THE ORIGIN OF GAMMA RAYS FROM GLOBULAR CLUSTERS

Journal Article · Wed Nov 10 04:00:00 EST 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/723/2/1219· OSTI ID:21471265

Cheng, K S ^[1]; Chernyshov, D O ^[2]; Dogiel, V A ^[3]; Hui, C Y ^[4]; Kong, A K. H. ^[5]

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1. Department of Physics, University of Hong Kong, Pokfulam Road (Hong Kong)
2. Moscow Institute of Physics and Technology, Institutskii lane, 141700 Moscow Region, Dolgoprudnii (Russian Federation)
3. I. E. Tamm Theoretical Physics Division of P. N. Lebedev Institute, Leninskii pr, 53, 119991 Moscow (Russian Federation)
4. Department of Astronomy and Space Science, Chungnam National University, Daejeon (Korea, Republic of)
5. Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu, Taiwan (China)

Fermi has detected gamma-ray emission from eight globular clusters (GCs). It is commonly believed that the energy sources of these gamma rays are millisecond pulsars (MSPs) inside GCs. Also it has been standard to explain the spectra of most Fermi Large Area Telescope pulsars including MSPs resulting from the curvature radiation (CR) of relativistic electrons/positrons inside the pulsar magnetosphere. Therefore, gamma rays from GCs are expected to be the collection of CR from all MSPs inside the clusters. However, the angular resolution is not high enough to pinpoint the nature of the emission. In this paper, we calculate the gamma rays produced by the inverse Compton (IC) scattering between relativistic electrons/positrons in the pulsar wind of MSPs in the GCs and background soft photons including cosmic microwave/relic photons, background star lights in the clusters, the galactic infrared photons, and the galactic star lights. We show that the gamma-ray spectrum from 47 Tucanae can be explained equally well by upward scattering of either the relic photons, the galactic infrared photons, or the galactic star lights, whereas the gamma-ray spectra from the other seven GCs are best fitted by the upward scattering of either the galactic infrared photons or the galactic star lights. We also find that the observed gamma-ray luminosity is correlated better with the combined factor of the encounter rate and the background soft photon energy density. Therefore, the IC scattering may also contribute to the observed gamma-ray emission from GCs detected by Fermi in addition to the standard CR process. Furthermore, we find that the emission region of high-energy photons from GCs produced by the IC scattering is substantially larger than the cores of GCs with a radius >10 pc. The diffuse radio and X-rays emitted from GCs can also be produced by the synchrotron radiation and IC scattering, respectively. We suggest that future observations including radio, X-rays, and gamma rays with energy higher than 10 GeV and better angular resolution can provide better constraints for the models.

• OSTI ID: 21471265
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 723; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ANTILEPTONS
ANTIMATTER
ANTIPARTICLES
COSMIC RADIO SOURCES
ELECTROMAGNETIC RADIATION
ELECTRONS
ELEMENTARY PARTICLES
EMISSION
ENERGY DENSITY
ENERGY RANGE
ENERGY SOURCES
EVOLUTION
FERMIONS
GAMMA RADIATION
GAMMA SPECTRA
IONIZING RADIATIONS
LEPTONS
LUMINOSITY
MATTER
OPTICAL PROPERTIES
PHOTON EMISSION
PHYSICAL PROPERTIES
POSITRONS
PULSARS
RADIATIONS
RELATIVISTIC RANGE
SCATTERING
SPECTRA
STAR CLUSTERS
STAR EVOLUTION
STARS
TELESCOPES