arXiv subjects

Karl Wette

Publications and source records attributed to Karl Wette.

At least 19 recordsLinked to original sources

arXiv2026

Search for Long-Transient Gravitational Waves from Supernova SN2023ixf using GFH-v2 Pipeline

We present a directed search for long-transient gravitational waves from the possible newborn magnetar remnant of SN2023ixf, a nearby Type II core-collapse supernova in the M101 galaxy. The analysis uses LIGO Hanford and Livingston data from Engineering Run 15, using coincident data lying within the on-source window associated with the supernova. We target signals from a rapidly rotating, non-axisymmetric neutron star whose spin-down is dominated by gravitational-wave emission, producing a power-law decrease in frequency and a corresponding decrease in strain amplitude. The search is performed with the GFH-v2 pipeline, based on the Generalized Frequency Hough transform. No candidate survives the coincidence and follow-up analysis. We therefore set upper limits on the maximum detectable distance as a function of initial frequency and ellipticity. For the highest ellipticity interval, the 90% upper limits reach distances of about 1-2.5~Mpc across most of the analysed band. Although these limits are below the distance to M101, the search provides the first application of GFH-v2 to a nearby core-collapse supernova and characterizes its performance on real detector data.

astro-ph.IM↗

arXiv2026

Mitigation of Incoherent Spectral Lines via Adaptive Coherence Analysis for Continuous Gravitational-Wave Searches

The sensitivity of continuous gravitational-wave searches is strictly limited by non-Gaussian spectral artefacts that accumulate coherent power over long observation baselines. In this paper, we present an unsupervised mitigation framework based on adaptive network coherence analysis. Unlike traditional veto methods that discard entire frequency bands, our pipeline selectively suppresses local artefacts while preserving global potentially astrophysical signals. We validate the method using Advanced LIGO O3 data, analysing the cleaning performance across integration times of 1, 3, and 5 days. For the 5-day dataset, the pipeline identifies and mitigates 89\% and 77\% of the total spectral lines in the Hanford and Livingston detectors, respectively, while effectively preserving the coherent population consistent with astrophysical morphologies. This is achieved while modifying less than 7\% of the analysis bandwidth spanning 20~Hz to 2000~Hz. Rigorous statistical verification demonstrates that the mitigation effectively suppresses the non-Gaussian tail of the noise distribution while strictly preserving the statistical integrity of coherent signal candidates. By recovering detector sensitivity in parameter spaces previously contaminated by the spectral forest, this framework provides a robust preprocessing strategy for all-sky searches.

gr-qc↗

arXiv2026

Search for continuous gravitational waves from neutron stars in five globular clusters in the first part of the fourth LIGO-Virgo-KAGRA observing run

We present the results of directed searches for continuous gravitational waves from unknown neutron stars in five Milky Way globular clusters. We carry out these searches in the LIGO data from the first eight months of the fourth LIGO-Virgo-KAGRA observing run using the WEAVE semicoherent program, which sums matched-filter detection-statistic values over many time segments spanning the observation period. No gravitational wave signal is detected in the search band of 20-475 Hz. Injections of simulated continuous wave signals in the data indicate that we achieve the most sensitive results to date across most of the explored parameter space volume, obtaining median 95% confidence level upper limits as low as $\sim 4.2 \times 10^{-26}$ near 282 Hz for NGC 6397. We also derive upper limits on neutron star ellipticity and $r$-mode amplitudes, reaching $\lesssim 10^{-5}$ and $\lesssim 10^{-3}$, respectively, at frequencies above 200 Hz.

gr-qc↗

arXiv2024

Methodology for constraining ultralight vector bosons with gravitational wave searches targeting merger remnant black holes

