Fast-cooling Synchrotron Prompt Emission from Internal Shocks in GRB 241030A

Abstract We present a time-resolved joint Swift–Fermi spectral study of GRB 241030A ( z = 1.411) that cleanly isolates the synchrotron origin of its prompt emission and favors a matter-dominated internal-shock scenario. The light curve shows two episodes separated by a quiescent gap. Episode 1 (0–45 s) is well described by a single power law with photon index ≃ –3/2, consistent with the fast-cooling synchrotron slope below the peak. Episode 2 (100–200 s) exhibits two robust spectral breaks: a low-energy break at E b ∼ 2–3 keV that remains nearly constant in time, and a spectral peak E p that tracks the flux within pulses but steps down between them. The photon indices below and above E b cluster around −2/3 and −3/2, respectively, as expected for fast-cooling synchrotron emission. The burst displays an unusually small (consistent with zero) spectral lag across Fermi Gamma-ray Burst Monitor bands. At later times (≳230 s), the spectrum softens toward ∼–2.7, as expected when the observing band lies above both ν m and ν c . These behaviors are difficult to reconcile with a globally magnetized outflow with a decaying field, which naturally produces hard-to-soft E p evolution, growing ν c , and appreciable lags. By contrast, internal shocks with a roughly steady effective magnetic field and a time-variable minimum electron Lorentz factor (equivalently, for example, a varying fraction of accelerated electrons) simultaneously account for: (1) the stable E b ; (2) the intensity-tracking yet stepped-down E p ; (3) the canonical −2/3 and −3/2 slopes; and (4) the near-zero lag.