Research

Gamma-ray bursts (GRBs) are enigmatic and energetic stellar explosions, releasing enormous energy of ∼10⁵¹ erg as gamma rays within a short timescale of seconds to minutes. A key milestone in GRB studies is the consensus that the emission comes from relativistic jets reaching Lorentz factors of Γ ≳ 100. One of the major problems in high-energy astrophysics is to understand the acceleration and emission mechanisms of these powerful and fast relativistic jets. For these problems, the fractions of magnetic energy, rest-mass energy, and thermal energy contained in the jet are crucial. To obtain these fractions, it is important to estimate the radius R and Lorentz factor Γ of the jet at the dissipation region.

However, these quantities are difficult to estimate because the main burst, or prompt emission, lasts only 10–100 seconds, and the gamma-ray spectrum does not provide clear diagnostic features. This has kept the problem unsolved for decades. In this context, I address this problem by focusing on the period 100–1000 seconds after the prompt emission. During this time, X-ray telescopes often detect jet-related emissions known as X-ray flares (XFs) and extended emissions (EEs). This time window is a unique opportunity to study the jet and the central engine.

Recently developed instruments, such as SVOM and CTAO, can respond within minutes and detect these emissions in the ultraviolet and very-high-energy gamma-ray bands, offering new opportunities to probe the jet during XFs and EEs. Moreover, simultaneous high-energy neutrino observations may provide further clues to the dissipation mechanism. My research theoretically calculates the expected multiwavelength and multi-messenger signals from the dissipation region during XFs and EEs. I find that very-high-energy gamma rays, ultraviolet emission, and high-energy neutrinos are especially useful for constraining the dissipation radius and the Lorentz factor. Comparing these theoretical predictions with future multi-messenger observations of XFs and EEs will provide important clues to the jet composition and the nature of the central engine in GRBs.