Ultralight bosons are a hypothetical class of particles predicted under various extensions of Standard Model physics. As a result of the superradiance mechanism, we expect ultralight bosons, should they exist in certain mass ranges, to form macroscopic clouds around rotating black holes, so that we can probe their existence by looking for the long-transient gravitational wave emission produced by such clouds. In this paper, we propose a statistically robust framework for constraining the existence of ultralight vector bosons in the absence of detecting such a signal from searches targeting merger remnant black holes, effectively marginalizing over the uncertainties present in the properties of the target black holes. We also determine the impact of weak kinetic mixing with the ordinary photon and vector mass generation through a hidden Higgs mechanism on the constraining power of these searches. We find that individual follow-up searches, particularly with the next-generation gravitational wave detectors, can probe regions of parameter space for such models where robust constraints are still lacking.

gr-qc↗

arXiv2024

Gravitational Wave Searches for Post-Merger Remnants of GW170817 and GW190425

We present the results of two searches for gravitational waves from the post-merger remnants of the binary neutron star coalescence events GW170817 and GW190425. The searches are fully coherent over 1800~s of data from the 2nd (for GW170817) and 3rd (for GW190425) observing runs of the LIGO and Virgo observatories. The searches compute the matched filter $\mathcal{F}$-statistic, and use a piecewise model of the rapidly changing frequency evolution appropriate for young neutron stars. No detection is claimed. The peak root-sum-squared strain upper limit at 50\% detection probability ($h_{\text{rss}}^{50\%})$ of both searches occurs at 1700~Hz and is estimated at $1.64 \times 10^{-22}~\text{Hz}^{-1/2}$ for GW170817, and $1.0 \times 10^{-22}~\text{Hz}^{-1/2}$ for GW190425. This is the first gravitational wave search for a neutron star remnant of GW190425.

gr-qc↗

arXiv2023

Population synthesis and parameter estimation of neutron stars with continuous gravitational waves and third-generation detectors

Precise measurement of stellar properties through the observation of continuous gravitational waves from spinning non-axisymmetric neutron stars can shed light onto new physics beyond terrestrial laboratories. Although hitherto undetected, prospects for detecting continuous gravitational waves improve with longer observation periods and more sensitive gravitational wave detectors. We study the capability of the Advanced Laser Interferometer Gravitational-Wave Observatory, and the Einstein Telescope to measure the physical properties of neutron stars through continuous gravitational wave observations. We simulate a population of Galactic neutron stars, assume continuous gravitational waves from the stars have been detected, and perform parameter estimation of the detected signals. Using the estimated parameters, we infer the stars' moments of inertia, ellipticities, and the components of the magnetic dipole moment perpendicular to the rotation axis. The estimation of the braking index proved challenging and is responsible for the majority of the uncertainties in the inferred parameters. Using the Einstein Telescope with an observation period of 5 yrs, point estimates using median can be made on the moments of inertia with error of ~ 10 - 100% and on the ellipticities with error of ~ 5 - 50%, subject to the inference of the braking index. The perpendicular magnetic dipole moment could not be accurately inferred for neutron stars that emit mainly gravitational waves.

gr-qc↗

arXiv2023

Piecewise frequency model for searches for long-transient gravitational waves from young neutron stars

In this work we characterise the performance of a new search technique designed to be sensitive to the remnants of binary neutron star systems. Sensitivity estimates of the new method on simulated data are competitive against those of other work. Previous searches for a gravitational-wave signal from a possible neutron star remnant of the binary neutron star merger event GW170817 have focused on short ($<500$~s) and long duration (2.5~hr -- 8~day) signals. To date, no such post-merger signal has been detected. We introduce a new piecewise model which has the flexibility to accurately follow gravitational-wave signals which are rapidly evolving in frequency, such as those which may be emitted from young neutron stars born from binary neutron star mergers or supernovae. We investigate the sensitivity and computational cost of this piecewise model when used in a fully coherent 1800-second $\mathcal{F}$-statistic search on simulated data containing possible signals from the GW170817 remnant. The sensitivity of the search using the piecewise model is determined using simulated data, with noise consistent with the LIGO second observing run. Across a 100--2000~Hz frequency band, the model achieves a peak sensitivity of $h_{\text{rss}}^{50\%} = 4.4 \times 10^{-23} \text{Hz}^{-1/2}$ at 200~Hz, competitive with other methods. The computational cost of conducting the search, over a bank of $1.1 \times 10^{12}$ templates, is estimated at 10 days running on 100 CPU's.

