Sub-torque-balance Upper Limits on Continuous Gravitational Waves from Scorpius X-1

Abstract We present the results of a search for continuous gravitational waves (GWs) from the low-mass X-ray binary Scorpius X-1 using LIGO data from the first part of the fourth LIGO–Virgo–KAGRA observing run. By applying the resampling version of the cross-correlation pipeline to search for signal frequencies f 0 between 25 and 200 Hz (corresponding to neutron star spin frequencies of 12.5–100 Hz for GWs due to triaxiality, or ∼15–20 to ∼120–150 Hz for GWs due to r -modes), we are able to achieve longer coherence times than before, and for the first time set upper limits below the standard torque balance level, independent of neutron star spin inclination, for 50 Hz ≲ f 0 ≲ 200 Hz. The most sensitive upper limits on the gravitational wave amplitude h 0 , at the upper end of the frequency band searched, approach 5 × 10 −26 marginalized over inclination angle and 2 × 10 −26 assuming the most favorable inclination. The marginalized upper limits correspond to a sensitivity depth of 70–75 Hz −1/2 , improving sensitivity considerably over previous searches. Expressed as constraints on the triaxial deformation of the neutron star, the limits correspond to an ellipticity of 3 × 10 −5 if the GW frequency f 0 is 75 Hz and 3 × 10 −6 if f 0 = 200 Hz, approaching deformations that could be supported by ordinary nuclear matter. Outliers from the search were ruled out as potential signals by a combination of hierarchical follow-up and analysis of additional data from later in the observing run.

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Journal
The Astrophysical Journal Letters
Published
2026-10-13
DOI
https://doi.org/10.3847/2041-8213/aea296
Primary Topic
Pulsars and Gravitational Waves Research
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article
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article

Sub-torque-balance Upper Limits on Continuous Gravitational Waves from Scorpius X-1

S. Adhicary, F. Acernese, M. Agathos, S. Babak et al.
The Astrophysical Journal Letters
Pulsars and Gravitational Waves Research
article

Sub-torque-balance Upper Limits on Continuous Gravitational Waves from Scorpius X-1

S. Adhicary, F. Acernese, M. Agathos, S. Babak, S. W. Ballmer, N. Arnaud, Adrian Abac, M. C. Araya, S. Assis de Souza Melo, Simone Albanesi, T. M. Baptiste, N. Aritomi, R. X. Adhikari, Luca Baiotti, S. K. Apple, P. Astone, G. Baldi, C. Alléné, W. G. Anderson, M. Arogeti, C. Badger, Tomasz Baka, G. Ashton, L. Aiello, P. Ajith, E. Z. Appavuravther, K. AultONeal, A. Amato, S. Bagnasco, Stefano Ansoldi, I. Abouelfettouh, F. Attadio, R. Bajpai, Giulio Ballardin, S. B. Anderson, M. Ball, M. Assiduo, N. Aggarwal, V. K. Adkins, P Baral, S. M. Aston, N. Baldicchi, M. Baratti, C. Amra, M. Andia, P. A. Altin, S Banagiri, N Adhikari, F Aubin, A Araya, C Adamcewicz, S Bae, A Ain, S Aronson, J Areeda, J. M. Antelis, I.-L. Ahrend, K.Ackley, T.Akutsu, M. Andrés-Carcasona, K. A. Baker, S. Al-Kershi, L. Asprea, S. Aggarwal, J. L. Andrey, S. Alvarez-Lopez, S. Antier, S. Appert, G. Balbi, J. Anglin, L. Albers, W. Ali, O. D. Aguiar, W. Amar, A. Agapito, E. A. Avila, D. Bankar, D. Adhikari, D. Agarwal, J. Anna, B. Banerjee, F. Amicucci, Y. Aso, A. Adam, S. Afroz, T. Baker, F. Arciprete, M. Arca Sedda, G. Avallone, O. Amarasinghe, F. Armato, C. Anand, A Allocca, M Aoumi, S Al-Shammari, T Andrić, M Ando, A Ananyeva, S Armstrong, K Arai
article en

