Session J13: Continuous Gravitational Waves and Gravitational Wave Background

Bayesian $\mathcal{F}$-statistic-based parameter estimation of continuous gravitational waves from known pulsars

Continuous gravitational wave (CW) signals from spinning, deformed neutron stars are yet to be detected. Searches targeted at known pulsars use pulsar timing observations to infer the phase evolution parameters of the CW signals they emit. We present a new method and implementation to obtain Bayesian posteriors on the amplitude parameters {h₀, cos ι, Ψ, φ_o} of continuous-gravitational waves from known pulsars. The method combines modern Bayesian parameter estimation techniques with the well-established $\mathcal{F}$-statistic framework. 

We further explore the benefits of a likelihood function that is analytically marginalized over φ_o, which avoids signal degeneracy problems in the Ψ-φ_o subspace and improves overall accuracy. The method is tested on simulated signals, CW hardware injections in Advanced-LIGO detector data, and using percentile-percentile (PP) self-consistency tests of the posteriors. As a first application, we use the method on CW emission from PSR J1526-2744, a recently discovered millisecond pulsar. 

We find no evidence for a signal and obtain a Bayesian 95% upper limit on the gravitational-wave amplitude that is consistent with a previous frequentist upper limit.   

Continuous gravitational wave atlas

Continuous waves from non-axisymmetric neutron stars are orders of magnitude weaker than transient events from black hole and neutron star collisions. However, because continuous waves persist, it is possible to improve sensitivity by integrating months of data at a great computational cost. The latest Falcon search has produced an atlas containing results of such integrations for every direction on the sky and every analyzed band. Both upper limits and signal-to-noise ratios are provided, including the new polarization specific upper limits. Some of the results have been followed up, but the majority are unexplored.

The atlas data is provided in a new MVL file format that allows analysis on small computers, such as your notebook. There is also an MVL version of Gaia DR3 data, which can be used to explore Falcon atlas, or on its own.

We will present the new results, and show examples of using MVL files with data from Falcon atlas and Gaia DR3.   

Probing More Deeply in an All-Sky Search for Continuous Gravitational Waves in the LIGO O3 Data Set

The LIGO-Virgo O3 data set offers not only detection of now-familiar compact binary mergers of distant black holes and neutron stars, but potentially the detection of much weaker but continuous radiation from nearby rapidly spinning, non-axisymmetric neutron stars in the galaxy. All-sky searches for such radiation from previously unknown stars using necessarily long data sets are computationally challenging and have given rise to several different approaches. We describe here the application of the well established PowerFlux program to a new all-sky search of the LIGO data from the O3 observing run. In order to probe the O3 data more deeply than in a previously published O3 PowerFlux search, we apply loose coherence and coherent summing of LIGO Hanford and Livingston data in the first stage of the hierarchical search. The details of the search, its chosen parameters, and results will be presented.   

A new method to search for long-duration gravitational wave signals

We focus on long, but not infinite duration continuous wave signals and investigate the statistical properties of the detection statistic in the presence of a transient signal searched with a template grid set up for an always-ON continuous signal. We show how this affects the signal-to-noise ratio in real data in the presence of the noise. Based on this research, we propose a search scheme useful when there are no electromagnetic observations to inform on the time of occurrence of the signal.   

Deep Einstein@Home search for Continuous Gravitational Waves from the Central Compact Objects in the Supernova Remnants Cas A, Vela Jr. and G347.3-0.5 using LIGO public O3 data

We perform a search for continuous nearly monochromatic gravitational waves from the central compact objects associated with the supernova remnants (SNRs) Cas A, Vela Jr. and G347.3 using LIGO O3 public data.

The searches for all three targets have the board frequency range from 20 and 1500 Hz. The search is deployed on the volunteer computing project Einstein@Home for six monthes, with thousands of participants donating compute cycles to make this endeavour possible. After the search is done, we perform multi-stages sub-threshold follow-up searches for over 10 million most promising waveforms out of $10^{19}$ of the original search. 

