Periodic Reporting for period 1 - BINSKY (All-sky search for continuous gravitational waves from neutron stars in binary systems)
Reporting period: 2022-05-01 to 2024-04-30
Gravitational waves from compact binary coalescences are now routinely detected, but other types of gravitational waves await to be discovered. Rotating neutron stars with an asymmetry around their rotating axis give rise to continuous gravitational waves, which differ from the already detected gravitational waves in their long duration (present during the entire observing runs) and expected weaker gravitational-wave amplitude (only through the integration of many months or years of data, dependent on the asymmetry of the source, can the signal-to-noise ratio grow to detectable strengths). Although many searches for continuous waves have been carried out, no detection has been achieved yet. The first detection of a continuous gravitational wave could be the next major discovery in gravitational-wave astronomy, probing different physics such as: the behaviour of matter at extreme conditions; the equation of state at densities above nuclear density that cannot be reached in any laboratory; the geometrical shape of neutron stars; and fundamental physics with tests of general relativity.
To date, only ~2800 neutron stars have been detected but it is believed that there are around hundreds of millions in the Milky Way. More than half of the known neutron stars that are located in the sensitive frequency region of the gravitational-wave detectors are part of binary systems. Neutron stars in binary systems may be more likely to have the asymmetries needed to emit detectable continuous waves, since they might be accreting matter from their companions, a process that provides a natural asymmetry. This increases the chances of detecting a continuous wave signal, making all-sky searches of continuous waves from unknown binary systems one of the most promising scenarios.
All-sky surveys for continuous wave signals from unknown neutron stars in binary systems are probably the most challenging search in gravitational-wave science. For this reason, they
have been carried out seldom and at lower sensitivity compared to surveys from signals from isolated neutron stars. With this project we want to address this deficiency, developing a method to carry out these searches with improved sensitivity and decreased computational cost. Furthermore, this project aims to make the first detection of a continuous wave signal, searching for signals from unknown neutron stars in binary systems.
To date, only ~2800 neutron stars have been detected but it is believed that there are around hundreds of millions in the Milky Way. More than half of the known neutron stars that are located in the sensitive frequency region of the gravitational-wave detectors are part of binary systems. Neutron stars in binary systems may be more likely to have the asymmetries needed to emit detectable continuous waves, since they might be accreting matter from their companions, a process that provides a natural asymmetry. This increases the chances of detecting a continuous wave signal, making all-sky searches of continuous waves from unknown binary systems one of the most promising scenarios.
All-sky surveys for continuous wave signals from unknown neutron stars in binary systems are probably the most challenging search in gravitational-wave science. For this reason, they
have been carried out seldom and at lower sensitivity compared to surveys from signals from isolated neutron stars. With this project we want to address this deficiency, developing a method to carry out these searches with improved sensitivity and decreased computational cost. Furthermore, this project aims to make the first detection of a continuous wave signal, searching for signals from unknown neutron stars in binary systems.
The first task we carried out was to improve and expand the (up to that point) most sensitive pipeline, BinarySkyHough. We compared the computational efficiency and sensitivity of different semicoherent detection statistics, as shown in two of the figures attached to this summary, which show that our new improved pipeline (BinarySkyHouF) is more sensitive and at the same time more computationally efficient. The main improvements were: 1) the usage of demodulated detection statistics, which allow the usage of longer coherent segments, increasing the sensitivity and giving more flexibility to the pipeline; 2) the per-detector data are combined coherently, thereby reducing the computational cost (by a factor equal to the number of detectors) and further improving sensitivity for searches with more than two detectors; 3) the new pipeline has explicit control over the mismatch in the coherent stage, allowing one to perform lower mismatch searches than before, for example, when following up an interesting candidate or targeting a particularly interesting smaller region of parameter space; 4) one can now explicitly search over binary orbital parameters such as the eccentricity and argument of periapse or higher-order frequency derivatives. These results are published in Physical Review D, available here: https://journals.aps.org/prd/pdf/10.1103/PhysRevD.106.084035(opens in new window)
