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We introduce a new capability of the Neil Gehrels Swift Observatory, dubbed “continuous commanding,” that achieves 10 s latency response time on orbit to unscheduled target-of-opportunity requests received on the ground. We show that this will allow Swift to respond to premerger (early-warning) gravitational-wave (GW) detections, rapidly slewing the Burst Alert Telescope (BAT) across the sky to place the GW origin in the BAT field of view at or before merger time. This will dramatically increase the GW/gamma-ray burst (GRB) codetection rate and enable prompt arcminute localization of a neutron star merger. We simulate the full Swift response to a GW early-warning alert, including input sky maps produced at different early-warning times, a complete model of the Swift attitude control system, and a full accounting of the latency between the GW detectors and the spacecraft. 60 s of early warning can double the rate of a prompt GRB detection with arcminute localization, and 140 s guarantees observation anywhere on the unocculted sky, even with localization areas ≫1000 deg2. While 140 s is beyond current GW detector sensitivities, 30–70 s is achievable today. We show that the detection yield is now limited by the latency of LIGO/Virgo cyberinfrastructure and motivate a focus on its reduction. Continuous commanding has been integrated as a general capability of Swift, significantly increasing its versatility in response to the growing demands of time-domain astrophysics. We demonstrate this potential on an externally triggered fast radio burst (FRB), slewing 81° across the sky, and collecting X-ray and UV photons from the source position <150 s after the trigger was received from the Canadian Hydrogen Intensity Mapping Experiment, thereby setting the earliest and deepest such constraints on high-energy activity from nonrepeating FRBs. The Swift Team invites the community to consider and propose novel scientific applications of ultra-low-latency UV, X-ray, and gamma-ray observations.

Advanced LIGO's first observing run marked the birth of gravitational-wave astronomy through the first detection of gravitational waves from coalescing black holes-GW150914. Advanced LIGO's second and Advanced Virgo's first observing run marked the birth of multimessenger astronomy with first joint observations of gravitational and electromagnetic radiation associated with coalescing neutron stars-GW170817. The electromagnetic observations included detection of a burst of gamma rays produced by the merger, and a kilonova powered by the radioactive decay of r-process nuclei synthesized in the neutron star coalescence ejecta. Gravitational waves from compact binary coalescences carry fingerprints of the sources that generated them. Studying them allows us to test Einstein’s general relativity in the strongest regimes, where it has never been tested before, and study matter at densities beyond reach of the most powerful laboratories on our planet. Moreover, we can gain insight about the evolution of stars, galaxies and even the Universe as a whole by studying the merger rate of compact objects. Joint electromagnetic and gravitational-wave observations help develop our understanding of the physical processes that occur in such systems, and provide a new method of probing cosmological parameters.GW170817 was detected by the GstLAL pipeline in low-latency making the extensive electromagnetic followup possible. The GstLAL pipeline is a matched filtering pipeline that uses compact binary coalescence waveform models to filter the data from gravitational-wave detectors in the time-domain. It can detect gravitational waves from coalescing compact binaries in near real time and provide point estimates for binary parameters. This thesis describes the methods, developments, and the results from the GstLAL pipeline over the course of the first two observing runs of Advanced LIGO, focusing on the contributions made by the author. We also present a study about the prospects of observing a cosmological stochastic background which is expected to be buried under the astrophysical background from the population of coalesceing compact binaries with third-generation gravitational-wave detectors.