gr-qc↗

arXiv2023

Methods and prospects for gravitational wave searches targeting ultralight vector boson clouds around known black holes

Ultralight bosons are predicted in many extensions to the Standard Model and are popular dark matter candidates. The black hole superradiance mechanism allows for these particles to be probed using only their gravitational interaction. In this scenario, an ultralight boson cloud may form spontaneously around a spinning black hole and extract a non-negligible fraction of the black hole's mass. These oscillating clouds produce quasi-monochromatic, long-duration gravitational waves that may be detectable by ground-based or space-based gravitational wave detectors. We discuss the capability of a new long-duration signal tracking method, based on a hidden Markov model, to detect gravitational wave signals generated by ultralight vector boson clouds, including cases where the signal frequency evolution timescale is much shorter than that of a typical continuous wave signal. We quantify the detection horizon distances for vector boson clouds with current- and next-generation ground-based detectors. We demonstrate that vector clouds hosted by black holes with mass $\gtrsim 60 M_{\odot}$ and spin $\gtrsim 0.6$ are within the reach of current-generation detectors up to a luminosity distance of $\sim 1$ Gpc. This search method enables one to target vector boson clouds around remnant black holes from compact binary mergers detected by gravitational-wave detectors. We discuss the impact of the sky localization of the merger events and demonstrate that a typical remnant black hole reasonably well-localized by the current generation detector network is accessible in a follow-up search.

gr-qc↗

arXiv2023

Searches for continuous gravitational waves from neutron stars: A twenty-year retrospective

Seven years after the first direct detection of gravitational waves, from the collision of two black holes, the field of gravitational wave astronomy is firmly established. A first detection of continuous gravitational waves from rapidly-spinning neutron stars could be the field's next big discovery. I review the last twenty years of efforts to detect continuous gravitational waves using the LIGO and Virgo gravitational wave detectors. I summarise the model of a continuous gravitational wave signal, the challenges to finding such signals in noisy data, and the data analysis algorithms that have been developed to address those challenges. I present a quantitative analysis of 297 continuous wave searches from 80 papers, published from 2003 to 2022, and compare their sensitivities and coverage of the signal model parameter space.

gr-qc↗

arXiv2023

What are neutron stars made of? Gravitational waves may reveal the answer

Neutron stars are one of the most mysterious wonders in the Universe. Their extreme densities hint at new and exotic physics at work within. Gravitational waves could be the key to unlocking their secrets. In particular, a first detection of gravitational waves from rapidly-spinning, deformed neutron stars could yield new insights into the physics of matter at extreme densities and under strong gravity. Once a first detection is made, a critical challenge will be to robustly extract physically interesting information from the detected signals. In this essay, we describe initial research towards answering this challenge, and thereby unleashing the full power of gravitational waves as an engine for the discovery of new physics.

gr-qc↗

arXiv2023

Inferring neutron star properties with continuous gravitational waves

Detection of continuous gravitational waves from rapidly-spinning neutron stars opens up the possibility of examining their internal physics. We develop a framework that leverages a future continuous gravitational wave detection to infer a neutron star's moment of inertia, equatorial ellipticity, and the component of the magnetic dipole moment perpendicular to its rotation axis. We assume that the neutron star loses rotational kinetic energy through both gravitational wave and electromagnetic radiation, and that the distance to the neutron star can be measured, but do not assume electromagnetic pulsations are observable or a particular neutron star equation of state. We use the Fisher information matrix and Monte Carlo simulations to estimate errors in the inferred parameters, assuming a population of gravitational-wave-emitting neutron stars consistent with the typical parameter domains of continuous gravitational wave searches. After an observation time of one year, the inferred errors for many neutron stars are limited chiefly by the error in the distance to the star. The techniques developed here will be useful if continuous gravitational waves are detected from a radio, X-ray, or gamma-ray pulsar, or else from a compact object with known distance, such as a supernova remnant.

