Evidence for a high-energy tail in the gamma-ray spectra of globular clusters

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 vs. 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-year $\text{Fermi}$-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 $\text{in toto}$, 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 $\pm$ 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.

16+9 pages, 13+4 figures, 4+1 tables. match the published version in MNRAS

References in corpus (24)

• The ATNF Pulsar Catalogue
• N-body modeling of globular clusters: Masses, mass-to-light ratios and intermediate-mass black holes
• High-Energy Gamma Rays from the Milky Way: Three-Dimensional Spatial Models for the Cosmic-Ray and Radiation Field Densities in the Interstellar Medium
• Fermi Large Area Telescope Fourth Source Catalog Data Release 2
• The $\gamma$-ray Emission of Star-Forming Galaxies
• Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda?
• Millisecond Pulsars and Black Holes in Globular Clusters
• The age of the young bulge-like population in the stellar system Terzan5: linking the Galactic bulge to the high-z Universe
• The global mass functions of 35 Galactic globular clusters: I. Observational data and correlations with cluster parameters
• Strong constraints on thermal relic dark matter from Fermi-LAT observations of the Galactic Center
• Six millisecond pulsars detected by the Fermi Large Area Telescope and the radio/gamma-ray connection of millisecond pulsars
• The Gamma-Ray Pulsar Population of Globular Clusters: Implications for the GeV Excess
• The Relationship Between Globular Cluster Mass, Metallicity, and Light Element Abundance Variations
• Pulsed Gamma-rays from the millisecond pulsar J0030+0451 with the Fermi Large Area Telescope
• Evidence of TeV Halos Around Millisecond Pulsars
• Comparing the Galactic Bulge and Galactic Disk Millisecond Pulsars
• Pulsars in Globular Clusters
• Millisecond Pulsars Modify the Radio-SFR Correlation in Quiescent Galaxies
• Cherenkov Telescope Array sensitivity to the putative millisecond pulsar population responsible for the Galactic center excess
• The Emission Physics of Millisecond Pulsars
• Inverse Compton emission from millisecond pulsars in the Galactic bulge
• Deep observations of the globular cluster M15 with the MAGIC telescopes
• Gamma-ray emission from globular clusters 2MS-GC01, IC 1257, NGC5904, NGC6656 and FSR1735
• Identifying the brightest Galactic globular clusters for future observations by H.E.S.S. and CTA

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• Cherenkov Telescope Array sensitivity to the putative millisecond pulsar population responsible for the Galactic center excess
• Discovery of gamma-ray pulsations of PSR J1835$-$3259B in the Globular Cluster NGC 6652
• Gamma-ray spectral properties of the Galactic globular clusters: constraint on the numbers of millisecond pulsars
• Likely detection of gamma-ray pulsations of PSR~J1717+4308A in NGC~6341 and implication of the gamma-ray millisecond pulsars in globular clusters
• The Andromeda Gamma-Ray Excess: Background Systematics of the Millisecond Pulsars and Dark Matter Interpretations
• On the gamma-ray emission from the core of the Sagittarius dwarf galaxy
• Timing of Pulsars in the Globular Cluster Omega Centauri
• A blazar candidate for the Fermi source 4FGL J1848.7-0129
• A possible GeV-TeV connection in the direction of the Globular Cluster UKS 1
• Characterizing the Gamma-ray Emission Properties of the Globular Cluster M5 with the Fermi-LAT
• Search for TeV emission from spider millisecond pulsars with HAWC
• Measuring the smearing of the Galactic 511 keV signal: positron propagation or supernova kicks?
• Discovery and Timing of the First Millisecond Pulsar in NGC 6316
• Gamma-Ray Millisecond Pulsars: Off-pulse Emission Characteristics, Phase-Resolved Pseudo-Luminosity--Cutoff Energy Correlation, and High-energy Pulsed Emission