Distant blazar OP 313 emits very high-energy gamma rays above 100 GeV

June 3, 2026 report Distant blazar OP 313 emits very high-energy gamma rays above 100 GeV Tomasz Nowakowski Author Stephanie Baum Scientific Editor Robert Egan Associate Editor An international team of astronomers have employed one of the Large-Sized Telescopes (LSTs) at the Cherenkov Telescope Array Observatory (CTAO) to observe a distant blazar known as OP 313. Results of the observational campaign, published May 26 on the arXiv preprint server, shed more light on the behavior and nature of this object. A general view of blazars Blazars are extremely compact quasi-stellar objects (quasars) associated with supermassive black holes (SMBHs) at the centers of active, giant elliptical galaxies. They are the most luminous and extreme subclass of active galactic nuclei (AGNs). The characteristic features of blazars are highly collimated relativistic jets oriented very close to our line of sight. Based on their optical emission properties, astronomers generally divide blazars into two classes: flat-spectrum radio quasars (FSRQs) that feature prominent and broad optical emission lines, and BL Lacertae objects (BL Lacs), which do not. OP 313 draws the attention of astronomers Initially identified in 1959 as a variable star, OP 313 (also known as B2 1308+326) is a blazar of the FSRQ class at a redshift of approximately 0.997. It is so far the most distant quasar detected at very high energy (VHE) gamma rays. A group of astronomers, led by Kenshin Abe of the Tokai University in Japan, decided to take a closer look at OP 313. For this purpose, they used the Large-Sized Telescope prototype (LST-1), which is the first of the four LSTs that will be part of CTAO at the Roque de los Muchachos Observatory on the Canary island of La Palma (Spain). "LST-1 is equipped with a 23-m diameter mirror dish and a camera composed of 1,855 high quantum-efficiency photomultipliers. These characteristics provide LST-1 with a large collection area and high sensitivity, enabling the detection of gamma-ray events down to energies of about 20 GeV, and making LST-1 the ideal instrument for the observation of distant VHE gamma-ray emitters," the researchers explained. Observations identify VHE gamma-ray emission The observations identified significant VHE gamma-ray emission from OP 313 during a flare that occurred in December 2023. The emission was at a level of 0.3 Crab Units above 100 GeV, with no significant night-wise variability detected. According to the study, the observed spectrum displays a very soft behavior, as expected due to the large extragalactic background light (EBL) attenuation for such a distant source. The flare brightness in the high-energy gamma-ray band was found to be a factor 50 above the average emission seen by observations with NASA's Fermi spacecraft. The astronomers also constructed and modeled the broadband spectral energy distribution (SED) of OP 313 within the framework of a two-zone leptonic model where the gamma-ray emission is produced via inverse-Compton scattering of the broad line region, accretion disk and dusty torus photon fields. "The combination of SSC [synchrotron self-Compton] and EC [external Compton] scattering (of the DT [dusty torus] and BLR [broad line region] fields) provides a good description of the spectral shape from optical to VHE gamma-rays, but requires the region to be relatively close to the outer edge of the BLR," concluded the authors of the paper. Written for you by our author Tomasz Nowakowski, edited by Stephanie Baum, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details K. Abe et al, Detection of the distant quasar OP 313 with the first Large-Sized Telescope of CTAO, arXiv (2026). DOI: 10.48550/arxiv.2605.26681 Journal information: arXiv © 2026 Science X Network