The afterglow of GRB 190114C — the first gamma-ray burst detected at teraelectronvolt energies — was monitored at radio/mm frequencies (ALMA, ATCA, uGMRT) and in the optical from about 1 to 140 days after the burst. The light curves violate the temporal/spectral closure relations of the standard synchrotron forward-shock model, and fitting an adiabatic blast wave requires the shock microphysical parameters to evolve: the fraction of shock energy in non-thermal electrons falling as ϵe∝t−0.4\epsilon_e \propto t^{-0.4} and the magnetic fraction growing as ϵB∝t+0.1\epsilon_B \propto t^{+0.1} (uniform-medium branch) or t+0.76t^{+0.76} (wind branch). No physical mechanism producing this evolution is established, and the two ambient profiles give comparable Bayesian evidence while implying kinetic energies differing by an order of magnitude. The problem is to determine whether the inferred time-dependent microphysics is real shock physics — and identify its mechanism — or an artifact of neglected standard effects (synchrotron self-Compton cooling, reverse-shock mm emission, low-frequency self-absorption, simplified jet dynamics), and thereby break the ambient-medium degeneracy.

Background

A gamma-ray burst (GRB) afterglow is the fading multi-wavelength emission produced when a relativistically expanding blast wave sweeps up and shocks the ambient medium. In the standard model (Sari, Piran & Narayan 1998), electrons accelerated to a power-law energy distribution at the external forward shock — the shock front ahead of the ejecta — radiate synchrotron emission in shock-amplified magnetic fields. Two microphysical parameters set the luminosity: ϵe\epsilon_e, the fraction of shock energy carried by the non-thermal electron population, and ϵB\epsilon_B, the fraction in magnetic fields; both are normally treated as constants. The temporal decay index α\alpha and spectral index β\beta of the afterglow are then locked together by 'closure relations' that depend on whether the ambient density is uniform (an interstellar-medium, ISM, profile) or falls as r−2r^{-2} (the wind blown by a massive progenitor star; Chevalier & Li 2000). Closure relations are the standard consistency test of afterglow modeling. The constant-ϵe\epsilon_e, ϵB\epsilon_B assumption ultimately rests on expectations from particle-in-cell (PIC) simulations — first-principles kinetic simulations of collisionless shocks — in which shock-acceleration efficiency and magnetic-field amplification typically saturate toward quasi-steady values. GRB 190114C (z=0.425z = 0.425) was the first GRB detected at teraelectronvolt (TeV) energies, by the MAGIC imaging atmospheric Cherenkov telescopes; its TeV emission is generally attributed to synchrotron self-Compton (SSC) radiation, in which the same shock-accelerated electrons up-scatter the synchrotron photons they emitted. Misra et al. (2021) observed the afterglow with ALMA, ATCA and uGMRT from ~1 to ~140 days, plus optical photometry to ~25 days, and found that the light curves do not follow the standard closure relations. Reconciling an adiabatic blast wave with the data demands time-dependent microphysics: ϵe∝t−0.4\epsilon_e \propto t^{-0.4} together with ϵB∝t+0.1\epsilon_B \propto t^{+0.1} (ISM) or ϵB∝t+0.76\epsilon_B \propto t^{+0.76} (wind). The two medium profiles yield comparable Bayesian evidence yet very different energetics — the ISM branch requires a kinetic energy Ek,iso≈1.9×1054E_{k,\mathrm{iso}} \approx 1.9 \times 10^{54} erg, nearly an order of magnitude above the prompt energy, while the wind branch matches it — and the absence of a detected jet break (the achromatic steepening expected when the relativistic jet's edge becomes visible) leaves the beaming-corrected energy and opening angle only bounded. Three further gaps compound the inference: the shallow early 97 GHz decay near t∼0.1t \sim 0.1 day is not reproduced by the reverse-shock prescriptions explored (a reverse shock crosses the ejecta from behind and can dominate early radio/mm emission); the low-frequency (≲1.4\lesssim 1.4 GHz) bands show residual mismatch because the self-absorption optical depth τm\tau_m (absorption of the synchrotron radiation by the emitting electron population itself) and the fraction ζe\zeta_e of electrons participating in the non-thermal distribution were assumed rather than fitted; and SSC cooling was omitted from the model fits even though SSC is the leading explanation of the contemporaneous TeV emission. In established relativistic-shock physics there is no accepted mechanism that makes ϵB\epsilon_B grow and ϵe\epsilon_e decay over the two decades in observer time spanned by the data — which is why the result, if real, would be a direct observational window on collisionless-shock microphysics.

Problem Statement

For the afterglow of GRB 190114C: establish, with a quantitatively validated physical model, whether the time dependence of the forward-shock microphysical parameters inferred from the ALMA/ATCA/uGMRT/optical campaign — ϵe∝t−0.4\epsilon_e \propto t^{-0.4} with ϵB∝t+0.1\epsilon_B \propto t^{+0.1} (uniform medium) or ϵB∝t+0.76\epsilon_B \propto t^{+0.76} (wind medium) — reflects genuine collisionless-shock physics, and if so identify the mechanism that produces it; or alternatively show that a model with constant microphysical parameters, supplemented by explicit treatment of whichever of these neglected effects the fit requires (synchrotron self-Compton cooling, reverse-shock emission at early mm times, low-frequency self-absorption and thermal-electron absorption, more realistic jet dynamics including lateral expansion), fits the same multi-band light curves and spectra and restores consistency with the closure relations. The answer must be decided jointly with, and must state, which ambient density profile (uniform ISM versus wind) the data and modeling support — including the implied isotropic-equivalent kinetic energy Ek,isoE_{k,\mathrm{iso}} (together with whatever bound the data place on the beaming-corrected energy) — and must extend to the early-time behavior (the shallow 97 GHz decay near 0.1 day) that the reverse-shock prescriptions of Misra et al. (2021) could not reproduce. These deliverables share one scientific objective: a single physically self-consistent account of the full broadband afterglow of GRB 190114C.

References

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