The NANOGrav Nine-year Data Set: Observations, Arrival Time Measurements, and Analysis of 37 Millisecond Pulsars (1505.07540v2)

Abstract: We present high-precision timing observations spanning up to nine years for 37 millisecond pulsars monitored with the Green Bank and Arecibo radio telescopes as part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project. We describe the observational and instrumental setups used to collect the data, and methodology applied for calculating pulse times of arrival; these include novel methods for measuring instrumental offsets and characterizing low signal-to-noise ratio timing results. The time of arrival data are fit to a physical timing model for each source, including terms that characterize time-variable dispersion measure and frequency-dependent pulse shape evolution. In conjunction with the timing model fit, we have performed a Bayesian analysis of a parameterized timing noise model for each source, and detect evidence for excess low-frequency, or "red," timing noise in 10 of the pulsars. For 5 of these cases this is likely due to interstellar medium propagation effects rather than intrisic spin variations. Subsequent papers in this series will present further analysis of this data set aimed at detecting or limiting the presence of nanohertz-frequency gravitational wave signals.

Overview of NANOGrav Nine-year Data Set: Observations, Arrival Time Measurements, and Analysis of 37 Millisecond Pulsars

The paper details the comprehensive analysis undertaken by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. It presents high-precision timing observations spanning nine years for 37 millisecond pulsars, with data collected using the Green Bank Telescope and the Arecibo Observatory. The primary objective of this paper is to contribute towards the detection of gravitational waves, particularly those in the nanohertz frequency range, through pulsar timing arrays (PTAs).

Methodology

The methodology section elaborates on the observational setup and techniques employed for this extensive effort. Key methods include:

1. Observational Data Collection: Utilizing various instrumentation setups, including ASP/GASP and PUPPI/GUPPI systems, to record pulsar data at different frequencies, enabling thorough coverage across the nanohertz spectrum.
2. Timing Data and Models: Arrival times were extracted using Fourier-domain techniques, and the data were fit against physical timing models. These models account for various phenomena such as time-variable dispersion measures and frequency-dependent pulse shape evolution.
3. Bayesian Analysis of Timing Noise: A sophisticated Bayesian approach was taken to model timing noise, identifying and characterizing excess low-frequency red noise in 10 of the pulsars. This analysis considered both intrinsic pulsar spin variations and interstellar medium propagation effects.
4. Offset and Noise Characterization: Instrumental offsets were precisely measured using novel cross-correlation methods, further refined by calculations involving signal path determination. Additionally, a parameterized noise model accounting for radiometer noise, pulse jitter, and long-term timing noise was developed.

Results

The paper achieves several significant outcomes:

• Detection of Red Noise: Evidence was found for red noise in 10 pulsars, with 5 cases attributable to interstellar medium effects rather than intrinsic pulsar properties. This finding is crucial for filtering potential sources that could emulate gravitational waves.
• Data Set Calibration: The paper meticulously calibrates the data over nine years and outlines methods for managing frequency-dependent profile evolution and timing offsets due to instrumental changes.
• Implications for Gravitational Wave Detection: The extensive nine-year dataset enhances the sensitivity of PTAs towards gravitational wave detection, aiding in characterizing and constraining gravitational wave backgrounds from supermassive black hole binaries, among other sources.

Implications and Future Work

By expanding the dataset and developing robust noise models, this paper sets the stage for subsequent analyses focused on direct gravitational wave detection and other astrophysical explorations such as pulsar binary system dynamics and interstellar medium properties. It highlights how pulsar timing can refine gravitational wave measurements, offering a complementary approach to other observational modalities in the astrophysical community.

NANOGrav's ongoing work promises to widen the array of detectable gravitational phenomena, thereby expanding our understanding of the universe. Expected future developments include enhanced sensitivity through continually improved instrumentation and extended time spans, contributing progressively more detailed assessments of gravitational wave physics.

In sum, this paper serves as a cornerstone contribution to gravitational wave research, fostering advancements in pulsar timing techniques and presenting clear pathways for future scientific inquiry within the field of gravitational astronomy.