Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. 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. 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 $(0.7 < \Gamma <1.4)$ 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 $(1.0 < \Gamma < 1.7)$, 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. The observation of high-energy gamma-ray emission from a globular cluster thus provides a reliable independent method to assess their millisecond pulsar populations that can be used to make constraints on the original neutron star X-ray binary population, essential for understanding the importance of binary systems in slowing the inevitable core collapse of globular clusters.
Deep Dive into A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope.
Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. 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. 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 $(0.7 < \Gamma <1.4)$ 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 $(1.0 < \Gamma < 1.7)$, however the presence of an exponential cut-off can
arXiv:1003.3588v2 [astro-ph.GA] 18 Aug 2010
Astronomy & Astrophysics manuscript no. 14458
c⃝ESO 2018
October 30, 2018
A population of gamma-ray emitting globular clusters seen with the
Fermi Large Area Telescope
A. A. Abdo1,2, M. Ackermann3, M. Ajello3, L. Baldini4, J. Ballet5, G. Barbiellini6,7, D. Bastieri8,9, R. Bellazzini4,
R. D. Blandford3, E. D. Bloom3, E. Bonamente10,11, A. W. Borgland3, A. Bouvier3, T. J. Brandt12,13, J. Bregeon4,
M. Brigida14,15, P. Bruel16, R. Buehler3, S. Buson8, G. A. Caliandro14,15, R. A. Cameron3, P. A. Caraveo17,
S. Carrigan9, J. M. Casandjian5, E. Charles3, S. Chaty5, A. Chekhtman1,18, C. C. Cheung1,2, J. Chiang3, S. Ciprini11,
R. Claus3, J. Cohen-Tanugi19, J. Conrad20,21,22, M. E. DeCesar23,24, C. D. Dermer1, F. de Palma14,15, S. W. Digel3,
E. do Couto e Silva3, P. S. Drell3, R. Dubois3, D. Dumora25,26, C. Favuzzi14,15, P. Fortin16, M. Frailis27,28,
Y. Fukazawa29, P. Fusco14,15, F. Gargano15, D. Gasparrini30, N. Gehrels23, S. Germani10,11, N. Giglietto14,15,
F. Giordano14,15, T. Glanzman3, G. Godfrey3, I. Grenier5, M.-H. Grondin25,26, J. E. Grove1, L. Guillemot54,
S. Guiriec31, D. Hadasch32, A. K. Harding23, E. Hays23, P. Jean12, G. J´ohannesson3, T. J. Johnson23,24, W. N. Johnson1,
T. Kamae3, H. Katagiri29, J. Kataoka33, M. Kerr34, J. Kn¨odlseder12, M. Kuss4, J. Lande3, L. Latronico4, S.-H. Lee3,
M. Lemoine-Goumard25,26, M. Llena Garde20,21, F. Longo6,7, F. Loparco14,15, M. N. Lovellette1, P. Lubrano10,11,
A. Makeev1,18, M. N. Mazziotta15, P. F. Michelson3, W. Mitthumsiri3, T. Mizuno29, C. Monte14,15, M. E. Monzani3,
A. Morselli35, I. V. Moskalenko3, S. Murgia3, M. Naumann-Godo5, P. L. Nolan3, J. P. Norris36, E. Nuss19, T. Ohsugi37,
N. Omodei3, E. Orlando38, J. F. Ormes36, B. Pancrazi12, D. Parent1,18, M. Pepe10,11, M. Pesce-Rollins4, F. Piron19,
T. A. Porter3, S. Rain`o14,15, R. Rando8,9, A. Reimer39,3, O. Reimer39,3, T. Reposeur25,26, J. Ripken20,21, R. W. Romani3,
M. Roth34, H. F.-W. Sadrozinski40, P. M. Saz Parkinson40, C. Sgr`o4, E. J. Siskind41, D. A. Smith25,26, P. Spinelli14,15,
M. S. Strickman1, D. J. Suson42, H. Takahashi37, T. Takahashi43, T. Tanaka3, J. B. Thayer3, J. G. Thayer3,
L. Tibaldo8,9,5,44, D. F. Torres32,45, G. Tosti10,11, A. Tramacere3,46,47, Y. Uchiyama3, T. L. Usher3, V. Vasileiou48,49,
C. Venter50, N. Vilchez12, V. Vitale35,51, A. P. Waite3, P. Wang3, N. Webb12, B. L. Winer13, Z. Yang20,21,
T. Ylinen52,53,21, and M. Ziegler40
(Affiliations can be found after the references)
Received March 18, 2010; accepted August 13, 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-offcan 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.
Key words. Pulsars: general – Globular clusters: NGC 104, NGC 5139, NGC 6266, NGC 6388, Terzan 5, NGC 6440, NGC 6441, NGC 6541,
NGC 6624, NGC 6626, NGC 6652, NGC 6752, NGC 7078 – Gamma rays: observations
1. Introduction
With their typical ages of ∼1010 years, globular clusters form
the most ancient constituents of our Milky Way Galaxy. These
gravitationally bound concentrations of ten thousand to one mil-
lion stars are surprisingly stable against collapse which implies
some sou
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