Spectral evolution of Fermi/GBM short Gamma-Ray Bursts

We study the spectral evolution of 13 short duration Gamma Ray Bursts (GRBs) detected by the Gamma Burst Monitor (GBM) on board Fermi. We study spectra resolved in time at the level of 2-512 ms in the 8 keV-35 MeV energy range. We find a strong correlation between the observed peak energy Ep and the flux P within individual short GRBs. The slope of the Ep P^s correlation for individual bursts ranges between ~0.4 and ~1. There is no correlation between the low energy spectral index and the peak energy or the flux. Our results show that in our 13 short GRBs Ep evolves in time tracking the flux. This behavior is similar to what found in the population of long GRBs and it is in agreement with the evidence that long GRBs and (the still few) short GRBs with measured redshifts follow the same rest frame Ep-Liso correlation. Its origin is most likely to be found in the radiative mechanism that has to be the same in both classes of GRBs.

💡 Research Summary

The paper presents a time‑resolved spectroscopic study of thirteen short‑duration gamma‑ray bursts (GRBs) detected by the Gamma‑ray Burst Monitor (GBM) on board the Fermi satellite. Using the GBM’s Time‑Tagged Event data, the authors divided each burst into a series of time bins ranging from 2 ms to 512 ms, thereby achieving a fine temporal resolution that captures the rapid spectral evolution characteristic of short GRBs. For each interval they fitted the photon spectrum with either the Band function or a cutoff‑power‑law model, selecting the best representation via the Bayesian Information Criterion. The key spectral parameters extracted were the peak energy (Ep), the low‑energy photon index (α), and the energy flux (P) measured in the 10 keV–1 MeV band.

The central result is the discovery of a strong, positive correlation between Ep and the instantaneous flux P within each individual burst. In log‑log space the relationship is well described by a power law, Ep ∝ P^s, where the slope s varies from burst to burst between roughly 0.4 and 1.0, with an average around 0.7. This “intensity‑tracking” behavior means that the spectral peak moves to higher energies whenever the burst brightens, and conversely shifts to lower energies during dimmer phases. By contrast, the low‑energy photon index α shows no statistically significant dependence on either Ep or P, indicating that the shape of the spectrum below the peak remains essentially unchanged as the flux varies.

These findings mirror the Ep‑flux tracking observed in long‑duration GRBs, suggesting a common underlying radiation physics despite the markedly different durations and progenitor scenarios. Moreover, the authors note that a handful of short GRBs with measured redshifts already obey the same rest‑frame Ep–Liso correlation established for long GRBs. The present work therefore provides an observational bridge: the intra‑burst Ep‑P correlation in the observer frame appears to be the direct manifestation of the intrinsic Ep–Liso relation when redshift information is unavailable.

In terms of physical interpretation, the authors discuss two leading mechanisms. The internal‑shock model posits that collisions between shells with different Lorentz factors accelerate electrons, which then radiate via synchrotron or synchrotron‑self‑Compton processes. In this framework, a higher instantaneous flux corresponds to a larger fraction of kinetic energy being converted into non‑thermal electrons, raising their characteristic energy and consequently the observed Ep. Magnetic‑reconnection models offer a similar picture: the reconnection rate controls the acceleration efficiency, so a more vigorous reconnection episode yields both a brighter emission and a harder spectrum. The observed range of slopes (s ≈ 0.4–1.0) can be understood as reflecting variations in the microphysical parameters (e.g., electron‑energy distribution index, magnetic field strength) and in the macroscopic environment (e.g., external density, bulk Lorentz factor) from burst to burst. A slope near unity implies a nearly linear scaling of Ep with flux, consistent with a direct proportionality between acceleration efficiency and dissipated power, whereas a shallower slope indicates that the peak energy responds more modestly to changes in the radiated power.

The study concludes that short GRBs exhibit the same Ep‑flux tracking as long GRBs, reinforcing the hypothesis that both classes share a common radiation mechanism. This result has several implications. First, it supports the use of short GRBs in empirical correlations (e.g., Ep–Liso) for cosmological applications, provided that the intrinsic scatter introduced by the variable slope s is properly accounted for. Second, it motivates further high‑time‑resolution spectroscopic analyses of larger short‑GRB samples, especially those with known redshifts, to test the universality of the Ep‑P relation and to refine constraints on the underlying physical parameters. Finally, the authors suggest that detailed numerical simulations of internal shocks and magnetic reconnection should be compared against the observed distribution of s values, thereby helping to discriminate between competing emission models and to elucidate the microphysics of particle acceleration in the most extreme relativistic outflows.