Session X10: Pulsar Timing

Live

The North American Observatory for Nanohertz Gravitational Waves (NANOGrav) collaboration is fifteen years into a program of long-term, high-precision millisecond pulsar timing. Our goals are to detect and characterize nanohertz gravitational waves (i.e., periods of many years) by measuring their effect on observed pulse arrival times. We presently observe 79 pulsars at least once a month using Arecibo Observatory, the Green Bank Telescope, and the VLA. In addition, daily observations of these pulsars have recently begun with CHIME. We target pulsars with timing precision of 1 $\mu$s or better, and we achieve precision better than 100 ns in the best cases. Observing a large number of pulsars will allow robust measurement of gravitational waves via correlations in the timing of pairs of pulsars depending on their separation on the sky. We pool data from telescopes worldwide via the International Pulsar Timing Array (IPTA) collaboration, further increasing our sensitivity. We will summarize the observing program and data releases. We will describe new timing wideband techniques that will allow for increased efficiency in gravitational wave searches. We will report on synergistic results from our data set, including new measurements of a massive neutron star.   

Live

Supermassive black hole binaries form in galaxy mergers and emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays.~We have searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GWs~from such systems. We placed limits on the GW strain, and used these limits to constrain the properties of supermassive black hole binaries~in the local Universe. We also used our results to constrain the merger history of nearby massive galaxies.   

Live

We develop a new method for characterizing the Bayesian response of a pulsar timing array (PTA) detector to the presence of a stochastic gravitational wave background (SGWB) in the presence of real, potentially unmodeled noise processes. This method involves the injection of a range of SGWB amplitudes into the PTA dataset and recovery of these signals through the Bayesian detection pipeline. Applying this method to the NANOGrav 11-yr dataset, we find that while this dataset would have made a conclusive detection (Bayes' factor > 100) of a common red process at the published GW-strain amplitude upper limit of 1.45e-15, it would have begun to see hints of the SGWB in the common red process (Bayes' factors > 20) at GW-strain amplitudes greater than ~9e-16. We also quantify how the parameter estimation depends on the significance of the SGWB signal in the dataset.   

Live

Pulsar timing arrays are galactic-scale low-frequency gravitational wave observatories sensitive to the nanohertz frequency band. The primary source of gravitational radiation in this regime is expected to be a stochastic background, formed from the cosmic population of supermassive black hole binaries. In this talk, I will discuss the current state-of-the-art detection approaches to searching for a gravitational wave background in pulsar timing data and present the results obtained by analyzing the 12.5-year data release from the North American Observatory for Gravitational Waves (NANOGrav). Additionally, I will discuss some advanced noise modeling techniques which have improved our sensitivity.   

Live

After a burst of gravitational waves (GWs) passes through an assemblage of free-falling masses, their relative positions are permanently altered by a phenomenon commonly called "memory". Memory is of theoretical interest since it is intimately related to the asymptotic symmetries of General Relativity and is of observational interest since it is feasibly detectable by both ground-based GW detectors like LIGO and pulsar timing arrays like NANOGrav. It has been shown that large-scale, high-precision astrometric surveys such as Gaia can have comparable sensitivity to the nanohertz frequency GWs that pulsar timing arrays are working to detect and characterize. Motivated by the possibility of GW detection through precision astrometry, I will describe the pattern of astrometric deflections and proper motions caused by a GW burst with memory. These memory-induced deflections will cause weak mixing of power between multipole moments in the temperature fluctuations of the cosmic microwave background. Since memory-induced deflections persist indefinitely after a GW burst has passed over the Earth, the effect of many GW bursts with memory can accumulate and possibly grow over cosmologically-long time scales.   

Live

Massive black hole (BH) binaries are one of the most promising gravitational wave (GW) sources for pulsar timing arrays (PTAs) and the Laser Interferometer Space Antenna (LISA). However, much is still not known about the BH population; in particular, the rates of BH binary inspiral and merger are highly uncertain. We describe our recent work to address these pressing issues in advance of LISA, using hydrodynamics simulations to characterize the EM and GW signatures of BH binary inspiral, spin evolution, and GW recoil. Nuclear obscuration during galaxy mergers likely plays a significant role in the elusive nature of BH pairs; we demonstrate that a high fraction of infrared-selected merging galaxies should contain BH pairs resolvable with JWST or with future X-ray imaging. Detections of such systems in the coming years will provide important constraints on GW source modeling in advance of low-frequency GW detections with PTAs and LISA. Finally, we discuss the prospects for identifying candidate recoiling BHs; such objects would provide another EM signature of BH mergers and constrain LISA event rate predictions.   

Live

Gravitational waves emitted during a binary black hole coalescence offer direct insights into the mass and spins of the black holes. The inaugural decade of gravitational wave astronomy will provide such mass-spin constraint for thousands of black holes, providing a unique opportunity to probe a wide range of astrophysical processes. Here we report a new phenomenological fit that captures a fundamental trend between the effective spins and total-mass of the binaries. We discuss its implication for the heaviest binary black holes observed so far.   

On Demand

The Christodoulou memory, which is a nonlinear memory effect sourced by the oscillatory component of the gravitational wave stress tensor, produces a growing, nonoscillatory change in the gravitational wave strain. This results in the permanent displacement of a pair of freely falling test masses after the gravitational wave has passed. Merger of supermassive black holes is a powerful source of gravitational wave with Chritodoulou memory effect. This effect can be potentially seen in pulsar timing observations. Initiatives to observe Burst with memory signal have assumed the signal to be a step function of a given amplitude, which is valid in situations like supernova explosion. But events like merger of supermassive black holes happens on the time scale of few days depending upon the mass. We investigate how the effect of Spin changes the timing residuals over a two weeks period of interval. We find that when the burst with memory( BWM) signals are not assumed to be a step function and replaced with the actual memory growth functions can significantly chage the expected results. Our results shows that spin contribution plays an important role while computing the residuals.