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Title: Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?
Authors: M. Ackermann
M. Ajello
A. Albert
L. Baldini
J. Ballet
G. Barbiellini
D. Bastieri
R. Bellazzini
E. Bissaldi
E. D. Bloom
R. Bonino
E. Bottacini
T. J. Brandt
J. Bregeon
P. Bruel
R. Buehler
R. A. Cameron
R. Caputo
M. Caragiulo
P. A. Caraveo
E. Cavazzuti
C. Cecchi
E. Charles
A. Chekhtman
G. Chiaro
S. Ciprini
F. Costanza
S. Cutini
F. D'Ammando
F. De Palma
R. Desiante
S. W. Digel
N. Di Lalla
M. Di Mauro
L. Di Venere
C. Favuzzi
S. Funk
P. Fusco
F. Gargano
N. Giglietto
F. Giordano
M. Giroletti
T. Glanzman
D. Green
I. A. Grenier
L. Guillemot
S. Guiriec
K. Hayashi
X. Hou
G. Jóhannesson
T. Kamae
J. Knödlseder
A. K.H. Kong
M. Kuss
G. La Mura
S. Larsson
L. Latronico
J. Li
F. Longo
F. Loparco
P. Lubrano
S. Maldera
D. Malyshev
A. Manfreda
P. Martin
M. N. Mazziotta
P. F. Michelson
N. Mirabal
W. Mitthumsiri
T. Mizuno
M. E. Monzani
A. Morselli
I. V. Moskalenko
M. Negro
Deutsche Elektronen-Synchrotron
Clemson University
Los Alamos National Laboratory
Università di Pisa
Institut de Recherche sur les Lois Fondamentales de l'Univers
Istituto Nazionale di Fisica Nucleare, Roma
Università degli Studi di Trieste
Istituto Nazionale Di Fisica Nucleare, Sezione di Padova
Università degli Studi di Padova
Istituto Nazionale Di Fisica Nucleare, Sezione di Pisa
Kavli Institute for Particle Astrophysics and Cosmology
Università degli Studi di Torino
NASA Goddard Space Flight Center
Laboratoire Univers et Particules de Montpellier
Laboratoire Leprince-Ringuet
Santa Cruz Institute for Particle Physics
Università degli Studi di Bari
INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan
Agenzia Spaziale Italiana
Università degli Studi di Perugia
George Mason University, Fairfax Campus
Istituto Di Radioastronomia, Bologna
Alma Mater Studiorum Università di Bologna
Università degli Studi di Napoli Federico II
Università degli Studi di Udine
Erlangen Centre for Astroparticle Physics
University of Maryland
Universite d'Orleans
Institut National des Sciences de l'Univers
Nagoya University
Chinese Academy of Sciences
National Tsing Hua University
University Science Institute Reykjavik
University of Tokyo
CNRS Centre National de la Recherche Scientifique
Université de Toulouse
The Royal Institute of Technology (KTH)
Oskar Klein Centre for Cosmoparticle Physics
Universitat Autònoma de Barcelona
Mahidol University
Hiroshima University
University of Denver
Max Planck Institut für Physik (Werner-Heisenberg-Institut)
Osservatorio Astronomico di Trieste
Medizinische Universitat Innsbruck
Stockholms universitet
NYCB Realtime Computing Inc.
Max Planck Institut für Kernphysik
Institucio Catalana de Recerca I Estudis Avancats
Naval Research Laboratory
Keywords: Earth and Planetary Sciences
Issue Date: 20-Feb-2017
Citation: Astrophysical Journal. Vol.836, No.2 (2017)
Abstract: © 2017. The American Astronomical Society. All rights reserved. The Fermi Large Area Telescope (LAT) has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using 2 yr of observations with the Fermi LAT, Local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range , presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of 2.0 × 10-12 erg cm-2 s-1 on the 0.1-100 GeV energy flux (95% confidence level). This revised upper limit remains consistent with the observed correlation between gamma-ray luminosity and star formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10σ. Its spectrum is consistent with a power law with photon index Γ = 2.4 ± 0.1stat+syst and a 0.1-100 GeV energy flux of (5.6 × 0.6stat+syst) × 10-12erg cm-2s-1. M31 is detected to be extended with a 4σ significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0.°4 and no offset from the center of the galaxy, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic center excess found in Fermi-LAT observations of the MW.
ISSN: 15384357
Appears in Collections:Scopus 2016-2017

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