# Testing the binary hypothesis: pulsar timing constrains on supermassive black hole binary candidates

**Authors:** A. Sesana, Z. Haiman, B. Kocsis, L. Z. Kelley  
**arXiv:** [1703.10611](https://arxiv.org/abs/1703.10611)  
**Date:** 2017-03-30  
**Summary page:** https://www.summarizepaper.com/en/paper/1703.10611v1/

## Key points

- Time domain astronomy has evolved significantly, enhancing our comprehension of the Universe.
- Key observational programs like CRTS and PTF have surveyed millions of celestial objects, facilitating variability studies.
- Identification of potential periodic sources among quasars, with 111 candidates from CRTS and 33 from PTF proposed as supermassive black hole binaries (SMBHBs).
- Calculations of SMBHB merger rates based on identified candidates have implications for gravitational wave background (GWB) models at nano-Hertz frequencies.
- Estimated GWB levels from observed data surpass existing upper limits by a significant margin, challenging current models and theories.
- Possible mechanisms like eccentricity and environmental interactions offer limited explanations for the observed discrepancy in GWB levels.
- Proposed scenarios to reconcile observations with theory include reevaluating SMBHB candidate validity or fundamental binary system parameters.
- Resolving discrepancies between observations and theoretical predictions is crucial for understanding supermassive black hole dynamics and cosmic evolution.

## Summary

The field of time domain astronomy has undergone a significant transformation in recent years, leading to a deeper understanding of the Universe. Key observational programs such as the Catalina Real-time Transient Survey (CRTS) and the Palomar Transient Factory (PTF) have played a crucial role in this revolution by surveying millions of celestial objects over extended periods. These surveys have enabled researchers to conduct variability studies on large statistical samples, shedding light on the dynamic nature of various astronomical phenomena. One particularly intriguing outcome of these surveys is the identification of potentially periodic sources within a vast pool of quasars. In the case of CRTS, approximately 250,000 quasars were scrutinized, resulting in a catalogue of 111 sources that have been proposed as candidates for supermassive black hole binaries (SMBHBs). Similarly, an investigation using PTF data revealed 33 potential SMBHB candidates from a sample size of 33,000 objects. Under the assumption that these candidates indeed represent SMBHB systems, researchers have delved into calculating the implied merger rate of such binary systems. This information has been instrumental in constructing models for the expected gravitational wave background (GWB) at nano-Hertz frequencies, which can be probed using pulsar timing arrays (PTAs). Upon accounting for any potential incompleteness in the data analysis process, it was found that the estimated GWB surpasses existing upper limits set by PTAs by a substantial margin. Specifically, for the CRTS sample, the implied GWB exceeds current constraints by nearly an order of magnitude while for the PTF sample it surpasses limits by at least 2-3 times. This significant deviation from observational constraints poses a challenge to existing models and theories. Various factors such as eccentricity and interactions with surrounding environments have been considered as possible mechanisms to suppress the signal; however, they can only account for a modest reduction in signal strength within the relevant frequency range. This discrepancy between observed GWB levels and theoretical predictions suggests that further investigations are warranted to reconcile these findings. To address this tension between observations and theoretical expectations, several scenarios have been proposed. One possibility is that most or all of the identified SMBHB candidates are false positives. Alternatively, it may be necessary to reconsider fundamental parameters such as black hole masses or mass ratios within binary systems. The resolution of this discrepancy holds profound implications for our understanding of supermassive black hole dynamics and their role in shaping cosmic evolution. Overall, these findings underscore both the promise and challenges associated with probing transient phenomena in astrophysics and highlight the need for continued exploration and refinement in our quest to unravel the mysteries of the Universe's most enigmatic inhabitants - supermassive black holes.