Directional Search for Persistent Gravitational Waves: Results from the First Part of LIGO-Virgo-KAGRA's Fourth Observing Run
Using data from the first part of LIGO-Virgo-KAGRA's fourth observing run, this study employs directional radiometer techniques to find no evidence of persistent gravitational-wave signals, thereby establishing the most stringent constraints to date on the strain amplitude and flux of narrowband, broadband, and extended sources across the sky.
Original authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu, S. Albanesi, W. Ali, S. Al-Kershi, C. Alléné, A. Allocca, S. Al-Shammari, P. A. Altin, S. Alvarez-Lopez, W. Amar, O. Amarasinghe, A. Amato, F. Amicucci, C. Amra, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Andia, M. Ando, M. Andrés-Carcasona, T. Andrić, J. Anglin, S. Ansoldi, J. M. Antelis, S. Antier, M. Aoumi, E. Z. Appavuravther, S. Appert, S. K. Apple, K. Arai, A. Araya, M. C. Araya, M. Arca Sedda, J. S. Areeda, N. Aritomi, F. Armato, S. Armstrong, N. Arnaud, M. Arogeti, S. M. Aronson, G. Ashton, Y. Aso, L. Asprea, M. Assiduo, S. Assis de Souza Melo, S. M. Aston, P. Astone, F. Attadio, F. Aubin, K. AultONeal, G. Avallone, E. A. Avila, S. Babak, C. Badger, S. Bae, S. Bagnasco, L. Baiotti, R. Bajpai, T. Baka, A. M. Baker, K. A. Baker, T. Baker, G. Baldi, N. Baldicchi, M. Ball, G. Ballardin, S. W. Ballmer, S. Banagiri, B. Banerjee, D. Bankar, T. M. Baptiste, P. Baral, M. Baratti, J. C. Barayoga, B. C. Barish, D. Barker, N. Barman, P. Barneo, F. Barone, B. Barr, L. Barsotti, M. Barsuglia, D. Barta, A. M. Bartoletti, M. A. Barton, I. Bartos, A. Basalaev, R. Bassiri, A. Basti, M. Bawaj, P. Baxi, J. C. Bayley, A. C. Baylor, P. A. Baynard, M. Bazzan, V. M. Bedakihale, F. Beirnaert, M. Bejger, D. Belardinelli, A. S. Bell, D. S. Bellie, L. Bellizzi, W. Benoit, I. Bentara, J. D. Bentley, M. Ben Yaala, S. Bera, F. Bergamin, B. K. Berger, S. Bernuzzi, M. Beroiz, D. Bersanetti, T. Bertheas, A. Bertolini, J. Betzwieser, D. Beveridge, G. Bevilacqua, N. Bevins, R. Bhandare, R. Bhatt, D. Bhattacharjee, S. Bhattacharyya, S. Bhaumik, V. Biancalana, A. Bianchi, I. A. Bilenko, G. Billingsley, A. Binetti, S. Bini, C. Binu, S. Biot, O. Birnholtz, S. Biscoveanu, A. Bisht, M. Bitossi, M. -A. Bizouard, S. Blaber, J. K. Blackburn, L. A. Blagg, C. D. Blair, D. G. Blair, N. Bode, N. Boettner, G. Boileau, M. Boldrini, G. N. Bolingbroke, A. Bolliand, L. D. Bonavena, R. Bondarescu, F. Bondu, E. Bonilla, M. S. Bonilla, A. Bonino, R. Bonnand, A. Borchers, S. Borhanian, V. Boschi, S. Bose, V. Bossilkov, Y. Bothra, A. Boudon, L. Bourg, M. Boyle, A. Bozzi, C. Bradaschia, P. R. Brady, A. Branch, M. Branchesi, I. Braun, T. Briant, A. Brillet, M. Brinkmann, P. Brockill, E. Brockmueller, A. F. Brooks, B. C. Brown, D. D. Brown, M. L. Brozzetti, S. Brunett, G. Bruno, R. Bruntz, J. Bryant, Y. Bu, F. Bucci, J. Buchanan, O. Bulashenko, T. Bulik, H. J. Bulten, A. Buonanno, K. Burtnyk, R. Buscicchio, D. Buskulic, C. Buy, R. L. Byer, G. S. Cabourn