gr-qc↗

arXiv2022

Implementation of a new weave-based search pipeline for continuous gravitational waves from known binary systems

Scorpius X-1 (Sco X-1) has long been considered one of the most promising targets for detecting continuous gravitational waves with ground-based detectors. Observational searches for Sco X-1 have achieved substantial sensitivity improvements in recent years, to the point of starting to rule out emission at the torque-balance limit in the low-frequency range \sim 40--180 Hz. In order to further enhance the detection probability, however, there is still much ground to cover for the full range of plausible signal frequencies \sim 20--1500 Hz, as well as a wider range of uncertainties in binary orbital parameters. Motivated by this challenge, we have developed BinaryWeave, a new search pipeline for continuous waves from a neutron star in a known binary system such as Sco X-1. This pipeline employs a semi-coherent StackSlide F-statistic using efficient lattice-based metric template banks, which can cover wide ranges in frequency and unknown orbital parameters. We present a detailed timing model and extensive injection-and-recovery simulations that illustrate that the pipeline can achieve high detection sensitivities over a significant portion of the parameter space when assuming sufficiently large (but realistic) computing budgets. Our studies further underline the need for stricter constraints on the Sco X-1 orbital parameters from electromagnetic observations, in order to be able to push sensitivity below the torque-balance limit over the entire range of possible source parameters.

gr-qc↗

arXiv2022

Deep exploration for continuous gravitational waves at 171--172 Hz in LIGO second observing run data

We pursue a novel strategy towards a first detection of continuous gravitational waves from rapidly-rotating deformed neutron stars. Computational power is focused on a narrow region of signal parameter space selected by a strategically-chosen benchmark. We search data from the 2nd observing run of the LIGO Observatory with an optimised analysis run on graphics processing units. While no continuous waves are detected, the search achieves a sensitivity to gravitational wave strain of $h_0 = 1.01{\times}10^{-25}$ at 90% confidence, 24% to 69% better than past searches of the same parameter space. Constraints on neutron star deformity are within theoretical maxima, thus a detection by this search was not inconceivable.

gr-qc↗

arXiv2022

Graphics processing unit implementation of the F-statistic for continuous gravitational wave searches

The $\mathcal{F}$-statistic is a detection statistic used widely in searches for continuous gravitational waves with terrestrial, long-baseline interferometers. A new implementation of the $\mathcal{F}$-statistic is presented which accelerates the existing "resampling" algorithm using graphics processing units (GPUs). The new implementation runs between 10 and 100 times faster than the existing implementation on central processing units without sacrificing numerical accuracy. The utility of the GPU implementation is demonstrated on a pilot narrowband search for four newly discovered millisecond pulsars in the globular cluster Omega Centauri using data from the second Laser Interferometer Gravitational-Wave Observatory observing run. The computational cost is $17.2$ GPU-hours using the new implementation, compared to 1092 core-hours with the existing implementation.

gr-qc↗

arXiv2021

Template Lattices for a Cross-Correlation Search for Gravitational Waves from Scorpius X-1

We describe the application of the lattice covering problem to the placement of templates in a search for continuous gravitational waves from the low-mass X-Ray binary Scorpius X-1. Efficient placement of templates to cover the parameter space at a given maximum mismatch is an application of the sphere covering problem, for which an implementation is available in the LatticeTiling software library. In the case of Sco X-1, potential correlations, in both the prior uncertainty and the mismatch metric, between the orbital period and orbital phase, lead to complications in the efficient construction of the lattice. We define a shearing coordinate transformation which simultaneously minimizes both of these sources of correlation, and allows us to take advantage of the small prior orbital period uncertainty. The resulting lattices have a factor of about 3 fewer templates than the corresponding parameter space grids constructed by the prior straightforward method, allowing a more sensitive search at the same computing cost and maximum mismatch.

