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

TL;DR

This study uses time-resolved, broadband spectroscopy from Swift and Fermi to test the radiative mechanism of GRB 241030A. The prompt emission displays fast-cooling synchrotron signatures, with Episode I showing a $- frac{3}{2}$ slope and Episode II revealing a near-constant low-energy break $E_b \\sim 2{-}3~\mathrm{keV}$ and a spectral peak $E_p$ that tracks flux within pulses while stepping down between pulses; spectral lags are effectively zero. The authors argue against a globally magnetized outflow with a decaying field, which would produce evolving $\nu_c$ and lags, and instead advocate an internal-shock picture with a roughly steady magnetic field and time-varying minimum electron Lorentz factor (or electron acceleration efficiency). They derive internal-shock scalings for $\nu_m$ and $\nu_c$, show how variations in $\epsilon_e/\xi$ can reproduce the observed $E_p$ and $E_b$ evolution, and imply a baryonic jet composition. Overall, GRB 241030A provides strong observational support for fast-cooling synchrotron prompt emission from internal shocks in a matter-dominated jet, with implications for microphysical parameters and jet composition in GRBs.

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 $ν_m$ and $ν_c$. These behaviors are difficult to reconcile with a globally magnetized outflow with a decaying field, which naturally produces hard-to-soft Ep 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, 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.

Figures (8)

• Figure 1: GRB 241030A light curves during prompt emission as observed by Swift and Fermi telescopes. The top two panels show the soft X-ray (0.3-10 keV) and hard X-ray (15-350 keV) light curves of XRT and BAT instruments onboard Swift satellite. XRT light curve starts from 80 s when it slewed to the position of the event. Next panel show the Fermi/GBM light curve in the energy band 8-1000 keV while its cumulative counts (CC) curve is shown underneath it in the bottom panel. A bin size of 1 sec is used for all three light curves. $T_{90}$ intervals for the full burst, its first and second episodes are marked in red, blue and green colors respectively.
• Figure 2: Energy-resolved light curves from combined selected GBM detectors from the first (left) and second (right) episodes of GRB 241030A. Due to faintness of burst during the first episode, GBM data was divided in 5 energy bands whereas good statistics were obtained in 13 energy band during the second episode. Light curves have been scaled for visual clarity.
• Figure 3: Energy dependent spectral lags between light curves from the first and second episodes of GRB 241030A. Positive values means soft energy photons arrive later than hard energy photons and vice versa. Error bars represent uncertainities at 1 $\sigma$ level.
• Figure 4: Placement of GRB 241030A in the peak luminosity versus rest frame spectral lags of GRBs. Type-I and type-II GRB population data points are shown in grey and cyan dots respectively. Best-fit correlations and 3$\sigma$ variation in the data for type II GRBs are shown by solid cyan line and shaded areas respectively.
• Figure 5: Evolution of different parameters from joint fitting in coarse spectral bins. Top panel shows the evolution of photon indices $\alpha_1$ and $\alpha_2$ while the middle panel illustrates the behavior of two break energies $E_b$ and $E_p$. Dash-dotted green and blue horizontal lines in the top panel indicate the theoretically expected values of photon indices $\alpha_1$=-0.67 and $\alpha_2$=-1.5. Bottom panel show the the value of breaks at different times with indices fixed at expected values. Light curve from the nearest GBM detecter to source direction is shown as grey background in all panels.