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New constraints on the Mid-IR EBL from the HESS discovery of VHE gamma rays from 1ES 0229+200
HESS Collaboration, F. Aharonian
TL;DR
The paper investigates VHE gamma-ray emission from 1ES 0229+200 and uses HESS observations to constrain the mid-infrared EBL. The detection supports an approximately lambda^-1 EBL spectrum near source-count lower limits, while the source's intrinsic spectrum remains hard with its high-energy peak above a few TeV.
Problem
1ES 0229+200 had not previously been detected at VHE energies, limiting probes of the near- to mid-infrared EBL where direct estimates are missing and models differ substantially.
Method
HESS observed 1ES 0229+200 above 580 GeV in 2005 and 2006, reconstructing intrinsic spectra under different EBL shapes and testing their compatibility with Gamma_int >= 1.5.
Results
The observations detect a 6.6sigma VHE signal, while EBL-compatible reconstructions support an approximately lambda^-1 MIR spectrum near galaxy-count lower limits and yield an intrinsically hard source spectrum.
Takeaways & Limitations
The intrinsic spectrum remains significantly hard up to approximately 10 TeV, placing the source's high-energy SED peak above a few TeV and constraining the diffuse 5-10mum background.
Takeaways & Limitations
The EBL constraints rely on the assumption that the intrinsic spectrum is not harder than Gamma_int approximately 1.5; allowing Gamma_int approximately 1.0 relaxes them.
Abstract
from arXiv · showhide
AIMS: To investigate the very high energy (VHE: >100 GeV) gamma-ray emission from the high-frequency peaked BL Lac 1ES 0229+200. METHODS: Observations of 1ES 0229+200 at energies above 580 GeV were performed with the High Energy Stereoscopic System (HESS) in 2005 and 2006. RESULTS: 1ES 0229+200 is discovered by HESS to be an emitter of VHE photons. A signal is detected at the 6.6 sigma level in the HESS observations (41.8 h live time). The integral flux above 580 GeV is (9.4 +- 1.5 {stat} +- 1.9 {syst}) x 10^{-13} cm^{-2} s^{-1}, corresponding to ~1.8% of the flux observed from the Crab Nebula. The data show no evidence for significant variability on any time scale. The observed spectrum is characterized by a hard power law (Gamma = 2.50 +- 0.19 {stat} +- 0.10 {syst}) from 500 GeV to ~15 TeV. CONCLUSIONS: The high-energy range and hardness of the observed spectrum, coupled with the object's relatively large redshift (z=0.1396), enable the strongest constraints so far on the density of the Extragalactic Background Light (EBL) in the mid-infrared band. Assuming that the emitted spectrum is not harder than Gamma_{int} ~ 1.5, the HESS data support an EBL spectrum ~ lambda^{-1} and density close to the lower limit from source counts measured by Spitzer, confirming the previous indications from the HEGRA data of 1ES 1426+428 (z=0.129). Irrespective of the EBL models used, the intrinsic spectrum of 1ES 0229+200 is hard, thus locating the high-energy peak of its spectral energy distribution above a few TeV.
1. Introduction
1ES 0229+200 is a high-frequency peaked BL Lac at redshift z = 0.1396 that had been proposed as a VHE gamma-ray source but remained undetected. Its eventual multi-TeV detection provides information on the EBL.
- 1ES 0229+200 is a high-frequency peaked BL Lac hosted by an elliptical galaxy at redshift z = 0.1396.
- The source had been suggested as a potential VHE gamma-ray emitter based on its spectral energy distribution.
- Before this work, Whipple, HEGRA, Milagro, and HESS observations reported only upper limits on its VHE flux.
- The HESS discovery made 1ES 0229+200 the most distant object detected at multi-TeV energies, enabling constraints on the EBL.
2. Observations and Analysis Technique
HESS observed 1ES 0229+200 in 2005 and 2006, retaining 41.8 hours of quality-selected live time for standard analysis above a 580 GeV threshold.
- HESS collected 70.2 hours of observations in 2005 and 2006, of which 41.8 hours remained after data-quality selection.The observations had a mean zenith angle of 46°.
- Standard HESS event-selection criteria produced a post-analysis energy threshold of 580 GeV.
- The analysis used a 0.11° circular on-source region centered on 1ES 0229+200 and estimated background with the Reflected-Region method.
3. Results
HESS detected a significant, point-like VHE signal from 1ES 0229+200 and measured a hard, steady spectrum with no significant cutoff or break.
- 261 excess events produced a 6.6σ detection, and the fitted point source was consistent with the position of 1ES 0229+200.
- The observed spectrum follows a power law with photon index Γ = 2.50 ± 0.19stat ± 0.10syst and shows no significant cutoff or break.
- The integral flux above 580 GeV is (9.4±1.5stat±1.9syst)×10−13 cm−2 s−1, equivalent to approximately 1.8% of the Crab Nebula flux.
- The flux shows no evidence for significant variability on monthly, yearly, or nightly timescales.
4. Discussion
The HESS spectrum of 1ES 0229+200 constrains the MIR EBL toward a steep, low-density shape while implying an unusually hard intrinsic spectrum under tested EBL models.
- The VHE spectrum probes the poorly measured NIR-to-MIR EBL range because γ-γ absorption links TeV photons to wavelengths of roughly 2–20 µm.At z = 0.140, galaxy-evolution corrections change Γ by less than 0.08.
- De-absorption with the Stecker et al. models gives extremely hard intrinsic indices, Γint = 0.6 ± 0.3 and 0.1 ± 0.3.These values are difficult to explain with commonly used BL Lac emission scenarios without extreme assumptions.
- The Prim05 EBL model yields Γint = 1.92 ± 0.22 but lies below Spitzer MIR source-count lower limits, motivating modified EBL shapes.
- For approximately λ^-1 EBL shapes, the reconstructed intrinsic spectrum changes little because the optical depth has weak energy dependence across the observed range.The overall scaling limit is 0.8× P1.0, or 1.8× Prim05.
- The MIR EBL is constrained to a slope α ≳1.1 ± 0.25, with source-count-consistent shapes yielding Γint ∼1.5.Flatter MIR EBL shapes are not supported by the HESS data.
5. Conclusions
The HESS detection shows that 1ES 0229+200 has a hard intrinsic spectrum extending to roughly 10 TeV. This supports a MIR EBL near galaxy-count lower limits and places the SED peak above a few TeV, while motivating further contemporaneous observations.
- The HESS spectrum implies an MIR EBL approximately proportional to λ^-1 and close to Spitzer and ISO galaxy-count lower limits unless Γint is considerably harder than 1.5.This suggests resolved Spitzer sources account for most of the diffuse 5–10 µm background.
- The intrinsic spectrum remains significantly hard up to ∼10 TeV across EBL details, placing the high-energy SED peak above a few TeV.
- The low optical state may indicate a correspondingly low VHE state, although evidence for a VHE-optical flux correlation is limited.Further VHE and contemporaneous lower-energy observations are motivated for SED modeling.