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 I (0-45 s) is well described by a single power law with photon index $\simeq -3/2$, consistent with the fast-cooling synchrotron slope below the peak. Episode II (100-200 s), exhibits two robust spectral breaks: a low-energy break at $E_{b}$$\sim$$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 GBM bands. At later times ($\geq 230$ s), the spectrum softens toward $\sim-2.7$, as expected when the observing band lies above both $\nu_m$ and $\nu_c$. These behaviors are difficult to reconcile with a globally magnetized outflow with a decaying field, which naturally produces hard-to-soft Ep evolution, growing $\nu_c$, and appreciable lags. By contrast, internal shocks with a roughly steady effective magnetic field and a time-variable minimum electron Lorentz factor (equivalently, e.g., a varying fraction of accelerated electrons simultaneously account for (i) the stable $E_{b}$, (ii) the intensity-tracking yet step-down $E_{p}$, (iii) the canonical -2/3 and -3/2 slopes, and (iv) the near-zero lag.