A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background

Ajello, M.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Berretta, A.; Bhattacharyya, B.; Bissaldi, E.; Blandford, R. D.; Bloom, E.; Bonino, R.; Bruel, P.; Buehler, R.; Burns, E.; Buson, S.; Cameron, R. A.; Caraveo, P. A.; Cavazzuti, E.; Cibrario, N.; Ciprini, S.; Clark, C. J.; Cognard, I.; Coronado-Bl??zquez, J.; Crnogorcevic, M.; Cromartie, H.; Crowter, K.; Cutini, S.; D'Ammando, F.; De Gaetano, S.; de Palma, F.; Digel, S. W.; Di Lalla, N.; Fana Dirirsa, F.; Di Venere, L.; Dom??nguez, A.; Ferrara, E. C.; Fiori, A.; Franckowiak, A.; Fukazawa, Y.; Funk, S.; Fusco, P.; Gammaldi, V.; Gargano, F.; Gasparrini, D.; Giglietto, N.; Giordano, F.; Giroletti, M.; Green, D.; Grenier, I. A.; Guillemot, L.; Guiriec, S.; Gustafsson, M.; Harding, A. K.; Hays, E.; Hewitt, J. W.; Horan, D.; Hou, X.; J??hannesson, G.; Keith, M. J.; Kerr, M.; Kramer, M.; Kuss, M.; Larsson, S.; Latronico, L.; Li, J.; Longo, F.; Loparco, F.; Lovellette, M. N.; Lubrano, P.; Maldera, S.; Manfreda, A.; Mart??-Devesa, G.; Mazziotta, M. N.; Mereu, I.; Michelson, P. F.; Mirabal, N.; Mitthumsiri, W.; Mizuno, T.; Monzani, M. E.; Morselli, A.; Negro, M.; Nieder, L.; Ojha, R.; Omodei, N.; Orienti, M.; Orlando, E.; Ormes, J. F.; Paneque, D.; Parthasarathy, A.; Pei, Z.; Persic, M.; Pesce-Rollins, M.; Pillera, R.; Poon, H.; Porter, T. A.; Principe, G.; Racusin, J. L.; Rain??, S.; Rando, R.; Rani, B.; Ransom, S. M.; Ray, P. S.; Razzano, M.; Razzaque, S.; Reimer, A.; Reimer, O.; Roy, J.; S??nchez-Conde, M.; Saz Parkinson, P. M.; Scargle, J.; Scotton, L.; Serini, D.; Sgr??, C.; Siskind, E. J.; Smith, D. A.; Spandre, G.; Spiewak, R.; Spinelli, P.; Stairs, I.; Suson, D. J.; Swihart, S. J.; Tabassum, S.; Thayer, J. B.; Theureau, G.; Torres, D. F.; Troja, E.; Valverde, J.; Wadiasingh, Z.; Wood, K.; Zaharijas, G.

doi:10.1126/science.abm3231

After 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 x 10(-14) at a frequency of 1 year(-1). The sensitivity is expected to scale with t(obs), the observing time span, as t(obs)(-13/6). This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.