GRB 241030A: a bright afterglow challenging forward shock emission

Abstract

Gamma-Ray Burst GRB 241030A (z = 1.411) exhibited a bright afterglow (similar to GRB 221009A), detected across gamma-ray, X-ray, UV, and optical bands, providing a probe of GRB afterglow physics. We compiled multi-wavelength observations spanning from a minute to a week after the prompt emission, processing the data through a unified photometry pipeline. We analysed the observations both analytically and using Bayesian inference with two independent models. Our models assume that the afterglow emission arises from the strong forward shock of a laterally structured jet, with possible contributions from synchrotron self-Compton (SSC) scatterings. Our models reproduce X-ray to optical data, favouring a jet propagating into a constant-density interstellar medium, with a viewing angle within the jet core. However, both analyses require parameter values that are extreme compared to expectations from standard theory. In particular, our results imply extremely energetic jets despite regular prompt energy, leading to a very inefficient prompt emission. Furthermore, the jets are inefficient at accelerating particles, with low electron and magnetic energy fractions, leading to significant SSC emission. Our analyses indicate that the jets have large opening angles and propagate in high-density media. If the afterglow is indeed powered by radiation emitted behind a strong forward shock, our results place GRB 241030A within a sub-class of GRBs characterised by extreme kinetic energies, large jet opening angles, and very low prompt emission efficiencies, with strong SSC radiation. These predictions are difficult to reconcile with typical expectations from other GRBs. We therefore suggest that the afterglow of GRB 241030A is not solely powered by forward shock emission.

Figures (8)

• Figure 1: Light curves of GRB 241030A, corrected for Galactic and host extinction. Observations after the vertical dashed line (shown at 400 s post T0) were used for the afterglow analysis in Section \ref{['section:MWafterglowanalysis']}. Grey stars are GRB 221009A r' band magnitude from 2023ApJ...948L..12K, adjusted to redshift 1.411 with k-correction, -2.5 magnitude for comparison with the GRB 241030A r' light curve (see Section \ref{['sec:bayesian_syn']}).
• Figure 2: SED of the afterglow of GRB 241030A at $t - t_0 \sim 3~\mathrm{h}$. The dashed line corresponds to the intrinsic broken power law model. The solid line represents the best fit to the data, and includes the absorption in the X-ray band.
• Figure 3: Light curve fits for selected filters from the NMMA inference with the afterglowpy gaussian jet model. Observed data and their uncertainties are shown as error bars; the solid lines indicate the median value of the posterior sample at each time. The shaded contours correspond to the 68% and 95% quantiles of the posterior distribution. The two transparent XRT data points before $10^3$ s were not included in the fit.
• Figure 4: Observed spectrum at $t_\mathrm{obs} = 8 \times 10^4~\mathrm{s}$. The blue error bars show the fitted data points with $7.5\times10^4~\mathrm{s} < t_\mathrm{obs} < 8.5\times10^4~\mathrm{s}$ (optical and X-ray data). The solid curve represents the spectrum computed with the best-fit set of parameters found when SSC is not included in the model. The dashed curve represents the spectrum computed with the same parameters, this time including SSC.
• Figure 5: Light curve fits from the inference with the afterglow model from Pellouin:2024gqj for selected filters. Observed data and their errors are shown, and the solid line indicates the median value of the posterior sample at each time. The shaded contours correspond to the $68\%$ and $95\%$ quantiles of the posterior distribution.