Unresolved Gamma-Ray Sky through its Angular Power Spectrum

Ackermann M, Ajello M, Baldini L, Ballet J, Barbiellini G, Bastieri D, Bellazzini R, Bissaldi E, Blandford RD, Bonino R, Bottacini E, Bregeon J, Bruel P, Buehler R, Burns E, Buson S, Cameron RA, Caputo R, Caraveo PA, Cavazzuti E, Chen S, Chiaro G, Ciprini S, Costantin D, Cuoco A, Cutini S, D'Ammando F, Luque PDLT, De Palma F, Desai A, Digel SW, Di Lalla N, Di Mauro M, Di Venere L, Dirirsa FF, Favuzzi C, Franckowiak A, Fukazawa Y, Funk S, Fusco P, Gargano F, Gasparrini D, Giglietto N, Giordano F, Giroletti M, Green D, Grenier IA, Guillemot L, Guiriec S, Horan D, Johannesson G, Kuss M, Larsson S, Latronico L, Li J, Liodakis I, Longo F, Loparco F, Lubrano P, Magill JD, Maldera S, Malyshev D, Manfreda A, Mazziotta MN, Mereu I, Michelson PF, Mitthumsiri W, Mizuno T, Monzani ME, Morselli A, Moskalenko IV, Negro M, Nuss E, Orienti M, Orlando E, Palatiello M, Paliya VS, Paneque D, Persic M, Pesce-Rollins M, Petrosian V, Piron F, Porter TA, Principe G, Raino S, Rando R, Razzano M, Razzaque S, Reimer A, Reimer O, Serini D, Sgro C, Siskind EJ, Spandre G, Spinelli P, Suson DJ, Tajima H, Takahashi M, Thayer JB, Tibaldo L, Torres DF, Troja E, Venters TM, Vianello G, Wood K, Yassine M, Zaharijas G, Ammazzalorso S, Fornengo N, Regis M (2018)

Publication Type: Journal article

Publication year: 2018


Book Volume: 121

Journal Issue: 24

DOI: 10.1103/PhysRevLett.121.241101


The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi-large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission [unresolved gamma-ray background (UGRB)] below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This Letter presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi-LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with similar to 3.7 sigma significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55 +/- 0.23 and 1.86 +/- 0.15.

Authors with CRIS profile

Involved external institutions

Deutsches Elektronen-Synchrotron DESY DE Germany (DE) Clemson University US United States (USA) (US) University of Pisa / Università di Pisa (UniPi) IT Italy (IT) National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) IT Italy (IT) Stanford University US United States (USA) (US) University of Montpellier / Université Montpellier FR France (FR) École Polytechnique - Université Paris-Saclay FR France (FR) National Aeronautics and Space Administration (NASA) US United States (USA) (US) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) IT Italy (IT) Italian Space Agency / Agenzia Spaziale Italiana (ASI) IT Italy (IT) University of Padua / Universita degli Studi di Padova IT Italy (IT) Politecnico di Bari IT Italy (IT) University of Johannesburg ZA South Africa (ZA) Hiroshima University / 広島大学 JP Japan (JP) Commissariat à l'énergie atomique FR France (FR) University of Orléans / Université d'Orléans FR France (FR) University of Iceland (UI) / Háskóli Íslands IS Iceland (IS) AlbaNova University Centre / Kungliga Tekniska Högskolan AlbaNova Universitetscentrum SE Sweden (SE) University of Maryland US United States (USA) (US) NYCB Real-Time Computing, Inc. US United States (USA) (US) Purdue University Northwest / Purdue University Calumet US United States (USA) (US) Università degli Studi di Trieste IT Italy (IT) Università degli Studi di Perugia IT Italy (IT) Mahidol University / มหาวิทยาลัยมหิดล TH Thailand (TH) Max-Planck-Institut für Physik (Werner-Heisenberg-Institut, MPP) / Max Planck Institute for Physics DE Germany (DE) Université Fédérale de Toulouse Midi-Pyrénées FR France (FR) Institute of Space Sciences (IEEC-CSIC) ES Spain (ES) Praxis, Inc. US United States (USA) (US)

How to cite


Ackermann, M., Ajello, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D.,... Regis, M. (2018). Unresolved Gamma-Ray Sky through its Angular Power Spectrum. Physical Review Letters, 121(24). https://doi.org/10.1103/PhysRevLett.121.241101


Ackermann, M., et al. "Unresolved Gamma-Ray Sky through its Angular Power Spectrum." Physical Review Letters 121.24 (2018).

BibTeX: Download