Teraelectronvolt emission from the $γ$-ray burst GRB 190114C

Abstract: Gamma-ray bursts (GRBs) of the long-duration class are the most luminous sources of electromagnetic radiation known in the Universe. They are generated by outflows of plasma ejected at near the speed of light by newly formed neutron stars or black holes of stellar mass at cosmological distances. Prompt flashes of MeV gamma rays are followed by longer-lasting afterglow emission from radio waves to GeV gamma rays, due to synchrotron radiation by energetic electrons in accompanying shock waves. Although emission of gamma rays at even higher, TeV energies by other radiation mechanisms had been theoretically predicted, it had never been detected previously. Here we report the clear detection of GRB 190114C in the TeV band, achieved after many years of dedicated searches for TeV emission from GRBs. Gamma rays in the energy range 0.2--1 TeV are observed from about 1 minute after the burst (at more than 50 standard deviations in the first 20 minutes). This unambiguously reveals a new emission component in the afterglow of a GRB, whose power is comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the TeV and X-ray bands points to processes such as inverse Compton radiation as the mechanism of the TeV emission, while processes such as synchrotron emission by ultrahigh-energy protons are disfavoured due to their low radiative efficiency.

• The paper presents a breakthrough by detecting TeV gamma-ray emission from GRB 190114C, marking the first observation of its kind.
• It demonstrates that inverse Compton scattering likely drives the high-energy component in the GRB afterglow, challenging synchrotron-only models.
• Rapid follow-up observations under suboptimal conditions highlight the potential of advanced telescopes in probing high-energy astrophysical phenomena.

Teraelectronvolt Emission from the Gamma-Ray Burst GRB 190114C

The paper under discussion presents an important observational study of the gamma-ray burst (GRB) 190114C, which has been detected in the teraelectronvolt (TeV) energy range for the first time. This detection was achieved through the use of the MAGIC telescopes, marking a significant milestone in GRB studies and providing a new perspective on the mechanisms behind high energy emissions.

Gamma-ray bursts are intense, high-energy events associated with the formation of stellar mass black holes or neutron stars. They emit radiation across a broad spectrum, from MeV γ-rays and spanning through radio waves and GeV γ-rays, typically generated by synchrotron radiation from accelerated electrons in blast waves. While emission at higher TeV energies had been theoretically anticipated, it had not been observed until the detection of GRB 190114C.

The observational campaign identified TeV γ-rays from GRB 190114C approximately one minute after the initial burst, with the signal's significance exceeding 50 standard deviations over the first 20-minute observation period. The presence of γ-rays in the 0.2–1 TeV range points to a hitherto undetected emission component during the GRB afterglow, suggesting power levels in the TeV range comparable to the synchrotron component.

The detaction contradicts assumptions where ultrahigh-energy proton synchrotron emissions were considered unlikely due to their low radiative efficiency. Instead, the observational data suggest inverse Compton processes playing a critical role in the TeV emission mechanism, aligning with existing models that describe interactions between accelerated electrons and ambient photons.

Observational Findings and Implications

The MAGIC telescopes captured γ-ray emission starting 57 seconds post-burst, with a total observation span of approximately 4.12 hours conducted under moonlight conditions—demonstrating the telescopes' capability to operate under suboptimal conditions for GRB studies. MAGIC's rapid response to alerts and technological adaptability are critical to such discoveries, highlighting the importance of sustained observation campaigns.

The spectra were corrected for absorption by the extragalactic background light (EBL), and the intrinsic spectra indicate a power-law distribution with a photon index near -2, suggesting that energy radiated is distributed evenly across the 0.2-1 TeV band and possibly beyond. The TeV energy flux persistence without irregular variabilities points to its afterglow association rather than the prompt phase.

From this detection arises evidence that electron-driven synchrotron emissions alone cannot account for the high-energy γ-rays observed, and thus, alternative leptonic processes (e.g., inverse Compton scattering) gain plausibility. Further analysis and data from additional GRBs are essential to refine these theories and verify whether similar mechanisms are widespread.

Future Directions

The detection of GRB 190114C in the TeV energy regime opens numerous avenues for further exploration in high-energy astrophysics. The implications of this discovery underscore the potential for discovering new components of energy emission in GRBs and related phenomena. The study also points toward a need for more advanced telescopic technologies and methodologies that can handle broader ranges of observational conditions and improve our understanding of cosmic γ-ray sources.

The role of the EBL in attenuating high-energy photon emissions provides an added layer for understanding the cosmological distribution of matter and radiation, making these observations valuable for investigating extragalactic phenomena and high-energy processes in general. Continued advancements and observations will conceivably allow the probing of distant cosmic processes and the natural accelerators of the Universe with increased detail and accuracy.