Fermi-LAT CollaborationMirabal, NestorNegro, MichelaValverde, Janethet al.2022-09-262022-09-262022-04-07Fermi-LAT Collaboration. "A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background." SCIENCE, 376, no. 6592 (7 Apr 2022): 521-523. DOI: 10.1126/science.abm3231https://doi.org/10.1126/science.abm3231http://hdl.handle.net/11603/25891Fermi-LAT Collaboration Authors: M. Ajello , W. B. Atwood , L. Baldini , J. Ballet , G. Barbiellini, , D. Bastieri, , R. Bellazzini , A. Berretta, B. Bhattacharyya, E. Bissaldi, R. D. Blandford, E. Bloom, R. Bonino, , P. Bruel, R. Buehler, E. Burns, S. Buson, R. A. Cameron, P. A. Caraveo, E. Cavazzuti , N. Cibrario, S. Ciprini, C. J. Clark, I. Cognard, J. Coronado-Blazquez ´ , , M. Crnogorcevic, H. Cromartie, K. Crowter, S. Cutini, F. D’Ammando, S. De Gaetano, , F. de Palma, S. W. Digel, N. Di Lalla, F. Fana Dirirsa, L. Di Venere, A. Dom´ınguez , E. C. Ferrara, A. Fiori , A. Franckowiak, Y. Fukazawa, S. Funk, P. Fusco, , V. Gammaldi, F. Gargano, D. Gasparrini, N. Giglietto, F. Giordano, M. Giroletti , D. Green, I. A. Grenier , L. Guillemot, S. Guiriec, M. Gustafsson, A. K. Harding , E. Hays, J.W. Hewitt, D. Horan, X. Hou, G. Johannesson ´ , M. J. Keith, M. Kerr† , M. Kramer, M. Kuss , S. Larsson, L. Latronico, J. Li, F. Longo, , F. Loparco, , M. N. Lovellette, P. Lubrano, S. Maldera, A. Manfreda , G. Mart´ı-Devesa, M. N. Mazziotta , I.Mereu, P. F. Michelson, N. Mirabal, W. Mitthumsiri, T. Mizuno, M. E. Monzani , A. Morselli, M. Negro, L. Nieder, R. Ojha, N. Omodei, M. Orienti, E. Orlando, , J. F. Ormes, D. Paneque, A. Parthasarathy† , Z. Pei , M. Persic, M. Pesce-Rollins , R. Pillera, H. Poon, T. A. Porter, G. Principe, J. L. Racusin, S. Raino` , R. Rando, , B. Rani, S. M. Ransom, P. S. Ray, M. Razzano , S. Razzaque, A. Reimer , O. Reimer, J. Roy, M. Sanchez-Conde ´ , P. M. Saz Parkinson, J. Scargle, L. Scotton , D. Serini, C. Sgro` , E. J. Siskind, D. A. Smith, G. Spandre , R. Spiewak, P. Spinelli, , I. Stairs, D. J. Suson, S. J. Swihart, S. Tabassum, J. B. Thayer, G. Theureau, , D. F. Torres, E. Troja, J. Valverde, Z. Wadiasingh, K. Wood, G. ZaharijasAfter large galaxies merge, their central supermassive black holes are expected to form binary systems. Their orbital motion should generate a gravitational wave background (GWB) at nanohertz frequencies. Searches for this background use pulsar timing arrays, which perform long-term monitoring of millisecond pulsars at radio wavelengths. We used 12.5 years of Fermi Large Area Telescope data to form a gamma-ray pulsar timing array. Results from 35 bright gamma-ray pulsars place a 95% credible limit on the GWB characteristic strain of 1.0 × 10−14 at a frequency of 1 year–1. The sensitivity is expected to scale with tobs, the observing time span, as t−13/6obs. This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.3 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0http://creativecommons.org/publicdomain/mark/1.0/Text