Davies, R. Cabrita, V. Cáceres-Barbosa, L. Cadonati, G. Cagnoli, C. Cahillane, A. Calafat, T. A. Callister, E. Calloni, S. R. Callos, G. Caneva Santoro, K. C. Cannon, H. Cao, L. A. Capistran, E. Capocasa, E. Capote, G. Capurri, G. Carapella, F. Carbognani, M. Carlassara, J. B. Carlin, T. K. Carlson, M. F. Carney, M. Carpinelli, G. Carrillo, J. J. Carter, G. Carullo, A. Casallas-Lagos, J. Casanueva Diaz, C. Casentini, S. Y. Castro-Lucas, S. Caudill, M. CavagliÃ, R. Cavalieri, A. Ceja, G. Cella, P. Cerdá-Durán, E. Cesarini, N. Chabbra, W. Chaibi, A. Chakraborty, P. Chakraborty, S. Chakraborty, S. Chalathadka Subrahmanya, J. C. L. Chan, M. Chan, K. Chang, S. Chao, P. Charlton, E. Chassande-Mottin, C. Chatterjee, Debarati Chatterjee, Deep Chatterjee, M. Chaturvedi, S. Chaty, K. Chatziioannou, A. Chen, A. H. -Y. Chen, D. Chen, H. Chen, H. Y. Chen, S. Chen, Yanbei Chen, Yitian Chen, H. P. Cheng, P. Chessa, H. T. Cheung, S. Y. Cheung, F. Chiadini, G. Chiarini, A. Chiba, A. Chincarini, M. L. Chiofalo, A. Chiummo, C. Chou, S. Choudhary, N. Christensen, S. S. Y. Chua, G. Ciani, P. Ciecielag, M. Cieślar, M. Cifaldi, B. Cirok, F. Clara, J. A. Clark, T. A. Clarke, P. Clearwater, S. Clesse, F. Cleva, E. Coccia, E. Codazzo, P. -F. Cohadon, S. Colace, E. Colangeli, M. Colleoni, C. G. Collette, J. Collins, S. Colloms, A. Colombo, C. M. Compton, G. Connolly, L. Conti, T. R. Corbitt, I. Cordero-Carrión, S. Corezzi, N. J. Cornish, I. Coronado, A. Corsi, R. Cottingham, M. W. Coughlin, A. Couineaux, P. Couvares, D. M. Coward, R. Coyne, A. Cozzumbo, J. D. E. Creighton, T. D. Creighton, P. Cremonese, S. Crook, R. Crouch, J. Csizmazia, J. R. Cudell, T. J. Cullen, A. Cumming, E. Cuoco, M. Cusinato, L. V. Da Conceição, T. Dal Canton, S. Dal Pra, G. Dálya, B. D'Angelo, S. Danilishin, S. D'Antonio, K. Danzmann, K. E. Darroch, L. P. Dartez, R. Das, A. Dasgupta, V. Dattilo, A. Daumas, N. Davari, I. Dave, A. Davenport, M. Davier, T. F. Davies, D. Davis, L. Davis, M. C. Davis, P. Davis, E. J. Daw, M. Dax, J. De Bolle, M. Deenadayalan, J. Degallaix, M. De Laurentis, F. De Lillo, S. Della Torre, W. Del Pozzo, A. Demagny, F. De Marco, G. Demasi, F. De Matteis, N. Demos, T. Dent, A. Depasse, N. DePergola, R. De Pietri, R. De Rosa, C. De Rossi, M. Desai, R. DeSalvo, A. DeSimone, R. De Simone, A. Dhani, R. Diab, M. C. Díaz, M. Di Cesare, G. Dideron, T. Dietrich, L. Di Fiore, C. Di Fronzo, M. Di Giovanni, T. Di Girolamo, D. Diksha, J. Ding, S. Di Pace, I. Di Palma, D. Di Piero, F. Di Renzo, Divyajyoti, A. Dmitriev, J. P. Docherty, Z. Doctor, N. Doerksen, E. Dohmen, A. Doke, A. Domiciano De Souza, L. D'Onofrio, F. Donovan, K. L. Dooley, T. Dooney, S. Doravari, O. Dorosh, W. J. D. Doyle, M. Drago, J. C. Driggers, L. Dunn, U. Dupletsa, P. -A. Duverne, D. D'Urso, P. Dutta