Merger rates of binary black holes, binary neutron stars, and neutron star-black hole binaries in the local Universe (i.e., redshift \\(z=0\\)), inferred from the Laser Interferometer Gravitational Wave Observatory (LIGO) and Virgo, are \\(16-130\\, Gpc^-3\\,yr^-1\\), \\(13-1900\\, Gpc^-3\\,yr^-1\\), and \\(7.4-320\\,Gpc^-3\\,yr^-1\\), respectively. These rates suggest that there is a significant chance that two or more of these signals will overlap with each other during their lifetime in the sensitivity-band of future gravitational-wave detectors such as the Cosmic Explorer and Einstein Telescope. The detection pipelines provide the coalescence time of each signal with an accuracy \\(O(10\\, ms)\\). We show that using a prior on the coalescence time from a detection pipeline, it is possible to correctly infer the properties of these overlapping signals with the current data-analysis infrastructure. We study different configurations of two overlapping signals created by non-spinning binaries, varying their time and phase at coalescence, as well as their signal-to-noise ratios. We conclude that, for the scenarios considered in this work, parameter inference is robust provided that their coalescence times in the detector frame are more than \\( 1-2 \\,s\\). Signals whose coalescence epochs lie within \\( 0.5\\,s\\) of each other suffer from significant biases in parameter inference, and new strategies and algorithms would be required to overcome such biases.

Gravitational waves from sub-solar mass inspiraling compact objects would provide almost smoking-gun evidence for primordial black holes (PBHs). We perform the first search for inspiraling planetary-mass compact objects in equal-mass and highly asymmetric mass-ratio binaries using data from the first half of the LIGO-Virgo-KAGRA third observing run. Though we do not find any significant candidates, we determine the maximum luminosity distance reachable with our search to be of \\(O(0.1-100)\\) kpc, and corresponding model-independent upper limits on the merger rate densities to be \\(O(10^3-10^-7)\\) kpc\\(^-3\\)yr\\(^-1\\) for systems with chirp masses of \\(O(10^-4-10^-2)M_\\), respectively. Furthermore, we interpret these rate densities as arising from PBH binaries and constrain the fraction of dark matter that such objects could comprise. For equal-mass PBH binaries, we find that these objects would compose less than 4-100% of DM for PBH masses of \\(10^-2M_\\) to \\(2 10^-3M_\\), respectively. For asymmetric binaries, assuming one black hole mass corresponds to a peak in the mass function at 2.5\\(M_\\), a PBH dark-matter fraction of 10% and a second, much lighter PBH, we constrain the mass function of the second PBH to be less than 1 for masses between \\(1.5 10^-5M_\\) and \\(2 10^-4M_\\). Our constraints, released on Zenodo, are robust enough to be applied to any PBH or exotic compact object binary formation models, and complement existence microlensing results. More details about our search can be found in our companion paper.

Gravitational waves from sub-solar mass primordial black holes could be detected in LIGO, Virgo and KAGRA data. Here, we apply a method originally designed to look for rapidly spinning-down neutron stars, the generalized frequency-Hough transform, to search for planetary-mass primordial black holes using data from the first half of the third observing run of advanced LIGO. In this companion paper to arXiv:2402.19468, in which the main results of our search are presented, we delve into the details of the search methodology, the choices we have made regarding the parameter space to explore, the follow-up procedure we use to confirm or reject possible candidates returned in our search, and a comparison of our analytic procedure of generating upper limits to those obtained through injections.

We present a novel method to conduct targeted gravitational-wave searches for compact binary mergers using the GstLAL inspiral pipeline. By incorporating sky localization and timing information from external electromagnetic triggers, we enhance the sensitivity of the search for sub-threshold gravitational-wave signals associated with events such as short gamma-ray bursts. Our approach modifies the standard likelihood ratio ranking statistic to include a sky localization prior, allowing for a more focused analysis on specific regions of the sky. We demonstrate the effectiveness of this method through injection studies, comparing the performance of the targeted search against the standard all-sky search configuration. The results show a significant improvement in detection efficiency for signals consistent with the provided sky location and timing, while maintaining control over false alarm rates. This targeted search framework enables rapid follow-up of electromagnetic transients, facilitating multi-messenger astronomy efforts in the era of advanced gravitational-wave detectors.