The follow-up searches are currently on going which are expected to be done before March, 2024. We yet don't know if we can find any signal candidates or not. However, even there is no candidate found in the end, we can set the upper limits on the amplitude of gravitational wave signals from these three targets  which are approximately 80% more constraining than the most sensitive seaches up to date [1].   

A (old) new science goal for deci-Hertz gravitational wave detectors

In the next few decades a number of space-based gravitational wave detectors sensitive in the deci-Hertz band should be operational. 

Among the main goals of such detectors are a number of "first detections" such as primordial gravitational waves, compact stellar mass inspirals and intermediate mass black hole mergers. 

Interestingly enough we find no mention of Continuous Gravitational Waves (CGWs). Such signals, which are yet to be detected, are thought to be emitted by rotating Neutron Stars (NSs) with a reasonably small degree of non-axisymmetry.

In a previous study of ours (arXiv:2303.04714) we show that, not only the frequency band currently covered by ground-based detectors is populated by less than ~ 0.5% of all NSs, but also how those sources who possess a strong magnetic field and thus are highly deformed in the scenario of a magnetically induced non-axisymmetry, all lie in the deci-Hertz band, outside the currently "visible" one.

Here we investigate on the possible role that deci-Hertz detectors can play for the cause of CGWs detection. We consider both blind and targeted searches for CGWs. 

In the former case we find that detecting unknown neutron stars will be possible if these have an almost purely toroidal magnetic field topology. This result is in line with what we find on a previous study of ours. 

In the case of known pulsars we cannot make detectability predictions, but we can explore the no-detection scenario. Already at the DECIGO pathfinder sensitivities we would be able to constrain spin-down ellipticities for ~ 300 known pulsars (currently we can do that for only ≈ 30), and of these ≈ 20 would result to have the percentage of CGW spin-down luminosity constrained down to values smaller than 0.1% (currently this is valid only for the Crab and Vela pulsars). Such estimations will increase of a factor ≈ 3 when considering DECIGO and BBO detectors.   

Understanding the impact of compact binary population uncertainties for the detection of the gravitational-wave background

The LIGO-Virgo-KAGRA collaboration has observed tens of binary black holes so far, which have been used in several studies to infer the features of the underlying binary black hole population. From these, it is possible to predict the overall gravitational-wave (GW) fractional energy density (Ω_GW) due to black holes throughout the Universe, and thus estimate the gravitational-wave background (GWB) spectrum we may measure in GW detectors. These predictions are fundamental in our forecasts for background detection and characterization, with both present and future instruments. The uncertainties in the inferred population strongly impact the predicted energy spectrum, and in this talk we present a new, flexible method to quickly calculate the energy spectrum for varying black hole population features such as the mass spectrum and redshift distribution. We discuss how uncertainties in these distributions impact our detection capabilities and present several caveats for background estimation.    

To hear or not to hear? Challenges to probing the early Universe with next-generation ground-based gravitational-wave observatories

The next generation of ground-based gravitational-wave detectors will look much deeper into the Universe and have unprecedented sensitivities and low-frequency capabilities. Especially alluring is the possibility of detecting an early-Universe cosmological stochastic background that could provide important insights into the beginnings of our Universe and fundamental physics at extremely high energies. However, even if next-generation detectors are sensitive to cosmological stochastic backgrounds, they will be masked by more dominant astrophysical backgrounds, namely the residual background from the imperfect subtraction of resolvable compact binary coalescences (CBCs) as well as the CBC background from individually unresolvable CBCs. Using our latest knowledge of masses, rates, and delay time distributions, we present a data-driven estimate of the unresolvable CBC background that will be seen by next-generation detectors. Accounting for statistical and systematic errors, this estimate quantifies an important piece in the CBC noise budget for next-generation detectors and can help inform detector design and subtraction algorithms. We compare our results with predictions for backgrounds from several cosmological sources in the literature, finding that the unresolvable background will likely be a significant impediment for many models. This motivates the need for simultaneous inference methods or other statistical techniques to detect early-Universe cosmological backgrounds.