[Paper Review] Search for TeV emission from the Fermi Bubbles at low Galactic latitudes with H.E.S.S. inner Galaxy survey observations

E. Moulin, Alessandro Montanari+2 more|arXiv (Cornell University)|Aug 23, 2021

Dark Matter and Cosmic Phenomena13 references4 citations

TL;DR

This study presents the first H.E.S.S. search for TeV gamma-ray emission from the base of the Fermi Bubbles at low Galactic latitudes using data from the Inner Galaxy Survey (2014–2020). Despite 546 hours of high-quality observations, no significant excess is detected, yielding a 95% confidence level upper limit of ~2×10⁻⁹ TeV⁻¹cm⁻²sr⁻¹ at 1 TeV, constraining hadronic and leptonic models of the bubbles' origin.

ABSTRACT

The Fermi Bubbles were discovered about a decade ago in the {\it Fermi}-LAT data as a double-lobe structure extending up to 55 deg. in Galactic latitudes above and below the Galactic Center. At the moment their origin is still unknown. The H.E.S.S. collaboration is currently performing the first ever survey in TeV gamma rays of the Milky Way inner region: the Inner Galaxy Survey. This survey is intended to achieve the best sensitivity to faint and diffuse emissions in a region of several degrees around the Galactic Centre. It provides an unprecedented sensitivity to dark matter signals, new diffuse emissions, and TeV outflows from the Galactic Centre. Understanding the properties of the Fermi Bubbles at low Galactic latitudes will provide key insights into their origin. We search for TeV emission at the base of the Fermi Bubbles using low-latitude spatial templates. The first results obtained with the 2014-2020 H.E.S.S. observations will be reported.

Motivation & Objective

• To search for very-high-energy (VHE) gamma-ray emission from the base of the Fermi Bubbles at low Galactic latitudes using H.E.S.S. data.
• To probe the origin of the Fermi Bubbles by testing hadronic and leptonic emission models via TeV observations.
• To leverage the H.E.S.S. Inner Galaxy Survey's sensitivity to detect faint and diffuse VHE signals near the Galactic Center.
• To assess whether the hard spectral index observed by Fermi-LAT up to 1 TeV persists in the VHE band.
• To provide critical constraints on models involving past AGN activity or star formation near the Galactic Center.

Proposed method

• The analysis uses 546 hours of high-quality H.E.S.S. observations from 2014 to 2020, collected via the Inner Galaxy Survey at zenith angles <40°.
• A region of interest (ROI) is defined where Fermi-LAT surface brightness exceeds 8.5 sr⁻¹, centered at l ≈ -1°, b ≈ 2°.
• The reflected background method is applied run-by-run to estimate background in symmetric OFF regions, ensuring equal solid angle and acceptance.
• Excluded regions (masks) are applied to avoid contamination from nearby VHE sources in signal and background regions.
• Energy-differential flux upper limits are computed at 95% confidence level in 0.2 dex energy bins, assuming a power-law index Γ = 1.9 and 20% systematic uncertainty.
• Joint spectral modeling with Fermi-LAT data is performed, assuming a power-law spectrum consistent with Fermi-LAT results above 10 GeV.

Experimental results

Research questions

• RQ1 Does the Fermi Bubble emission extend to TeV energies, as suggested by the hard Fermi-LAT spectrum up to 1 TeV?
• RQ2 Can the observed gamma-ray spectrum at the base of the bubbles be explained by hadronic or leptonic processes?
• RQ3 Is there evidence for a VHE component at the base of the Fermi Bubbles that would distinguish between AGN-like outbursts and star formation-driven outflows?
• RQ4 What is the sensitivity of H.E.S.S. to diffuse VHE emission from the Galactic Center region, particularly near the base of the Fermi Bubbles?
• RQ5 How do the H.E.S.S. upper limits constrain the intrinsic spectral shape and emission mechanisms of the Fermi Bubbles?

Key findings

• No significant gamma-ray excess is detected in the H.E.S.S. region of interest, indicating no statistically significant TeV emission from the base of the Fermi Bubbles.
• The 95% confidence level upper limit on the differential flux at 1 TeV is ~2×10⁻⁹ TeV⁻¹cm⁻²sr⁻¹, consistent with the absence of a strong VHE component.
• The upper limit is computed under the assumption of a power-law spectrum with index Γ = 1.9, matching the Fermi-LAT spectral index at high energies.
• The analysis accounts for a 20% systematic uncertainty in flux calibration, ensuring robust upper limit estimation.
• The results are consistent with the Fermi-LAT spectrum remaining hard up to 1 TeV, but do not confirm a VHE component.
• The study sets a stringent constraint on hadronic models, disfavoring strong proton-proton interactions in the bubble medium at TeV energies.