J/MNRAS/507/5161 Gamma-ray spectra of globular clusters in MW (Song+, 2021)
Evidence for a high-energy tail in the gamma-ray spectra of globular clusters.
Song D., Macias O., Horiuchi S., Crocker R.M., Nataf D.M.
<Mon. Not. R. Astron. Soc. 507, 5161-5176 (2021)>
=2021MNRAS.507.5161S 2021MNRAS.507.5161S (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Clusters, globular ; Pulsars ; Gamma rays ;
Abundances, [Fe/H] ; Stars, masses ; Photometry ; Infrared ;
Optical ; Ultraviolet ; X-ray sources ; Photometry ;
Stars, distances
Keywords: pulsars: general - globular clusters: general - gamma-rays: general
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. 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.
Description:
In Milky Way's GCs, we are searching for the evidence of inverse
Compton (IC) γ-ray emission in the GeV-TeV energy range. Here,
motivated by the increasing number of GCs detected in γ-rays, we
perform a collective statistical study of their properties in order to
gain insight into the nature of their high-energy emission.
In the recently published Fermi-LAT fourth source catalogue
(Abdollahi et al. 2020ApJS..247...33A 2020ApJS..247...33A, Cat. J/ApJS/247/33) (4FGL), 30
GCs have been detected in GeV γ-rays. We repeat the bin-by-bin
analysis of the 4FGL data for the 157 known Milky Way GCs in the
Harris (1996AJ....112.1487H 1996AJ....112.1487H, Cat. VII/195) catalogue. After our
Globular cluster sample construction (30 GCs {gamma]-rays detected
with 4FGL) and data analysis (please refer to sections 2.1 and 2.2 for
complete procedure), we report 127 additional GCs {gamma]-rays not
detected in the Harris (1996AJ....112.1487H 1996AJ....112.1487H, Cat. VII/195) catalogue.
Thus, the tablea1.dat regroups the parameters and γ-ray analysis
of these GCs.
In fact, those ones will be useful in the section 3 Correlation
analysis especially, the algorithm uses the upper limits of Fγ
and Lγ to perform maximum likelihood estimates and find the
best-fit parameters for the correlations with the other GC observables.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 68 127 Parameters and data analysis results of 127 GCs
not detected in the 4FGL data
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See also:
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
J/ApJ/766/136 : Stellar encounter rates in Galactic GCs (Bahramian+, 2013)
J/MNRAS/478/1520 : Milky Way globular clusters data (Baumgardt+, 2018)
J/ApJS/247/33 : The Fermi LAT fourth source catalog (4FGL) (Abdollahi+,2020)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- ID Cluster identification number (ID) (1)
13- 19 F7.2 --- Gammac ?=- Integrated stellar encounter rate
(Γc) (2)
21- 25 F5.2 [Sun] [Fe/H] ?=- Iron abundance ([Fe/H])
27- 31 F5.2 [10+5Msun] M* ?=- Stellar mass (M*) (3)
33- 36 F4.2 eV/cm3 uMW Galactic Milky Way photon field energy
density (uMW) (4)
38 A1 --- l_uT Lower limit on uTotal (l_uT)
40- 45 F6.2 eV/cm3 uT Total photon field energy density
(uTotal) (5)
47- 52 F6.2 kpc R Distance from the Sun (R_☉)
54 A1 --- l_Fgamma Upper limit flag on Fgamma (l_Fgamma)
56- 59 F4.2 10-8ph/cm2/s Fgamma 95 per cent confidence level γ-ray
upper limits photon flux (Fγ) (6)
61 A1 --- l_Lgamma Upper limit flag on Lgamma (l_Lgamma)
63- 68 F6.2 10+27W Lgamma 95 per cent confidence level γ-ray
upper limits luminosity (Lγ) (6)
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Note (1): Taken from Harris (1996AJ....112.1487H 1996AJ....112.1487H, Cat. VII/202) with properties
of 147 global clusters in the Milky Way.
Note (2): In this work, we adopt the stellar encounter rate estimated Γc
by Bahramian et al. (2013ApJ...766..136B 2013ApJ...766..136B, Cat. J/ApJ/766/136) which is
defined as the equation 2 in the section 2.1 Globular cluster sample
(4πσ-1∫ρ2r2dr) where σ is the velocity
dispersion at the core radius and ρ is the stellar density profile
of the cluster, and the line-of-sight integration is performed along
the half-light radius.
Note (3): Adopted from Baumgardt (2017MNRAS.464.2174B 2017MNRAS.464.2174B), Sollima & Baumgardt
(2017MNRAS.471.3668S 2017MNRAS.471.3668S), and Baumgardt & Hilker (2018MNRAS.478.1520B 2018MNRAS.478.1520B,
Cat. J/MNRAS/478/1520). They are estimated from N-body modelling of
the velocity dispersion and surface density profiles.
Note (4): In this work, we improve upon the Galactic radiation field model used
by Hui et al. (2011ApJ...726..100H 2011ApJ...726..100H) by extracting the energy density
of the interstellar radiation at the locations of the GCs from the 3D
interstellar radiation model in GALPROP v563 (Porter et al.
2017ApJ...846...67P 2017ApJ...846...67P; Johannesson et al. 2018ApJ...856...45J 2018ApJ...856...45J).
This is a fully 3D model that combines the CMB, infrared, and optical
photons of the Milky Way, denoted as uMW.
Note (5): In addition, photons from stars in the GCs are expected to make a
dominant contribution to the total, ambient radiation field. We
estimate this component by uGC = L*/4*π*c*rh2 where L*
and rh are the stellar luminosity and the half-light radius of the
GC. The uTotal is defined as the sum of uMW and uGC.
Note (6): For fluxes and L, we report their 95 per cent confidence level upper
limits by placing a putative point source at the sky location of the
GC and running a maximum-likelihood procedure in which we assume a
power-law spectrum dN/dE ∼ E-2. We applied the same pipeline used
on the set of detected GCs, photons are from 300 MeV to 500 GeV.
(see section 2.2 Data analysis).
The luminosity is estimated with 4*π*R2 times the energy flux.
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History:
From electronic version of the journal
(End) Luc Trabelsi [CDS] 24-Jul-2024