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-yr 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. Here, we find the luminosity of this power-law component is comparablemore »
- Authors:
-
[1];
[3];
- Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)
- Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States); Univ. of Tokyo, Chiba (Japan); Univ. of Amsterdam (The Netherlands)
- Australian National Univ., Canberra, ACT (Australia)
- Johns Hopkins Univ., Baltimore, MD (United States)
- Publication Date:
- Research Org.:
- Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)
- Sponsoring Org.:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- OSTI Identifier:
- 1837494
- Grant/Contract Number:
- SC0020262
- Resource Type:
- Accepted Manuscript
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Additional Journal Information:
- Journal Volume: 507; Journal Issue: 4; Journal ID: ISSN 0035-8711
- Publisher:
- Royal Astronomical Society
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 79 ASTRONOMY AND ASTROPHYSICS; pulsars: general; globular clusters: general; gamma-rays: general
Citation Formats
Song, Deheng, Macias, Oscar, Horiuchi, Shunsaku, Crocker, Roland M., and Nataf, David M. Evidence for a high-energy tail in the gamma-ray spectra of globular clusters. United States: N. p., 2021.
Web. doi:10.1093/mnras/stab2406.
Song, Deheng, Macias, Oscar, Horiuchi, Shunsaku, Crocker, Roland M., & Nataf, David M. Evidence for a high-energy tail in the gamma-ray spectra of globular clusters. United States. https://doi.org/10.1093/mnras/stab2406
Song, Deheng, Macias, Oscar, Horiuchi, Shunsaku, Crocker, Roland M., and Nataf, David M. Mon .
"Evidence for a high-energy tail in the gamma-ray spectra of globular clusters". United States. https://doi.org/10.1093/mnras/stab2406. https://www.osti.gov/servlets/purl/1837494.
@article{osti_1837494,
title = {Evidence for a high-energy tail in the gamma-ray spectra of globular clusters},
author = {Song, Deheng and Macias, Oscar and Horiuchi, Shunsaku and Crocker, Roland M. and Nataf, David M.},
abstractNote = {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 (3.8σ) 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. Here, 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.},
doi = {10.1093/mnras/stab2406},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 4,
volume = 507,
place = {United States},
year = {Mon Aug 23 00:00:00 EDT 2021},
month = {Mon Aug 23 00:00:00 EDT 2021}
}
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