoidal variability in their 1000-day light curves. By supplementing Gaia photometry with longer-baseline light curves from the Zwicky Transient Facility (ZTF) and the Catalina Real Time Transient Survey (CRTS), we test whether the reported periodic signals persist beyond the Gaia DR3 time window. We find that in all 116 cases with available ZTF data, the Gaia-inferred periodic model fails to predict subsequent variability, which appears stochastic rather than periodic. The periodic candidates thus overwhelmingly appear to be false positives; red noise contamination appears to be the primary source of false detections. We conclude that truly periodic and sinusoidal AGN variability is exceedingly rare, with at most a few in AGN exhibiting it on 100 to 1000 day timescales. Models predict that the Gaia AGN light curve sample should contain dozens of true SMBH binaries with periods within the observational baseline, so the lack of strictly periodic light curves in the sample suggests that most short-period binary AGN do not have light curves dominated by simple sinusoidal periodicity.

Paper Structure

This paper contains 11 sections, 2 equations, 22 figures.

Table of Contents

  1. Introduction
  2. Light curves
  3. Gaia
  4. ZTF
  5. CRTS
  6. Analysis
  7. Consistency of the periods
  8. Summary and Discussion
  9. Limits on strictly sinusoidal variability
  10. How many binary SMBHs are in the Gaia sample?
  11. All the light curves

Figures (22)

  • Figure 1: Long-term light curve of the source Gaia DR3 3870752104863883264, which Huijse2025 report as periodic with $P=520$ d and a Bayes factor $\log(B_{PR})= 3.32$, indicating strong preference of periodic variability over a damped random walk. Top panel shows the Gaia DR3 $G-$band light curve and posterior samples from a sinusoidal fit to it. Bottom panel combines this light curve and the best-fit sinusoid with long-term photometry from ZTF ($g-$ and $r-$ bands) and CRTS (unfiltered). Neither of these datasets is consistent with the sinusoidal model.
  • Figure 2: Best-fit periods measured from Gaia and ZTF light curves for all 116 sources with ZTF data. Errorbars show the formal uncertainties when fitting light curves with a sinusoid. The cutoff at $P_{\rm Gaia} \lesssim 700$ d is a consequence of cuts on the input sample. There is no correlation between the Gaia period and the dominant period measured in the ZTF light curves for the same objects five years later. Three (six) sources have best-fit periods that are consistent across the two surveys within one (two) sigma.
  • Figure 3: Light curves of 116 sources. Black points show Gaia DR3; red and green points show binned ZTF $r-$ and $g-$band data. Blue line shows the best-fit sinusoid, with the period fixed to the value inferred by Huijse2025. That period and the Gaia DR3 source ID are listed in each panel. In all cases, the sinsuidal model fails to predict the ZTF data.
  • Figure 4: Figure 3 (continued)
  • Figure 5: Figure 3 (continued)
  • ...and 17 more figures