Breaking the Redshift Barrier: H.E.S.S. and the Distant Blazar PKS 0346−27

In this episode, we dive into a groundbreaking discovery in high-energy astrophysics: the detection of the blazar PKS 0346−27 at a redshift of $z = 0.991$. This makes it one of the most distant objects ever detected in very-high-energy (VHE) gamma-rays ($E > 100$ GeV). We explore how the H.E.S.S. (High Energy Stereoscopic System) telescopes in Namibia managed to capture this elusive signal despite the thick "fog" of Extragalactic Background Light (EBL) that usually absorbs such distant photons.

Key Discussion Points:

• The Record-Breaking Detection: Why reaching a redshift of approximately 1 is a major milestone for gamma-ray astronomy and what it tells us about the evolution of the universe.
• A Tale of Two Flares: The strange two-day delay between the high-energy flare caught by the Fermi-LAT satellite and the very-high-energy flare detected by H.E.S.S..
• The Physics of the Jet: We break down the debate between leptonic and hadronic models. While electrons are the usual suspects, the data from PKS 0346−27 strongly favors a proton-synchrotron model, even though it requires jet power that temporarily exceeds the source’s Eddington limit.
• Multi-Wavelength Cooperation: How a global team used data from H.E.S.S., Fermi-LAT, the Swift Observatory, and the ATOM telescope to build a complete picture of this cosmic event.
• The "Synchrotron Mirror" Hypothesis: Exploring how stationary clouds near the black hole might be reflecting radiation back into the jet to create "orphan" VHE flares.

Technical Insight: The researchers found that a traditional leptonic model (based on electrons) would require "implausible" parameters, such as a Doppler factor exceeding 80, to explain the flare. This push toward hadronic models suggests that relativistic protons may play a much larger role in the most powerful jets in the universe than previously confirmed.

Featured Article: H.E.S.S. Collaboration, et al. (2026). "H.E.S.S. detection and multi-wavelength study of the $z \sim 1$ blazar PKS 0346−27." Astronomy & Astrophysics manuscript no. 0346.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Stefan Schwarzburg