Abstract

Abstract We present the results of a search for continuous gravitational waves (GWs) from the low-mass X-ray binary Scorpius X-1 using LIGO data from the first part of the fourth LIGO–Virgo–KAGRA observing run. By applying the resampling version of the cross-correlation pipeline to search for signal frequencies f 0 between 25 and 200 Hz (corresponding to neutron star spin frequencies of 12.5–100 Hz for GWs due to triaxiality, or ∼15–20 to ∼120–150 Hz for GWs due to r -modes), we are able to achieve longer coherence times than before, and for the first time set upper limits below the standard torque balance level, independent of neutron star spin inclination, for 50 Hz ≲ f 0 ≲ 200 Hz. The most sensitive upper limits on the gravitational wave amplitude h 0 , at the upper end of the frequency band searched, approach 5 × 10 −26 marginalized over inclination angle and 2 × 10 −26 assuming the most favorable inclination. The marginalized upper limits correspond to a sensitivity depth of 70–75 Hz −1/2 , improving sensitivity considerably over previous searches. Expressed as constraints on the triaxial deformation of the neutron star, the limits correspond to an ellipticity of 3 × 10 −5 if the GW frequency f 0 is 75 Hz and 3 × 10 −6 if f 0 = 200 Hz, approaching deformations that could be supported by ordinary nuclear matter. Outliers from the search were ruled out as potential signals by a combination of hierarchical follow-up and analysis of additional data from later in the observing run.

The Astrophysical Journal LettersVol. 1010(2)
Université Claude Bernard Lyon 1 (FR), University of Pisa (IT), Università di Camerino (IT), Leibniz University Hannover (DE), University of Rome Tor Vergata (IT), Australian National University (AU), Tata Institute of Fundamental Research (IN), Louisiana State University (US), California Institute of Technology (US), University of London (GB), University of Minnesota (US), Georgia Institute of Technology (US), Pennsylvania State University (US), University of Salerno (IT), California State University, Fullerton (US), University of Antwerp (BE), Universität Hamburg (DE), Istituto Nazionale di Fisica Nucleare (IT), Queen Mary University of London (GB), The University of Western Australia (AU), University of Oregon (US), University of Strathclyde (GB), University of Urbino (IT), University of Trento (IT), The Graduate University for Advanced Studies, SOKENDAI (JP), University of Washington (US), Université Paris Cité (FR), Aix-Marseille Université (FR), University of Perugia (IT), Université Paris-Saclay (FR), LIGO Scientific Collaboration (US), University of Florida (US), Maastricht University (NL), University of Warwick (GB), Istituto Nazionale di Fisica Nucleare, Trento Institute for Fundamental Physics And Applications (IT), National Institute for Subatomic Physics (NL), Istituto Nazionale di Fisica Nucleare, Sezione di Torino (IT), Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT), Centre de Physique Théorique (FR), Max Planck Institute for Gravitational Physics (DE), Istituto Nazionale di Fisica Nucleare, Sezione di Bologna (IT), Gran Sasso Science Institute (IT), Centre de Physique Théorique (FR), European Gravitational Observatory (IT), University of Wisconsin–Milwaukee (US), Monash University (AU), Inter-University Centre for Astronomy and Astrophysics (IN), University of Portsmouth (GB), Massachusetts Institute of Technology (US), Université de Strasbourg (FR), Université Savoie Mont Blanc (FR), Syracuse University (US), University of Naples Federico II (IT), The University of Tokyo (JP), Friedrich Schiller University Jena (DE), Cardiff University (GB), Instituto Nacional de Pesquisas Espaciais (BR), University of Genoa (IT), Embry–Riddle Aeronautical University (US), Sapienza University of Rome (IT), Korea Institute of Science & Technology Information (KR), UCLouvain (BE), The University of Osaka (JP), Tecnológico de Monterrey (MX)
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