The second task we carried out was the development of a semi-coherent follow-up and parameter estimation pipeline using stochastic samplers such as dynesty, using the well-known and widely used bilby Python package. The main results from this project were the expansion of the capabilities of the previously available pyfstat package in a number of ways: more flexibility in the choice of sampler and prior distribution, and a new convergence criterion. We showed that for a large number of dimensions one can perform searches of CWs with a greatly reduced computational cost as compared to a search that would use a template bank (as shown in one of the attached figures). Furthermore, we showed that it is possible to find the maximum posterior point for parameter-space regions much larger than previously thought. We focused on finding a good configuration of the ptemcee, dynesty, and nessai samplers in order to reduce the computational cost of a single followup stage, showing that these samplers can produce similar results with comparable computational efficiency. We also characterized the computational cost of a parameter estimation run, and showed the improved computational efficiency of the newly developed framework. These results have been accepted for publication in Physical Review D, and for the moment are available here: https://arxiv.org/pdf/2404.18608(opens in new window)
In order to achieve the second objective of our project (detect the first continuous gravitational-wave signal), we applied for computational time for a supercomputer through the PRACE platform. We obtained around 10 million hours of computing time in the Jülich supercomputer, which we have been using together with the previously discussed improved search pipeline to carry out two types of all-sky searches from unknown neutron stars in binary systems (shown in two of the attached figures): a broad search including higher frequencies with moderated sensitivity, and a narrow search with the best sensitivity ever obtained for this type of search. Unfortunately, the final results for these two searches have not yet been obtained, and will be published within the next months.
Additionally, during this project a new pipeline to carry out targeted searches has been developed, related to the previously discussed follow-up and parameter estimation development.efforts.
All of these results have been presented in multiple conferences, such as the first and second continuous gravitational waves workshops in Amsterdam and Hannover, the Spanish Astronomical Society 2022 meeting in Tenerife, the neutron stars workshop in Bonn, and the PHAROS 2022 conference in Rome.
The second task we carried out was the development of a semi-coherent follow-up and parameter estimation pipeline using stochastic samplers such as dynesty, using the well-known and widely used bilby Python package. The main results from this project were the expansion of the capabilities of the previously available pyfstat package in a number of ways: more flexibility in the choice of sampler and prior distribution, and a new convergence criterion. We showed that for a large number of dimensions one can perform searches of CWs with a greatly reduced computational cost as compared to a search that would use a template bank (as shown in one of the attached figures). Furthermore, we showed that it is possible to find the maximum posterior point for parameter-space regions much larger than previously thought. We focused on finding a good configuration of the ptemcee, dynesty, and nessai samplers in order to reduce the computational cost of a single followup stage, showing that these samplers can produce similar results with comparable computational efficiency. We also characterized the computational cost of a parameter estimation run, and showed the improved computational efficiency of the newly developed framework. These results have been accepted for publication in Physical Review D, and for the moment are available here: https://arxiv.org/pdf/2404.18608(opens in new window)
In order to achieve the second objective of our project (detect the first continuous gravitational-wave signal), we applied for computational time for a supercomputer through the PRACE platform. We obtained around 10 million hours of computing time in the Jülich supercomputer, which we have been using together with the previously discussed improved search pipeline to carry out two types of all-sky searches from unknown neutron stars in binary systems (shown in two of the attached figures): a broad search including higher frequencies with moderated sensitivity, and a narrow search with the best sensitivity ever obtained for this type of search. Unfortunately, the final results for these two searches have not yet been obtained, and will be published within the next months.
Additionally, during this project a new pipeline to carry out targeted searches has been developed, related to the previously discussed follow-up and parameter estimation development.efforts.
All of these results have been presented in multiple conferences, such as the first and second continuous gravitational waves workshops in Amsterdam and Hannover, the Spanish Astronomical Society 2022 meeting in Tenerife, the neutron stars workshop in Bonn, and the PHAROS 2022 conference in Rome.
All of the work we have described has gone beyond the previous state of the art. Our new BSHF pipeline is the most sensitive and efficient existing pipeline to carry out all-sky searches for unknown neutron stars in binary systems. We have also shown that our follow-up pipeline can bring down the cost of these searches, and also that using stochastic samplers is much more efficient that using template banks for a large number of dimensions, a result that had not been shown before. Lastly, when the results of our searches are published, they will represent both the broadest and most sensitive searches ever performed of their kind.