Roy, H. Duval, S. E. Dwyer, C. Eassa, M. Ebersold, T. Eckhardt, G. Eddolls, A. Effler, J. Eichholz, H. Einsle, M. Eisenmann, M. Emma, K. Endo, R. Enficiaud, L. Errico, R. Espinosa, M. Esposito, R. C. Essick, H. Estellés, T. Etzel, M. Evans, T. Evstafyeva, B. E. Ewing, J. M. Ezquiaga, F. Fabrizi, V. Fafone, S. Fairhurst, A. M. Farah, B. Farr, W. M. Farr, G. Favaro, M. Favata, M. Fays, M. Fazio, J. Feicht, M. M. Fejer, R. Felicetti, E. Fenyvesi, J. Fernandes, T. Fernandes, D. Fernando, S. Ferraiuolo, T. A. Ferreira, F. Fidecaro, P. Figura, A. Fiori, I. Fiori, M. Fishbach, R. P. Fisher, R. Fittipaldi, V. Fiumara, R. Flaminio, S. M. Fleischer, L. S. Fleming, E. Floden, H. Fong, J. A. Font, F. Fontinele-Nunes, C. Foo, B. Fornal, K. Franceschetti, F. Frappez, S. Frasca, F. Frasconi, J. P. Freed, Z. Frei, A. Freise, O. Freitas, R. Frey, W. Frischhertz, P. Fritschel, V. V. Frolov, G. G. Fronzé, M. Fuentes-Garcia, S. Fujii, T. Fujimori, P. Fulda, M. Fyffe, B. Gadre, J. R. Gair, S. Galaudage, V. Galdi, R. Gamba, A. Gamboa, S. Gamoji, D. Ganapathy, A. Ganguly, B. Garaventa, J. García-Bellido, C. García-Quirós, J. W. Gardner, K. A. Gardner, S. Garg, J. Gargiulo, X. Garrido, A. Garron, F. Garufi, P. A. Garver, C. Gasbarra, B. Gateley, F. Gautier, V. Gayathri, T. Gayer, G. Gemme, A. Gennai, V. Gennari, J. George, R. George, O. Gerberding, L. Gergely, Archisman Ghosh, Sayantan Ghosh, Shaon Ghosh, Shrobana Ghosh, Suprovo Ghosh, Tathagata Ghosh, J. A. Giaime, K. D. Giardina, D. R. Gibson, C. Gier, S. Gkaitatzis, J. Glanzer, F. Glotin, J. Godfrey, R. V. Godley, P. Godwin, A. S. Goettel, E. Goetz, J. Golomb, S. Gomez Lopez, B. Goncharov, G. González, P. Goodarzi, S. Goode, A. W. Goodwin-Jones, M. Gosselin, R. Gouaty, D. W. Gould, K. Govorkova, A. Grado, V. Graham, A. E. Granados, M. Granata, V. Granata, S. Gras, P. Grassia, J. Graves, C. Gray, R. Gray, G. Greco, A. C. Green, L. Green, S. M. Green, S. R. Green, C. Greenberg, A. M. Gretarsson, H. K. Griffin, D. Griffith, H. L. Griggs, G. Grignani, C. Grimaud, H. Grote, S. Grunewald, D. Guerra, D. Guetta, G. M. Guidi, A. R. Guimaraes, H. K. Gulati, F. Gulminelli, H. Guo, W. Guo, Y. Guo, Anuradha Gupta, I. Gupta, N. C. Gupta, S. K. Gupta, V. Gupta, N. Gupte, J. Gurs, N. Gutierrez, N. Guttman, F. Guzman, D. Haba, M. Haberland, S. Haino, E. D. Hall, E. Z. Hamilton, G. Hammond, M. Haney, J. Hanks, C. Hanna, M. D. Hannam, O. A. Hannuksela, A. G. Hanselman, H. Hansen, J. Hanson, S. Hanumasagar, R. Harada, A. R. Hardison, S. Harikumar, K. Haris, I. Harley-Trochimczyk, T. Harmark, J. Harms, G. M. Harry, I. W. Harry, J. Hart, B. Haskell, C. J. Haster, K. Haughian, H. Hayakawa, K. Hayama, M. C. Heintze, J. Heinze, J. Heinzel, H. Heitmann, F. Hellman, A. F. Helmling-Cornell, G. Hemming, O. Henderson-Sapir, M. Hendry, I. S. Heng, M. H. Hennig, C. Henshaw, M. Heurs, A. L. Hewitt, J. Heynen, J. Heyns, S. Higginbotham, S. Hild, S. Hill, Y. Himemoto, N. Hirata, C. Hirose, D. Hofman, B. E. Hogan, N. A. Holland, I. J