Kavli Affiliate: Shunsaku Horiuchi
| First 5 Authors: Deheng Song, Oscar Macias, Shunsaku Horiuchi, Roland M. Crocker, David M. Nataf
| Summary:
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
$textit{Fermi}$-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 $textit{in toto}$, 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.
| Search Query: ArXiv Query: search_query=au:”Shunsaku Horiuchi”&id_list=&start=0&max_results=10