A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope

¹ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
² National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA
³ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
⁵ Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France
⁶ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34127 Trieste, Italy
⁷ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
⁹ Dipartimento di Fisica “G. Galilei”, Università di Padova, 35131 Padova, Italy
¹⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
¹¹ Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
¹² Centre d’Étude Spatiale des Rayonnements, CNRS/UPS, BP 44346, 31028 Toulouse Cedex 4, France
e-mail: jurgen.knodlseder@cesr.fr; natalie.webb@cesr.fr; benoit.pancrazi@cesr.fr
¹³ Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
¹⁴ Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, 70126 Bari, Italy
¹⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
¹⁶ Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France
¹⁷ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, 20133 Milano, Italy
¹⁸ George Mason University, Fairfax, VA 22030, USA
¹⁹ Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, Montpellier, France
²⁰ Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²¹ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
²² Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation, Swedish
²³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁴ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁵ CNRS/IN2P3, Centre d’Études Nucléaires Bordeaux Gradignan, UMR 5797, 33175 Gradignan, France
²⁶ Université de Bordeaux, Centre d’Études Nucléaires Bordeaux Gradignan, UMR 5797, 33175 Gradignan, France
²⁷ Dipartimento di Fisica, Università di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, 33100 Udine, Italy
²⁸ Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, 34143 Trieste, Italy
²⁹ Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³⁰ Agenzia Spaziale Italiana (ASI) Science Data Center, 00044 Frascati (Roma), Italy
³¹ Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA
³² Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
³³ Research Institute for Science andEngineering, Waseda University, 3-4-1, Okubo, Shinjuku, 169-8555 Tokyo, Japan
³⁴ Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
³⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, 00133 Roma, Italy
³⁶ Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
³⁷ Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
³⁸ Max-Planck Institut für extraterrestrische Physik, 85748 Garching, Germany
³⁹ Institut für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
⁴⁰ Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
⁴¹ NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
⁴² Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA
⁴³ Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, 229-8510 Kanagawa, Japan
⁴⁵ Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
⁴⁶ Consorzio Interuniversitario per la Fisica Spaziale (CIFS), 10133 Torino, Italy
⁴⁷ INTEGRAL Science Data Centre, 1290 Versoix, Switzerland
⁴⁸ Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴⁹ Department of Physics and Center for Space Sciences and Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
⁵⁰ North-West University, Potchefstroom Campus, Potchefstroom 2520, South Africa
⁵¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133 Roma, Italy
⁵² Department of Physics, Royal Institute of Technology (KTH), AlbaNova, 106 91 Stockholm, Sweden
⁵³ School of Pure and Applied Natural Sciences, University of Kalmar, 391 82 Kalmar, Sweden
⁵⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 18 March 2010
Accepted: 13 August 2010

Abstract

Context. Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. The observation of this emission provides a powerful tool to assess the millisecond pulsar population of a cluster, is essential for understanding the importance of binary systems for the evolution of globular clusters, and provides complementary insights into magnetospheric emission processes.

Aims. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations.

Methods. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters.

Results. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices (0.7 < Γ < 1.4) and clear evidence for an exponential cut-off in the range 1.0 − 2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices (1.0 < Γ < 1.7), however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600 − 4700 MSPs in Galactic globular clusters, commensurate with previous estimates.

Conclusions. The observation of high-energy gamma-ray emission from globular clusters thus provides a reliable independent method to assess their millisecond pulsar populations.