Gravitational waves from binary supermassive black holes missing in pulsar observations (1509.07320v1)

Abstract: Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems will modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrain the characteristic amplitude of this background, $A_{\rm c,yr}$, to be < $1.0\times10^{-15}$ with 95% confidence. This limit excludes predicted ranges for $A_{\rm c,yr}$ from current models with 91-99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments, and that higher-cadence and shorter-wavelength observations would result in an increased sensitivity to gravitational waves.

• The paper establishes an upper limit of Ac,yr < 1.0×10⁻¹⁵ at 1 yr⁻¹ using 11 years of precise pulsar timing data.
• The study challenges existing models by revealing a lower-than-expected gravitational wave background from binary SMBHs and questioning standard merger rate assumptions.
• The research emphasizes the need for refined astrophysical models and enhanced PTA techniques, including future SKA observations, to capture elusive gravitational wave signals.

Analysis of Gravitational Wave Constraints from Pulsar Timing Observations of Supermassive Black Holes

The paper by Shannon et al. focuses on the non-detection of gravitational waves (GWs) from binary supermassive black holes (SMBHs) using pulsar timing array (PTA) observations. The research employs highly precise millisecond pulsar (MSP) data accumulated over 11 years from the Parkes Pulsar Timing Array project, aiming to limit the gravitational wave background (GWB) produced by the combined effect of numerous such binary systems.

Summary of Findings

Observational Constraints:

1. Characteristic Amplitude: The paper establishes an upper limit on the characteristic amplitude $A_{c,yr} < 1.0 \times 10^{-15}$ at a frequency of 1 $\text{yr}^{-1}$ with a 95% confidence level. This bound significantly excludes previously predicted amplitudes derived from galaxy and SMBH evolution models with a 91-99.7% probability.
2. GWB Non-detection: The absence of GWB detection implies the need to reassess some of the assumptions in existing models regarding galaxy merger rates, the formation, and evolution of SMBH binaries, and other GW emission processes.
3. Bounding the Cosmological Impact: Constraints on the GWB limit its fractional contribution to the critical density of the Universe, $\Omega_{\text{GW}} < 2.3 \times 10^{-10}$ at 0.2 $\text{yr}^{-1}$, reflecting a value six times more restrictive than previous studies.

Model Implications:

1. Binary Evolution: The limit suggests alternative mechanisms affecting SMBH evolution, including the environmental coupling of binary SMBHs with stars and gas, which could accelerate orbital decay and reduce the observable GWB amplitude.
2. Model Assumptions Reevaluation: The assumption that all galaxy mergers result in the formation of coalescing binary SMBHs and that such binaries are solely driven into the GW-emission phase by radiative losses is contested by these new constraints. Potential SMBH ejections due to gravitational recoil also require more consideration.
3. Astrophysical Factors: High GWB model predictions failing to align with the observed limits may result from uncertainties in galaxy merger rates, a lower-than-expected fraction of galaxies hosting SMBHs, or very efficient angular momentum loss mechanisms, such as dynamical friction or star-gas interactions.

Implications and Future Directions

The outcomes highlight the importance of refining galaxy evolution models to incorporate more complex dynamics of SMBH interactions with their environments. As current stochastic models predict a stronger GWB signal than observed, research should focus on understanding the exceptional environmental efficiency in binary orbital shrinkage.

Moreover, the paper emphasizes the potential of high-cadence, short-wavelength radio observations using advanced telescopes like the Square Kilometre Array (SKA) to enhance sensitivity to gravitational waves. Future work must explore the possibilities of detecting GW emissions from individual massive binary systems and expanding the sensitivity of PTAs to a broader frequency spectrum.

Conclusion

Shannon et al.'s research provides a crucial observational benchmark that challenges prevailing theoretical models of supermassive black hole evolution in galaxies. By establishing stringent limits on gravitational wave amplitudes, it necessitates a closer examination of SMBH binaries' physical assumptions and environmental interactions. The ongoing advancements in observation techniques and infrastructure promise further insights into the dynamical history of SMBHs and their roles in galactic evolution.