Showing papers by "Christopher J. Russell published in 2023"

Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar Timing Array

Abstract: Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 yr. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum hc=A(f/1yr−1)α , we measure A=3.1−0.9+1.3×10−15 and α = −0.45 ± 0.20, respectively (median and 68% credible interval). For a spectral index of α = −2/3, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of A=2.04−0.22+0.25×10−15 . However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on A that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with α = −2/3, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of p ≲ 0.02 (approx. 2σ). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.

The Parkes Pulsar Timing Array Third Data Release

Abstract: We present the third data release from the Parkes Pulsar Timing Array (PPTA) project. The release contains observations of 32 pulsars obtained using the 64-m Parkes “Murriyang” radio telescope. The data span is up to 18 years with a typical cadence of 3 weeks. This data release is formed by combining an updated version of our second data release with ∼ 3 years of more recent data primarily obtained using an ultra-wide-bandwidth receiver system that operates between 704 and 4032 MHz. We provide calibrated pulse profiles, flux-density dynamic spectra, pulse times of arrival, and initial pulsar timing models. We describe methods for processing such wide-bandwidth observations, and compare this data release with our previous release.

The Gravitational-wave Background Null Hypothesis: Characterizing Noise in Millisecond Pulsar Arrival Times with the Parkes Pulsar Timing Array

Abstract: The noise in millisecond pulsar (MSP) timing data can include contributions from observing instruments, the interstellar medium, the solar wind, solar system ephemeris errors, and the pulsars themselves. The noise environment must be accurately characterized in order to form the null hypothesis from which signal models can be compared, including the signature induced by nanohertz-frequency gravitational waves (GWs). Here we describe the noise models developed for each of the MSPs in the Parkes Pulsar Timing Array (PPTA) third data release, which have been used as the basis of a search for the isotropic stochastic GW background. We model pulsar spin noise, dispersion measure variations, scattering variations, events in the pulsar magnetospheres, solar wind variability, and instrumental effects. We also search for new timing model parameters and detected Shapiro delays in PSR J0614−3329 and PSR J1902−5105. The noise and timing models are validated by testing the normalized and whitened timing residuals for Gaussianity and residual correlations with time. We demonstrate that the choice of noise models significantly affects the inferred properties of a common-spectrum process. Using our detailed models, the recovered common-spectrum noise in the PPTA is consistent with a power law with a spectral index of γ = 13/3, the value predicted for a stochastic GW background from a population of supermassive black hole binaries driven solely by GW emission.

Searching for continuous Gravitational Waves in the second data release of the International Pulsar Timing Array

Abstract: The International Pulsar Timing Array 2nd data release is the combination of datasets from worldwide collaborations. In this study, we search for continuous waves: gravitational wave signals produced by individual supermassive black hole binaries in the local universe. We consider binaries on circular orbits and neglect the evolution of orbital frequency over the observational span. We find no evidence for such signals and set sky averaged 95% upper limits on their amplitude h 95 . The most sensitive frequency is 10nHz with h 95 = 9.1 10-15 . We achieved the best upper limit to date at low and high frequencies of the PTA band thanks to improved effective cadence of observations. In our analysis, we have taken into account the recently discovered common red noise process, which has an impact at low frequencies. We also find that the peculiar noise features present in some pulsars data must be taken into account to reduce the false alarm. We show that using custom noise models is essential in searching for continuous gravitational wave signals and setting the upper limit.

Respiratory culture organism isolation and test characteristics in children with tracheostomies with and without acute respiratory infection

Abstract: Among children with tracheostomies, little is known about how respiratory culture results differ between states with and without acute respiratory infections (ARI), or the overall test performance of respiratory cultures.

Variation in Inhaled Medication Use for Infants With Congenital Heart Disease and Bronchiolitis.

Abstract: BACKGROUND AND OBJECTIVES Current viral bronchiolitis guidelines exclude infants with congenital heart disease (CHD). Variations in the use of common therapeutics in this population and their associations with clinical outcomes are unknown. Our objective was to evaluate variations in (1) the use of β-2-agonists and hypertonic saline across hospitals among infants with CHD hospitalized with bronchiolitis, and (2) hospital-level associations between medication use and outcomes. METHODS We performed a multicenter retrospective cohort study using administrative data from 52 hospitals in the Pediatric Health Information System. We included infants ≤12 months old hospitalized from January 1, 2015 to June 30, 2019 for bronchiolitis with a secondary diagnosis of CHD. Primary exposures were the hospital-level proportion of days that patients received β-2-agonists or hypertonic saline. Linear regression models assessed the association between the primary exposure and length of stay, 7-day readmission, mechanical ventilation use, and ICU utilization, adjusting for patient covariates and accounting for clustering by center. RESULTS We identified 6846 index hospitalizations for bronchiolitis in infants with CHD. Overall, 43% received a β-2-agonist, and 23% received hypertonic saline. The proportion of days with the use of β-2-agonists (3.6% to 57.4%) and hypertonic saline (0.0% to 65.8%) varied widely across hospitals in our adjusted model. For both exposures, adjusted models revealed no association between days of use and patient outcomes. CONCLUSIONS For children with CHD hospitalized with bronchiolitis, hospital-level use of β-2-agonists and hypertonic saline varied widely, and their use was not associated with clinical outcomes.