gr-qc↗

arXiv2021

Geometric Approach to Analytic Marginalisation of the Likelihood Ratio for Continuous Gravitational Wave Searches

The likelihood ratio for a continuous gravitational wave signal is viewed geometrically as a function of the orientation of two vectors; one representing the optimal signal-to-noise ratio, the other representing the maximised likelihood ratio or $\mathcal{F}$-statistic. Analytic marginalisation over the angle between the vectors yields a marginalised likelihood ratio which is a function of the $\mathcal{F}$-statistic. Further analytic marginalisation over the optimal signal-to-noise ratio is explored using different choices of prior. Monte-Carlo simulations show that the marginalised likelihood ratios have identical detection power to the $\mathcal{F}$-statistic. This approach demonstrates a route to viewing the $\mathcal{F}$-statistic in a Bayesian context, while retaining the advantages of its efficient computation.

gr-qc↗

arXiv2021

Deep searches for X-ray pulsations from Scorpius X-1 and Cygnus X-2 in support of continuous gravitational wave searches

Neutron stars in low mass X-ray binaries are hypothesised to emit continuous gravitational waves that may be detectable by ground-based observatories. The torque balance model predicts that a higher accretion rate produces larger-amplitude gravitational waves, hence low mass X-ray binaries with high X-ray flux are promising targets for gravitational wave searches. The detection of X-ray pulsations would identify the spin frequency of these neutron stars, and thereby improve the sensitivity of continuous gravitational-wave searches by reducing the volume of the search parameter space. We perform a semi-coherent search for pulsations in the two low mass X-ray binaries Scorpius X-1 and Cygnus X-2 using X-ray data from the \textit{ Rossi X-ray Timing Explorer} Proportional Counter Array. We find no clear evidence for pulsations, and obtain upper limits (at $90\%$ confidence) on the fractional pulse amplitude, with the most stringent being $0.034\%$ for Scorpius X-1 and $0.23\%$ for Cygnus X-2. These upper limits improve upon those of Vaughan et al. (1994) by factors of $\sim 8.2$ and $\sim 1.6$ respectively.

astro-ph.HE↗

arXiv2021

The SPIIR online coherent pipeline to search for gravitational waves from compact binary coalescences

This paper presents the SPIIR pipeline used for public alerts during the third advanced LIGO and Virgo observation run (O3 run). The SPIIR pipeline uses infinite impulse response (IIR) filters to perform extremely low-latency matched filtering and this process is further accelerated with graphics processing units (GPUs). It is the first online pipeline to select candidates from multiple detectors using a coherent statistic based on the maximum network likelihood ratio statistic principle. Here we simplify the derivation of this statistic using the singular-value-decomposition (SVD) technique and show that single-detector signal-to-noise ratios from matched filtering can be directly used to construct the statistic for each sky direction. Coherent searches are in general more computationally challenging than coincidence searches due to extra search over sky direction parameters. The search over sky directions follows an embarrassing parallelization paradigm and has been accelerated using GPUs. The detection performance is reported using a segment of public data from LIGO-Virgo's second observation run. We demonstrate that the median latency of the SPIIR pipeline is less than 9 seconds, and present an achievable roadmap to reduce the latency to less than 5 seconds. During the O3 online run, SPIIR registered triggers associated with 38 of the 56 non-retracted public alerts. The extreme low-latency nature makes it a competitive choice for joint time-domain observations, and offers the tantalizing possibility of making public alerts prior to the merger phase of binary coalescence systems involving at least one neutron star.

gr-qc↗