. Hollows, D. E. Holz, L. Honet, D. J. Horton-Bailey, J. Hough, S. Hourihane, N. T. Howard, E. J. Howell, C. G. Hoy, C. A. Hrishikesh, P. Hsi, H. -F. Hsieh, H. -Y. Hsieh, C. Hsiung, S. -H. Hsu, W. -F. Hsu, Q. Hu, H. Y. Huang, Y. Huang, Y. T. Huang, A. D. Huddart, B. Hughey, V. Hui, S. Husa, R. Huxford, L. Iampieri, G. A. Iandolo, M. Ianni, G. Iannone, J. Iascau, K. Ide, R. Iden, A. Ierardi, S. Ikeda, H. Imafuku, Y. Inoue, G. Iorio, P. Iosif, M. H. Iqbal, J. Irwin, R. Ishikawa, M. Isi, K. S. Isleif, Y. Itoh, M. Iwaya, B. R. Iyer, C. Jacquet, P. -E. Jacquet, T. Jacquot, S. J. Jadhav, S. P. Jadhav, M. Jain, T. Jain, A. L. James, K. Jani, J. Janquart, N. N. Janthalur, S. Jaraba, P. Jaranowski, R. Jaume, W. Javed, A. Jennings, M. Jensen, W. Jia, J. Jiang, H. -B. Jin, G. R. Johns, N. A. Johnson, M. C. Johnston, R. Johnston, N. Johny, D. H. Jones, D. I. Jones, R. Jones, H. E. Jose, P. Joshi, S. K. Joshi, G. Joubert, J. Ju, L. Ju, K. Jung, J. Junker, V. Juste, H. B. Kabagoz, T. Kajita, I. Kaku, V. Kalogera, M. Kalomenopoulos, M. Kamiizumi, N. Kanda, S. Kandhasamy, G. Kang, N. C. Kannachel, J. B. Kanner, S. A. KantiMahanty, S. J. Kapadia, D. P. Kapasi, M. Karthikeyan, M. Kasprzack, H. Kato, T. Kato, E. Katsavounidis, W. Katzman, R. Kaushik, K. Kawabe, R. Kawamoto, D. Keitel, L. J. Kemperman, J. Kennington, F. A. Kerkow, R. Kesharwani, J. S. 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A. Mercer, L. Mereni, K. Merfeld, E. L. Merilh, J. R. Mérou, J. D. Merritt, M. Merzougui, C. Messick, B. Mestichelli, M. Meyer-Conde, F. Meylahn, A. Mhaske, A. Miani, H. Miao, C. Michel, Y. Michimura, H. Middleton, D. P. Mihaylov, S. J. Miller, M. Millhouse, E. Milotti, V. Milotti, Y. Minenkov, E. M. Minihan, Ll. M. Mir, L. Mirasola, M. Miravet-Tenés, C. -A. Miritescu, A. Mishra, C. Mishra, T. Mishra, A. L. Mitchell, J. G. Mitchell, S. Mitra, V. P. Mitrofanov, K. Mitsuhashi, R. Mittleman, O. Miyakawa, S. Miyoki, A. Miyoko, G. Mo, L. Mobilia, S. R. P. Mohapatra, S. R. Mohite, M. Molina-Ruiz, M. Mondin, M. Montani, C. J. Moore, D. Moraru, A. More, S. More, C. Moreno, E. A. Moreno, G. Moreno, A. Moreso Serra, S. Morisaki, Y. Moriwaki, G. Morras, A. Moscatello, M. Mould, B. Mours, C. M. Mow-Lowry, L. Muccillo, F. Muciaccia, D. Mukherjee, Samanwaya Mukherjee, Soma Mukherjee, Subroto Mukherjee, Suvodip Mukherjee, N. Mukund, A. Mullavey, H. Mullock, J. Mundi, C. L. Mungioli, M. Murakoshi, P. 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Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine the universe is a giant, cosmic radio station. For years, scientists have been trying to tune into a specific kind of broadcast: a "hum" of gravitational waves. These aren't the loud "chirps" from colliding black holes that make headlines; they are a persistent, background static—a continuous whisper from billions of invisible sources, or perhaps a single, steady note from a spinning neutron star.
