Journal article
Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
Monthly notices of the Royal Astronomical Society, Vol.507(4), pp.5161-5176
11/01/2021
DOI: 10.1093/mnras/stab2406
Abstract
Millisecond pulsars are very likely the main source of gamma-ray emission from globular clusters. However, the relative contributions of two separate emission processes - curvature radiation from millisecond pulsar magnetospheres versus inverse Compton emission from relativistic pairs launched into the globular cluster environment by millisecond pulsars - have long been unclear. To address this, we search for evidence of inverse Compton emission in 8-yr Fermi-LAT data from the directions of 157 Milky Way globular clusters. We find a mildly statistically significant (3.8 sigma) correlation between the measured globular cluster gamma-ray luminosities and their photon field energy densities. However, this may also be explained by a hidden correlation between the photon field densities and the stellar encounter rates of globular clusters. Analysed in toto, we demonstrate that the gamma-ray emission of globular clusters can be resolved spectrally into two components: (i) an exponentially cut-off power law and (ii) a pure power law. The latter component - which we uncover at a significance of 8.2 sigma - has a power index of 2.79 +/- 0.25. It is most naturally interpreted as inverse Compton emission by cosmic-ray electrons and positrons injected by millisecond pulsars. We find the luminosity of this power-law component is comparable to, or slightly smaller than, the luminosity of the curved component, suggesting the fraction of millisecond pulsar spin-down luminosity into relativistic leptons is similar to the fraction of the spin-down luminosity into prompt magnetospheric radiation.
Details
- Title: Subtitle
- Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
- Creators
- Deheng Song - Virginia TechOscar Macias - Virginia TechShunsaku Horiuchi - Virginia TechRoland M. Crocker - Australian National UniversityDavid M. Nataf - Johns Hopkins University
- Resource Type
- Journal article
- Publication Details
- Monthly notices of the Royal Astronomical Society, Vol.507(4), pp.5161-5176
- DOI
- 10.1093/mnras/stab2406
- ISSN
- 0035-8711
- eISSN
- 1365-2966
- Publisher
- Oxford Univ Press
- Number of pages
- 16
- Grant note
- JP17H04836; JP18H04340; JP18H04578; JP20K14463 / Japan Society for the Promotion of Science KAKENHI Grant; Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT); Japan Society for the Promotion of Science; Grants-in-Aid for Scientific Research (KAKENHI) DE-SC0020262 / United States Department of Energy; United States Department of Energy (DOE) DP190101258 / Australian Government through the Australian Research Council; Australian Research Council World Premier International Research Centre Initiative (WPI Initiative), Ministry of Education, Culture, Sports, Science and Technology, Japan 80NSSC19K0589 / National Aeronautics and SpacAdministration AST1908960; PHY-1914409 / National Science Foundation; National Science Foundation (NSF)
- Language
- English
- Date published
- 11/01/2021
- Academic Unit
- Physics and Astronomy
- Record Identifier
- 9984701730202771
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