What did the CTAO LST Collaboration and MAGIC Collaboration’s observations, published in Astronomy & Astrophysics on August 15, 2026, reveal about OP 313—a flat-spectrum radio quasar at redshift 0.997—including how the LST-1 and twin MAGIC telescopes detected its very-high-energy

The observations established OP 313 (z = 0.997) as the most distant blazar yet detected in very-high-energy (VHE, >100 GeV) gamma rays. During its December 2023 outburst, LST-1 and the two MAGIC telescopes measured significant VHE emission—about 0.3 Crab units above 100 GeV—despite the source lying roughly 8 billion light-years away. 3

• How it was detected: LST-1 followed a Fermi-LAT alert for heightened activity; its low energy threshold and large collection area made the detection possible. MAGIC observations independently supported and extended the VHE measurement. These atmospheric-Cherenkov telescopes infer gamma rays from the brief particle showers and Cherenkov flashes the photons produce in Earth’s atmosphere. 73

• Why the distance matters: At this redshift, the detected photons were emitted when the Universe was much younger, after the peak epoch of cosmic star formation (“cosmic noon”), and then propagated for about 8 billion years. OP 313 is the distance record holder specifically for a VHE blazar/AGN—not necessarily for every kind of VHE transient. 31

• A probe of diffuse cosmic light: Along that path, VHE gamma rays collide with optical-to-infrared photons of the extragalactic background light (EBL), the accumulated radiation from stars and galaxies. Such collisions can produce electron–positron pairs, removing gamma rays from the beam and imprinting an energy-dependent attenuation signature. Modeling the intrinsic spectrum jointly with the Fermi, LST-1, and MAGIC data therefore set strong upper limits on the EBL’s total density; for example, the LST-1 fit gave λIλ < 8.74 nW m⁻² sr⁻¹ at 0.6 μm. 3

• What powers the flare: The broadband data are consistent with highly relativistic electrons in OP 313’s jet inverse-Compton scattering lower-energy photons, boosting them into the gamma-ray band. The jet points close to our line of sight, so relativistic beaming makes this emission exceptionally bright. 3

• Instrumental significance: The result was LST-1’s first discovery while still in commissioning, demonstrating that the prototype can access the low-energy VHE regime essential for distant, strongly EBL-attenuated sources. The planned four-LST CTAO-North subarray at La Palma—scheduled for inauguration on October 15—is intended to reach roughly 20 GeV, improving sensitivity to still more distant gamma-ray emitters. 1245