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 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-year Fermi-LAT data from the directions of 157 Milky Way globular clusters. We find a mildly statistically significant (3.8σ) 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 8.2σ–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.

Gamma-ray spectral properties of the Galactic globular clusters: constraint on the numbers of millisecond pulsars

“…The fitting thus indicates that γ-ray emission in 27 of the GCs can be explained with MSPs contained in them. This result is different from that found by Song et al (2021), since our fitting results do not show the need of an additional power-law component (due to inverse-Compton scattering of infrared and optical photons). We note that several power-law like spectra (e.g., NGC 6093, NGC 6139, NGC 6304; see Appendix Figures B2, B3 & B4) can actually be fit with our PLEC-like spectral form, partly due to the large flux uncertainties, and in some cases the flux upper limits do require a curved spectrum (e.g., NGC 6304).…”

“…Thus both deviations are not sufficiently significant such that additional source of γ-ray emission would be definitely needed. Recently Song et al (2021) have applied a double-component (PLEC plus power law) model to spectral fitting for the γ-ray GCs and stated that a power law component is significantly present. Although the significances in our studies are not high, the deviations at the high-energy tails of the spectra of NGC 104 and Terzan 5 do hint the existence of an extra component in γ-ray emission from these two GCs.…”

“…The radio results seem to be consistent with our upper-limit results, suggesting that the GCs probably do have many MSPs. Considering their relatively high Γ e , these GC cases suggest that the number of MSPs in a GC may not be directly indicated by Γ e ; other factors could play important roles in determining the number of MSPs a GC contains (e.g., de Menezes et al 2019;Song et al 2021). Deep radio observations of these GCs would help clarify their cases.…”

“…From high-energy observations with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi), globular clusters (GCs) in our Milky Way have been identified as a class of γ-ray sources in the sky (Abdo et al 2009;Abdollahi et al 2020). Thus far, approximately 30 GCs have been detected at γ-rays (Abdo et al 2009(Abdo et al , 2010Kong et al 2010;Tam et al 2011;Zhou et al 2015;Zhang et al 2016;Lloyd et al 2018;de Menezes et al 2019;Song et al 2021; see also Table 1). Their γ-ray emission presumably arises from millisecond pulsars (MSPs) that are contained in.…”

“…Given the relatively well determined spectral shape for MSPs, γ-ray emission of the GCs can be studied by fitting their spectra with that of MSPs and thus the numbers of MSPs in the GCs can be estimated. Previously, γ-ray spectra of the GCs have been studied in detail by Lloyd et al (2018) and (Song et al 2021) through spectral fitting using different models. Spectral types different from PLEC or additional spectral components have been claimed to be present, resulting in suggestions that other types of high-energy sources in the GCs may +0.15 −0.16 and Ec = 1.45 +0.60 −0.35 GeV, while the 3σ region is indicated by the gray area.…”