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Probing the ATIC peak in the cosmic-ray electron spectrum with H.E.S.S
H. E. S. S. Collaboration, :, F. Aharonian
TL;DR
ATIC’s reported electron-spectrum excess, together with PAMELA’s positron-fraction rise, motivated dark-matter and nearby-source interpretations. This paper extends H.E.S.S. measurements down to 340 GeV and finds a smooth spectrum that excludes a pronounced ATIC-like peak while steepening near 1 TeV.
Problem
ATIC’s reported electron-spectrum peak, combined with PAMELA’s positron-fraction rise, prompted dark-matter and nearby-source interpretations requiring further measurement of the high-energy electron spectrum.
Method
H.E.S.S. extended its cosmic-ray electron-spectrum analysis toward lower energies using extragalactic fields, revised event selection, and improved hadronic-background rejection.
Results
The H.E.S.S. spectrum follows a broken power law with Γ1 = 3.0 ± 0.1, Γ2 = 4.1 ± 0.3, and Eb = 0.9 ± 0.1 TeV, showing no pronounced ATIC-like excess or sharp cutoff.
Takeaways & Limitations
The combined H.E.S.S. and FERMI measurements make an electron-spectrum feature in their overlap region unlikely, while the tested 620 GeV Kaluza–Klein scenario is incompatible with H.E.S.S. at 99% confidence.
Takeaways & Limitations
H.E.S.S. cannot test the rising section of the ATIC excess because its measurement starts only above 340 GeV; its energy-scale uncertainty is about 15%.
Abstract
from arXiv · showhide
The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has - together with the PAMELA detection of a rise in the positron fraction up to 100 GeV - motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H.E.S.S. starting at 340 GeV. While the overall electron flux measured by H.E.S.S. is consistent with the ATIC data within statistical and systematic errors, the H.E.S.S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H.E.S.S. data follow a power-law spectrum with spectral index of 3.0 +- 0.1 (stat.) +- 0.3 (syst.), which steepens at about 1 TeV.
1. Introduction
Very-high-energy cosmic-ray electrons cool rapidly, so their observed spectrum is expected to be shaped by a few nearby sources. The ATIC excess, combined with PAMELA’s positron-fraction rise, motivated dark-matter and nearby-source interpretations that H.E.S.S. could test.
- E ≳ 100 GeV cosmic-ray electrons lose energy rapidly through inverse Compton scattering and synchrotron radiation, limiting their cooling time and range.
- The resulting short propagation range implies that very-high-energy electrons must originate from a few nearby sources.
- ATIC reported an electron-spectrum excess appearing as a peak in E^3 Φ(E), approximated by a power law of index around 2 with a sharp cutoff near 620 GeV.
- Together with PAMELA’s positron-fraction rise, the ATIC peak motivated interpretations involving dark matter annihilation or a nearby pulsar.
- Imaging atmospheric Cherenkov telescopes offer much larger collection areas than balloon and satellite experiments, enabling high-statistics measurements of TeV electrons.
2. The low-energy extension of the H.E.S.S. electron measurement
H.E.S.S. extended its electron-spectrum measurement down to 340 GeV using a revised event selection, then fit the resulting spectrum with a broken power law. The spectrum is smooth, steepens near 1 TeV, and carries substantial systematic uncertainties from atmospheric and hadronic-interaction modeling.
- H.E.S.S. used extragalactic fields and excluded known or potential γ-ray sources to avoid γ-ray contamination of the electron signal.The diffuse extragalactic γ-ray contribution was estimated to be less than 6% under the stated blazar-spectrum assumptions.
- 340 GeV is the lowered analysis threshold enabled by the new data and event selection, with an effective collection area of ≈4×10^4 m^2.The analysis used 77 hours of good-quality data and achieved an effective exposure of ≈2.2 × 10^7 m^2 sr s at 340 GeV.
- The ζ distribution for 0.34–0.7 TeV showers was compared with simulated proton and electron distributions, using a best-fit electron–proton mixture and SIBYLL proton simulations.
- The spectrum is well described by a broken power law with normalization k = (1.5 ± 0.1) × 10^-4 TeV^-1 m^-2 sr^-1 s^-1 and break energy Eb = 0.9 ± 0.1 TeV.
- The fitted indices are Γ1 = 3.0 ± 0.1 below the break and Γ2 = 4.1 ± 0.3 above it, with a preferred sharp transition α < 0.3.
- Systematic uncertainties are about 30% for flux normalization, while the spectral-index uncertainty is ΔΓ(syst.) ≲ 0.3 and the H.E.S.S. energy-scale uncertainty is 15%.
3. Interpretation
H.E.S.S. finds no pronounced ATIC-like peak and instead observes a spectrum that steepens toward higher energies. The data are consistent with FERMI and conventional astrophysical electron populations, while the tested 620 GeV Kaluza–Klein scenario is difficult to reconcile with the measurement.
- The H.E.S.S. spectrum shows no indication of the excess and sharp cutoff reported by ATIC.An overall ATIC–H.E.S.S. consistency remains possible within the H.E.S.S. energy-scale uncertainty.
- The nominal H.E.S.S. data agree very well with FERMI up to 1 TeV, making an electron-spectrum feature in their overlap region unlikely.
- The tested Kaluza–Klein model uses a 620 GeV particle and an ATIC-fitted flux, but it cannot be easily reconciled with the H.E.S.S. measurement at the 99% confidence level.Figure 3 compares the KK signature, background model, and their sum with H.E.S.S. and balloon measurements.
- The spectrum steepens toward higher energies and is compatible with conventional astrophysical electron populations within injection-spectrum and propagation uncertainties.