Modeling the uncertainties of solar-system ephemerides for robust gravitational-wave searches with pulsar timing arrays (2001.00595v2)

Abstract: The regularity of pulsar emissions becomes apparent once we reference the pulses' times of arrivals to the inertial rest frame of the solar system. It follows that errors in the determination of Earth's position with respect to the solar-system barycenter can appear as a time-correlated bias in pulsar-timing residual time series, affecting the searches for low-frequency gravitational waves performed with pulsar timing arrays. Indeed, recent array datasets yield different gravitational-wave background upper limits and detection statistics when analyzed with different solar-system ephemerides. Crucially, the ephemerides do not generally provide usable error representations. In this article we describe the motivation, construction, and application of a physical model of solar-system ephemeris uncertainties, which focuses on the degrees of freedom (Jupiter's orbital elements) most relevant to gravitational-wave searches with pulsar timing arrays. This model, BayesEphem, was used to derive ephemeris-robust results in NANOGrav's 11-yr stochastic-background search, and it provides a foundation for future searches by NANOGrav and other consortia. The analysis and simulations reported here suggest that ephemeris modeling reduces the gravitational-wave sensitivity of the 11-yr dataset; and that this degeneracy will vanish with improved ephemerides and with the longer pulsar timing datasets that will become available in the near future.

• The paper demonstrates that uncertainties in solar-system ephemerides, particularly from Jupiter’s orbit, can mimic gravitational-wave signals.
• It employs the B AYES E PHEM model to reconcile discrepancies across different ephemeris datasets, reducing time-correlated biases in pulsar timing.
• Results show that addressing SSE uncertainties slightly reduces current gravitational-wave sensitivity, with improvements expected from enhanced data precision.

Modeling Uncertainties in Solar-System Ephemerides for Pulsar Timing Arrays

The paper, titled "Modeling the Uncertainties of Solar-System Ephemerides for Robust Gravitational-Wave Searches with Pulsar Timing Arrays," addresses a critical issue in the search for low-frequency gravitational waves using pulsar timing arrays (PTAs). The authors focus on the uncertainties inherent in solar-system ephemerides (SSEs), which have significant implications for the detection of gravitational waves.

Pulsar Timing Arrays have been employed to detect gravitational waves by leveraging the consistent emissions of millisecond pulsars. These emissions are analyzed by referencing the times of arrival (TOA) to the inertial rest frame of the solar system. However, errors in pinpointing Earth’s position concerning the solar-system barycenter can introduce time-correlated biases in pulsar-timing residuals. Such biases impact the gravitational-wave background upper limits and detection statistics derived from PTA data.

The paper introduces a model termed B AYES E PHEM, which aims to reconcile discrepancies that arise when different SSEs are employed in gravitational-wave searches. The authors highlight that Jupiter’s orbital parameters significantly influence the search for gravitational waves due to their impact on the SSE accuracy, making Jupiter's orbit a primary focus of their uncertainty model.

Key findings from the analysis include:

• Impact of Jupiter’s Orbit: The paper demonstrates that the position and trajectory errors associated with Jupiter's orbit lead to notable discrepancies in the calculated gravitational-wave amplitudes. These discrepancies can be large enough to simulate gravitational-wave signals, as the errors share similar periodic characteristics with the targeted gravitational-wave signal.
• Bridging SSE Discrepancies: Through the implementation of B AYES E PHEM, the authors addressed the SSE-related biases, achieving consistency across various SSEs, including DE421, DE430, DE435, DE436, and DE438.
• SSE Uncertainties vs. Gravitational-Wave Sensitivity: It is notable that addressing SSE uncertainties reduces the sensitivity of gravitational-wave detection within the 11-year dataset. However, this reduction in sensitivity will diminish as improved ephemerides and extended pulsar datasets become available.

The implications of this work are profound for future gravitational-wave detection. Accurate modeling of SSE uncertainties is paramount to ensuring the reliability of gravitational-wave searches. As PTAs continue to evolve with longer datasets and more sophisticated techniques, these models will facilitate more robust detections by mitigating ephemeris-induced errors.

The authors acknowledge that the abundance and precision of PTA data necessitate sophisticated modeling techniques to control TOA systematics rigorously. They note that the implementation of B AYES E PHEM offers a methodological framework for future PTA-based gravitational-wave searches, emphasizing that the priority should be on refining estimates of planetary orbits, notably Jupiter’s, which limits current gravitational-wave search sensitivities.

As PTAs capture increasingly larger datasets and achieve greater precision, we expect that B AYES E PHEM will play a pivotal role in ensuring accurate gravitational-wave detection and characterization. Furthermore, collaboration with solar-system dynamicists could enhance ephemeris models, providing clearer gravitational-wave signals unmarred by systematics of celestial mechanics.