This paper is the report card from the LIGO-Virgo-KAGRA team after they spent years listening to this cosmic radio with their most sensitive equipment yet. They scanned the entire sky, looking for two types of signals: a steady "beep" from a specific spot (like a lighthouse) and a diffuse "static" that might be stronger in some directions than others (like a storm cloud).
The Big News: The Radio is Quiet
The main finding is simple but profound: They didn't hear anything. After combing through data from the first three observing runs and the very first part of the fourth run (O4a), the team found no evidence of these persistent gravitational waves. The cosmic radio is silent in the frequency range they checked.
Because they didn't find a signal, they didn't just shrug and walk away. Instead, they set the strictest "volume limits" ever recorded. Think of it like a detective saying, "I didn't find the thief, but I can prove with 95% confidence that the thief isn't hiding in this room, and if they were, they would have to be quieter than a whisper."
What They Ruled Out
The paper explicitly argues against the idea that there are strong, persistent gravitational waves coming from specific places or filling the sky in a lumpy way.
- No Cosmic Lighthouses: They looked at five famous "hotspots" in the sky, including the Galactic Center, a supernova remnant called SN 1987A, and a star system named Scorpius X-1. They ruled out the possibility that these objects are screaming out gravitational waves at levels the team could detect.
- No Lumpy Static: They checked if the background "hum" of the universe was stronger in some directions (anisotropy) due to the large-scale structure of the cosmos. They found no evidence of these "hotspots" in the static.
- No New Noise: They did spot a weird blip in the data at a frequency of 92.8125 Hz. However, after running simulations and checking the data, they concluded this was likely a glitch in the detector's own machinery, not a signal from space. It's like hearing a squeak in your car and realizing it's the seatbelt, not an alien engine.
How Sure Are They?
The team is very confident in their "silence." They didn't just guess; they used a powerful statistical tool called a "radiometer" (think of it as a super-advanced metal detector for space) to scan the sky.
- They combined data from O1, O2, O3, and the first part of O4.
- They found that their sensitivity improved by about 1.12 to 1.26 times compared to previous searches.
- For the specific targets they watched, they set upper limits on the "strain amplitude" (how much the signal stretches space) ranging from roughly 1.1 × 10⁻²⁵ to 6.5 × 10⁻²⁴.
- For the whole sky, they say the signal (if it exists) must be weaker than 0.03 to 8.4 × 10⁻²⁴ in the frequency range of 20 to 160 Hz.
The "What If" Game
The authors also ran simulations to see what would happen if a signal were there. They found that if a signal existed at the levels they are looking for, their detectors would have seen it. Since they didn't, the "volume" of the universe's gravitational hum must be incredibly low.
They also looked at the "noise" in their data. They found that while their detectors are getting better, they still have to deal with "notches" (frequencies they have to ignore because of Earth's vibrations or machinery). By combining data from different years, they managed to fill in some of these holes, making their search even more thorough.
The Bottom Line
This paper is a victory for precision, even without a discovery. By proving that the universe is quieter than we thought in these specific ranges, the team has narrowed the search for the next big discovery. They haven't found the cosmic hum yet, but they've built a much better map of where it isn't, and they've set the bar higher for the next time they tune the radio. The search continues, but for now, the sky is quiet.
Technical Summary: Directional Search for Persistent Gravitational Waves (LIGO-P250038)
Problem and Motivation
The gravitational-wave background (GWB) is a diffuse signal arising from the incoherent superposition of numerous unresolved sources, potentially including compact binary coalescences, rotating neutron stars, and early-universe phenomena. While often modeled as isotropic, the GWB may exhibit anisotropies due to the large-scale structure of the Universe, propagation effects, or local clustering of sources (e.g., in the Galactic plane). Previous searches by the LIGO-Virgo-KAGRA (LVK) Collaboration were limited by approaches that either targeted specific directions across narrow frequency bins or scanned the entire sky averaged over wide frequency ranges. These methods could not simultaneously explore the full angular and spectral properties of potential signals. Furthermore, matched-filtering searches for continuous waves (CW) are computationally expensive and rely on specific signal models. This paper addresses these limitations by presenting a comprehensive directional search for persistent gravitational-wave sources using data from the first portion of the fourth observing run (O4a), combined with data from O1, O2, and O3.
