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1. Improved statistic to identify strongly lensed gravitational wave events

   https://doi.org/10.1093/mnras/stac1704  

   More, Anupreeta ; More, Surhud ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   As the number of detected gravitational wave sources increases with increased sensitivity of the gravitational wave observatories, observing strongly lensed pairs of events will become a real possibility. Lensed gravitational wave (GW) events will have very accurately measured time delays and magnification ratios. Suppose we identify the lens system corresponding to a GW event in the electromagnetic domain and also measure the redshifts of the lens and the host galaxy; in that case, we can use the GW event to constrain important astrophysical parameters of the lens system. As most lensing events have image separations that are significantly smaller than the GW event localization uncertainties, we must develop diagnostics that will aid in the robust identification of such lensed events. We define a new statistic based on the joint probability of lensing observables that can be used to discriminate lensed pairs of events from the unlensed ones. To this end, we carry out simulations of lensed GW events to infer the distribution of the relative time delays and relative magnifications subdivided by the type of lensed images. We compare this distribution to a similar one obtained for random unlensed event pairs. Our statistic can improve the search pipelines’ existingmore »ranking approach to down-select event pairs for joint parameter estimates. The distributions we obtain can further be used to define more informative priors in joint parameter estimation analyses for candidate lensed events.

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2. Implications of the search for optical counterparts during the second part of the Advanced LIGO’s and Advanced Virgo’s third observing run: lessons learned for future follow-up observations

   https://doi.org/10.1093/mnras/staa1925  

   Coughlin, Michael W ; Dietrich, Tim ; Antier, Sarah ; Almualla, Mouza ; Anand, Shreya ; Bulla, Mattia ; Foucart, Francois ; Guessoum, Nidhal ; Hotokezaka, Kenta ; Kumar, Vishwesh ; et al ( September 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Joint multimessenger observations with gravitational waves and electromagnetic (EM) data offer new insights into the astrophysical studies of compact objects. The third Advanced LIGO and Advanced Virgo observing run began on 2019 April 1; during the 11 months of observation, there have been 14 compact binary systems candidates for which at least one component is potentially a neutron star. Although intensive follow-up campaigns involving tens of ground and space-based observatories searched for counterparts, no EM counterpart has been detected. Following on a previous study of the first six months of the campaign, we present in this paper the next five months of the campaign from 2019 October to 2020 March. We highlight two neutron star–black hole candidates (S191205ah and S200105ae), two binary neutron star candidates (S191213g and S200213t), and a binary merger with a possible neutron star and a ‘MassGap’ component, S200115j. Assuming that the gravitational-wave (GW) candidates are of astrophysical origin and their location was covered by optical telescopes, we derive possible constraints on the matter ejected during the events based on the non-detection of counterparts. We find that the follow-up observations during the second half of the third observing run did not meet the necessary sensitivity tomore »constrain the source properties of the potential GW candidate. Consequently, we suggest that different strategies have to be used to allow a better usage of the available telescope time. We examine different choices for follow-up surveys to optimize sky localization coverage versus observational depth to understand the likelihood of counterpart detection.« less
3. Uncertainty and Bias of Cosmology and Astrophysical Population Model from Statistical Dark Sirens

   https://doi.org/10.3847/1538-4357/ac9da0  

   Yu, Hang ; Seymour, Brian ; Wang, Yijun ; Chen, Yanbei ( December 2022 , The Astrophysical Journal)

   Gravitational-wave (GW) radiation from a coalescing compact binary is a standard siren, as the luminosity distance of each event can be directly measured from the amplitude of the signal. One possibility to constrain cosmology using the GW siren is to perform statistical inference on a population of binary black hole (BBH) events. In essence, this statistical method can be viewed as follows. We can modify the shape of the distribution of observed BBH events by changing the cosmological parameters until it eventually matches the distribution constructed from an astrophysical population model, thereby allowing us to determine the cosmological parameters. In this work, we derive the Cramér–Rao bound for both cosmological parameters and those governing the astrophysical population model from this statistical dark siren method by examining the Fisher information contained in the event distribution. Our study provides analytical insights and enables fast yet accurate estimations of the statistical accuracy of dark siren cosmology. Furthermore, we consider the bias in cosmology due to unmodeled substructures in the merger rate and mass distribution. We find that a 1% deviation in the astrophysical model can lead to a more than 1% error in the Hubble constant. This could limit the accuracy ofmore »dark siren cosmology when there are more than 10⁴BBH events detected.

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4. Multi-messenger Approaches to Supermassive Black Hole Binary Detection and Parameter Estimation: Implications for Nanohertz Gravitational Wave Searches with Pulsar Timing Arrays

   https://doi.org/10.3847/1538-4357/ac1da9  

   Liu, Tingting ; Vigeland, Sarah J. ( November 2021 , The Astrophysical Journal)

   Abstract Pulsar timing array (PTA) experiments are becoming increasingly sensitive to gravitational waves (GWs) in the nanohertz frequency range, where the main astrophysical sources are supermassive black hole binaries (SMBHBs), which are expected to form following galaxy mergers. Some of these individual SMBHBs may power active galactic nuclei, and thus their binary parameters could be obtained electromagnetically, which makes it possible to apply electromagnetic (EM) information to aid the search for a GW signal in PTA data. In this work, we investigate the effects of such an EM-informed search on binary detection and parameter estimation by performing mock data analyses on simulated PTA data sets. We find that by applying EM priors, the Bayes factor of some injected signals with originally marginal or sub-threshold detectability (i.e., Bayes factor ∼1) can increase by a factor of a few to an order of magnitude, and thus an EM-informed targeted search is able to find hints of a signal when an uninformed search fails to find any. Additionally, by combining EM and GW data, one can achieve an overall improvement in parameter estimation, regardless of the source’s sky location or GW frequency. We discuss the implications for the multi-messenger studies of SMBHBs withmore »PTAs.« less
5. A Bayesian approach to high-fidelity interferometric calibration – I. Mathematical formalism

   https://doi.org/10.1093/mnras/stac1861  

   Sims, Peter H. ; Pober, Jonathan C. ; Sievers, Jonathan L. ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   High-fidelity radio interferometric data calibration that minimizes spurious spectral structure in the calibrated data is essential in astrophysical applications, such as 21 cm cosmology, which rely on knowledge of the relative spectral smoothness of distinct astrophysical emission components to extract the signal of interest. Existing approaches to radio interferometric calibration have been shown to impart spurious spectral structure to the calibrated data if the sky model used to calibrate the data is incomplete. In this paper, we introduce BayesCal: a novel solution to the sky-model incompleteness problem in interferometric calibration, designed to enable high-fidelity data calibration. The BayesCal data model supplements the a priori known component of the forward model of the sky with a statistical model for the missing and uncertain flux contribution to the data, constrained by a prior on the power in the model. We demonstrate how the parameters of this model can be marginalized out analytically, reducing the dimensionality of the parameter space to be sampled from and allowing one to sample directly from the posterior probability distribution of the calibration parameters. Additionally, we show how physically motivated priors derived from theoretical and measurement-based constraints on the spectral smoothness of the instrumental gains can be usedmore »to constrain the calibration solutions. In a companion paper, we apply this algorithm to simulated observations with a HERA-like array and demonstrate that it enables up to four orders of magnitude suppression of power in spurious spectral fluctuations relative to standard calibration approaches.

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