In astronomy, we frequently face the decision problem: does this data contain a signal? Typically, a statistical approach is used, which requires a threshold. The choice of threshold presents a common challenge in settings where signals and noise must be delineated, but their distributions overlap. Gravitational-wave astronomy, which has gone from the first discovery to catalogues of hundreds of events in less than a decade, presents a fascinating case study. For signals from colliding compact objects, the field has evolved from a frequentist to a Bayesian methodology. However, the issue of choosing a threshold and validating noise contamination in a catalogue persists. Confusion and debate often arise due to the misapplication of statistical concepts, the complicated nature of the detection statistics, and the inclusion of astrophysical background models. We introduce Conformal Prediction (CP), a framework developed in Machine Learning to provide distribution-free uncertainty quantification to point predictors. We show that CP can be viewed as an extension of the traditional statistical frameworks whereby thresholds are calibrated such that the uncertainty intervals are statistically rigorous and the error rate can be validated. Moreover, we discuss how CP offers a framework to optimally build a meta-pipeline combining the outputs from multiple independent searches. We introduce CP with a toy cosmic-ray detector, which captures the salient features of most astrophysical search problems and allows us to demonstrate the features of CP in a simple context. We then apply the approach to a recent gravitational-wave Mock Data Challenge using multiple search algorithms for compact binary coalescence signals in interferometric gravitational-wave data. Finally, we conclude with a discussion on the future potential of the method for gravitational-wave astronomy.

Detection of primordial gravitational-wave backgrounds generated during the early universe phase transitions is a key science goal for future ground-based detectors. The rate of compact binary mergers is so large that their cosmological population produces a confusion background that could masquerade the detection of potential primordial stochastic backgrounds. In this paper we study the ability of current and future detectors to resolve the confusion background to reveal interesting primordial backgrounds. The current detector network of LIGO and Virgo and the upcoming KAGRA and LIGO-India will not be able to resolve the cosmological compact binary source population and its sensitivity to stochastic background will be limited by the confusion background of these sources. We find that a network of three (and five) third generation (3G) detectors of Cosmic Explorer and Einstein Telescope will resolve the confusion background produced by binary black holes leaving only about 0.013\\% (respectively, 0.00075\\%) unresolved; in contrast, as many as 25\\% (respectively, 7.7\\%) of binary neutron star sources remain unresolved. Consequently, the binary black hole population will likely not limit observation of primordial backgrounds but the binary neutron star population will limit the sensitivity of 3G detectors to \\(_ GW 10^-11\\) at 10 Hz (respectively, \\(_ GW 3 10^-12\\)).

Stellar-mass binary black holes will sweep through the frequency band of the Laser Interferometer Space Antenna (LISA) for months to years before appearing in the audio-band of ground-based gravitational-wave detectors. One can expect several tens of these events up to a distance of \\(500 \\,Mpc\\) each year. The LISA signal-to-noise ratio for such sources even at these close distances will be too small for a blind search to confidently detect them. However, next generation ground-based gravitational-wave detectors, expected to be operational at the time of LISA, will observe them with signal-to-noise ratios of several thousands and measure their parameters very accurately. We show that such high fidelity observations of these sources by ground-based detectors help in archival searches to dig tens of signals out of LISA data each year.

Strong gravitational lensing of gravitational waves can produce duplicate signals separated in time with different amplitudes. We consider the case in which strong lensing produces superthreshold gravitational-wave events and weaker subthreshold signals buried in the noise background. We present the GstLAL-based TargetEd Subthreshold Lensing seArch search method for the subthreshold signals using reduced template banks targeting specific confirmed gravitational-wave events. We perform a simulation campaign to assess the performance of the proposed search method. We show that it can effectively uprank potential subthreshold lensed counterparts to the target gravitational-wave event. We also compare its performance to other alternative solutions to the posed problem and demonstrate that our proposed method outperforms the other solutions. The method described in this paper has already been deployed in the recent LVK Collaboration-wide search for lensing signatures of gravitational waves in the first half of LIGO/Virgo third observing run O3a [R. Abbott et al. (LIGO Scientific, Virgo Collaborations), Astrophys. J. 923, 14 (2021).].