Methodology
The analysis employs the gravitational-wave radiometer technique, utilizing cross-correlation between geographically separated detector pairs to estimate the power spectral density of the GW strain field, P(f,n^). The study implements four distinct analysis strategies:
- All-Sky All-Frequency (ASAF) Radiometer: This unmodeled search scans the entire sky and the frequency band (20–160 Hz) in narrow bins (1/32 Hz) using a HEALPix grid (Nside=16). It targets point-like, persistent narrowband sources, such as continuous waves from neutron stars or narrowband GWBs from unresolved sources, without prior assumptions on location or frequency.
- Targeted Narrowband Radiometer (NBR): This analysis focuses on five specific astrophysically motivated directions: Scorpius X-1, the Galactic Center, SN 1987A, Terzan 5, and NGC 6397. Unlike the ASAF search, it accounts for signal spreading caused by Earth's Doppler modulation and source characteristics by combining information from neighboring frequency bins via a running average.
- Broadband Radiometer (BBR): Designed to detect point-like sources with broadband spectra, this search assumes the GWB power spectral density can be factorized into frequency and angular components. It tests three spectral index models (α=0,2/3,3) corresponding to cosmological inflation, compact binary coalescence, and flat strain power spectra, respectively.
- Spherical Harmonics (SPH) Search: To probe spatially extended or diffuse sources, this analysis decomposes the sky map into spherical harmonic modes (Yℓm). It computes the angular power spectrum (Cℓ) up to ℓmax=3,4,16 (depending on the spectral index) using both auto- and cross-correlation estimators to mitigate shot-noise bias.
Data from O1, O2, O3, and O4a were preprocessed to remove non-Gaussian features, hardware injections, and instrumental artifacts. The analysis utilizes the PyStoch package and employs a maximum-likelihood estimator to generate "dirty maps" and "clean maps," accounting for detector noise and antenna patterns.
Key Results
No evidence of persistent gravitational-wave signals was found in any of the four analyses. Consequently, the authors set the most stringent upper limits (ULs) to date on such emissions:
- ASAF Search: For narrowband point sources, the sensitivity estimate for the effective strain amplitude (heff) across the sky and frequency range (20–160 Hz) lies between (0.03−8.4)×10−24. The combined O1–O4a dataset shows a median sensitivity improvement of approximately 1.26 compared to O1–O3 alone. A marginal outlier was identified at 92.8125 Hz in the O4a data, but further investigation suggests it is likely a detector noise artifact rather than an astrophysical signal.
- Targeted NBR Search: For the five specific targets (Scorpius X-1, Galactic Center, SN 1987A, Terzan 5, NGC 6397), the 95% confidence ULs on strain amplitude range from ∼1.1×10−25 to 6.5×10−24. These results represent a median improvement by a factor of 1.6–1.7 over previous O1–O3 results. For Scorpius X-1, the limits remain above the torque-balance level.
- BBR Search: The gravitational-wave flux (F95%,UL) at 25 Hz is constrained to be less than (0.008−5.5)×10−8 erg cm−2 s−1 Hz−1, depending on the sky direction and spectral index.
- SPH Search: Upper limits on the strain angular power spectrum are placed at Cℓ1/2<(0.63−17)×10−10 sr−1.
Significance and Claims
The paper claims that these results constitute the most stringent constraints to date on persistent gravitational-wave emissions from both point-like and extended sources. By combining data from four observing runs and employing a unified framework that supports both pixel-based and spherical harmonic analyses, the study significantly enhances the ability to probe anisotropic sources. The authors emphasize that while no signals were detected, the methodology allows for a comprehensive exploration of potential anisotropic signals without prior assumptions on their location or frequency content. The paper also notes that these results are part of a broader effort, with companion papers addressing isotropic GWB searches and their cosmological implications. The work demonstrates the improved sensitivity of the global detector network and sets the stage for future searches with increased data volume and detector sensitivity.
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