AnswersPublished19 sources
How OP 313 became the most distant blazar seen in very-high-energy gamma rays
OP 313 is a flat spectrum radio quasar at redshift z = 0.997, roughly 8 billion light years away, making it the most distant blazar or active galactic nucleus detected at very high energy gamma ray energies above 100... The discovery followed an unusually bright December 2023 flare identified by Fermi LAT.
Published byEdited with GPT-5.6 LunaImages generated with GPT Image 1.5
Research answer
Key takeaways
- OP 313 is a flat spectrum radio quasar at redshift z = 0.997, roughly 8 billion light years away, making it the most distant blazar or active galactic nucleus detected at very high energy gamma ray energies above 100...
- The discovery followed an unusually bright December 2023 flare identified by Fermi LAT.
- The surviving gamma rays helped researchers place upper limits on the density of extragalactic background light—the diffuse glow produced by stars and galaxies across cosmic history.
AI Prompt
openai.comCreate a landscape editorial hero image for this Studio Global article: What did the CTAO LST Collaboration and MAGIC Collaboration discover about the blazar OP 313—including its redshift, distance, December 2023. Article summary: OP 313 is a flat-spectrum radio quasar (a blazar) at redshift z = 0.997—roughly 8 billion light-years away—and its 2023 outburst made it the most distant active galactic nucleus/blazar yet detected at very-high-energy (V. Topic tags: general, academic, education, general web. Style: premium digital editorial illustration, source-backed research mood, clean composition, high detail, modern web publication hero. Use reference image context only for broad subject, composition, and topical grounding; do not copy the exact image. Avoid: logos, brand marks, copyrighted characters, real person likenesses, fake screenshots, UI text, readable text, watermarks, charts wi
OP 313 is a flat-spectrum radio quasar (FSRQ)—a type of blazar whose relativistic jet is aimed broadly toward Earth. With a redshift of z = 0.997—reported more precisely in some observations as z = 0.9973—it lies roughly 8 billion light-years away. Detecting very-high-energy (VHE) gamma rays from the source, at energies above 100 GeV, made OP 313 the most distant blazar—and active galactic nucleus—ever observed in this energy range. 1
2
4
There is one important qualification: OP 313 is the most distant blazar or AGN detected in VHE gamma rays, not the most distant VHE source of any kind. The gamma-ray burst GRB 201216C is farther away. 2
3
8
The December 2023 flare
The observation began after the Fermi Large Area Telescope (Fermi-LAT) detected increased activity from OP 313. LST-1 started observing the source around 9–10 December 2023, followed by coordinated observations across multiple wavelengths. 1
2
During the exceptionally bright flare, the VHE flux reached approximately 0.3 Crab Units above 100 GeV. LST-1 made the first detection of VHE emission from OP 313, while the two MAGIC telescopes independently confirmed the signal. Researchers combined the LST-1 and MAGIC data with lower-energy observations from other facilities, including Fermi-LAT. 1
5
6
The source was less active in January 2024. The flare showed correlated behaviour across much of the observed electromagnetic spectrum, while its radio emission remained comparatively stable. 1
Why detecting the signal was so difficult
Gamma rays above 100 GeV do not travel easily across cosmological distances. Along the way, they encounter the extragalactic background light (EBL): the diffuse optical and infrared glow produced by stars and galaxies throughout the history of the Universe. 1
4
7
A sufficiently energetic gamma-ray photon can collide with a photon from the EBL and produce an electron–positron pair:
[
\gamma + \gamma \rightarrow e^+ + e^-
]
This interaction removes the original gamma-ray photon from the beam reaching Earth. As a result, detecting VHE photons from an object at a redshift of about 1 means that some of its emission managed to cross roughly 8 billion light-years despite the expected absorption by the EBL. 1
2
4
That survival is what makes OP 313’s detection a distance record for VHE blazars and AGN. It also makes the source a useful probe of the otherwise faint cosmic light filling intergalactic space.
How the gamma rays were produced
Broadband modelling favours a leptonic scenario. In this picture, electrons moving at relativistic speeds inside the blazar’s jet scatter lower-energy photons through inverse Compton scattering, boosting them into high-energy gamma rays. 1
The seed photons are consistent with radiation fields in the quasar’s broad-line region and in a dusty torus surrounding the supermassive black hole that powers the active galactic nucleus. The study examines both a two-zone model and an alternative single-zone model in which the scattering occurs farther out, within the dusty-torus radiation field. 1
The EBL played a separate role. It did not generate the gamma rays at the source; instead, it attenuated some of them after they escaped the jet and travelled through intergalactic space.
What the detection reveals about the EBL
If the EBL were too dense, it would have absorbed more of OP 313’s gamma rays than researchers observed. The surviving emission therefore allows the team to place upper limits on the EBL intensity near 0.6 micrometres:
- λIλ < 8.74 nW m⁻² sr⁻¹ from the LST-1 analysis;
- λIλ < 14.7 nW m⁻² sr⁻¹ from the MAGIC data, including systematic uncertainties.
1
These figures are upper limits, not direct precision measurements of the EBL’s exact density. They indicate how much diffuse optical and near-infrared light can exist along the gamma rays’ path without eliminating the signal that reached Earth. 1
4
A first scientific result for LST-1
The discovery was the first scientific result from LST-1, the prototype Large-Sized Telescope of the Cherenkov Telescope Array Observatory (CTAO), while the instrument was still being commissioned. Its low energy threshold and large effective area were crucial for detecting such a distant source whose VHE emission had been weakened by EBL absorption. 4
6
LST-1 is a 23-metre telescope at CTAO’s northern site on La Palma, designed to cover approximately the 20 GeV to 3 TeV energy range. 4 Detecting OP 313 during commissioning demonstrated that the telescope can respond to transient activity and identify distant sources near the lower end of the VHE regime.
The planned northern CTAO array is expected to include four LSTs. Together, they should improve sensitivity, spectral studies and rapid follow-up of transient sources. The available evidence does not quantify a specific performance improvement for future flares resembling OP 313, however. 4
17
Studio Global AI
Continue your research
This page includes a source-backed answer you can continue inside Studio Global.
People also ask
What is the short answer to "How OP 313 became the most distant blazar seen in very-high-energy gamma rays"?
OP 313 is a flat spectrum radio quasar at redshift z = 0.997, roughly 8 billion light years away, making it the most distant blazar or active galactic nucleus detected at very high energy gamma ray energies above 100...
What are the key points to validate first?
OP 313 is a flat spectrum radio quasar at redshift z = 0.997, roughly 8 billion light years away, making it the most distant blazar or active galactic nucleus detected at very high energy gamma ray energies above 100... The discovery followed an unusually bright December 2023 flare identified by Fermi LAT.
What should I do next in practice?
The surviving gamma rays helped researchers place upper limits on the density of extragalactic background light—the diffuse glow produced by stars and galaxies across cosmic history.
Sources
indico.uchicago.eduOP313_TeVPA2024
mpp.mpg.deGamma-ray Astronomy: New learnings from blazar OP 313
20minutos.esCientíficos baten el récord de distancia en la observación de energía muy alta: un blázar a 8.000 millones de años luz