How LST-1 and MAGIC detected gamma rays from the most distant known VHE blazar

The observations reported by the CTAO LST Collaboration and the MAGIC Collaboration have established OP 313 as the most distant blazar yet detected in very-high-energy (VHE) gamma rays—those with energies above 100 GeV. The flat-spectrum radio quasar lies at redshift 0.997, roughly 8 billion light-years away. During an outburst in December 2023, the telescopes measured VHE emission at about 0.3 Crab units above 100 GeV. 3

A detection triggered by a flare

LST-1, the prototype Large-Sized Telescope of the Cherenkov Telescope Array Observatory, began observing OP 313 after NASA’s Fermi Large Area Telescope (Fermi-LAT) reported heightened activity. Its combination of a very low energy threshold and large collection area enabled it to detect the faint, highly attenuated signal from such a distant source. Observations with the two MAGIC telescopes independently supported and extended the measurement. 73

Neither LST-1 nor MAGIC observes gamma rays as a conventional optical telescope would. Instead, when a gamma ray enters Earth’s atmosphere, it produces a brief cascade of secondary particles. Those particles generate faint flashes of blue Cherenkov light, which the ground-based telescopes record and analyse to reconstruct the original gamma ray.

A message from the young Universe

Because of OP 313’s distance, the photons detected during the flare began their journey when the Universe was far younger than it is today. The source is seen from an era after “cosmic noon”—the period when cosmic star formation reached its peak—and the gamma rays then travelled for about 8 billion years before reaching Earth. 31

The record is specific: OP 313 is the most distant blazar or active galactic nucleus detected at VHE energies. It is not necessarily the most distant object ever observed in every category of VHE transient.

Gamma rays as a backlight for cosmic radiation

The long journey also turned the signal into a probe of the extragalactic background light, or EBL. This is the accumulated optical-to-infrared radiation produced by stars and galaxies throughout cosmic history.

As VHE gamma rays pass through the EBL, they can collide with lower-energy photons and convert into electron–positron pairs. That process removes some gamma rays from the beam and produces an energy-dependent weakening, or attenuation, in the spectrum measured at Earth. By comparing the observed spectrum with models of OP 313’s intrinsic emission, researchers can estimate how much intervening background light is allowed.

Combining Fermi, LST-1 and MAGIC data produced stringent upper limits on the EBL’s total density. For example, the LST-1 analysis found an upper limit of λIλ < 8.74 nW m⁻² sr⁻¹ at a wavelength of 0.6 μm. 3

What powered the flare?

The broadband observations are consistent with a leptonic model in which highly relativistic electrons inside OP 313’s jet boost lower-energy photons to gamma-ray energies through inverse-Compton scattering. The jet is aimed relatively close to Earth, so relativistic beaming concentrates and amplifies the radiation in our direction. 3

That combination—a rapidly moving electron population, a powerful jet and favourable viewing geometry—helps explain how a source so distant could produce a detectable VHE signal despite the strong absorption expected during its journey.

A milestone for LST-1—and a preview of what comes next

The OP 313 result was LST-1’s first scientific discovery while the telescope was still in commissioning. It demonstrates that the prototype can reach the low-energy end of the VHE range, where distant sources are especially valuable but their higher-energy photons are more strongly suppressed by the EBL.

The planned four-telescope CTAO-North LST subarray at La Palma is designed to improve this capability, with sensitivity extending to roughly 20 GeV. Its inauguration is scheduled for 15 October 2026. By collecting more of the lower-energy signal, the completed array should improve the prospects for detecting and studying even more distant gamma-ray emitters. 1245