Exploring the central engines of gamma-ray bursts from prompt light curves

TL;DR

This work addresses discriminating GRB central engines between Blandford–Znajek (BZ) and neutrino-dominated accretion flow (NDAF) by using the prompt-light-curve decay slope $d$. It derives theoretical decays from BH spin evolution under BZ and NDAF, and fits Swift/BAT FRED pulses with the Kocevski–Ryde–Liang function to extract $d$, comparing to model predictions where $d \approx 1.67$ for BZ and $d$ spans $3.7$–$7.8$ for NDAF. Applying this to 85 GRBs, they find 15 consistent with BZ and 22 with NDAF, while most events cluster in $2 < d < 4$, implying hybrid or transitional accretion regimes. The results establish a robust link between prompt-light-curve decay and central-engine physics, highlighting the decay slope as a diagnostic for jet-launching mechanisms and motivating refined, hybrid models and multiwavelength/multimessenger tests. Collectively, this work advances our understanding of GRB jet engines and provides a practical observational handle to distinguish between distinct energy-extraction channels.

Abstract

Hyperaccreting stellar-mass black hole systems are leading candidates for the central engines of gamma-ray bursts (GRBs). Their jets are thought to be powered by either the Blandford-Znajek (BZ) process or neutrino-dominated accretion flows (NDAFs), but discriminating between these mechanisms remains challenging. To address this, we propose using the luminosity decay slope (parameter d) of GRB light curves to distinguish between the BZ and NDAF mechanisms, thereby linking the light-curve morphology to the central engine physics. By analysing 85 single-peaked GRBs with fast-rise, exponential-decay (FRED) profiles observed by Swift/BAT using 64 ms background-subtracted light curves, we fit the decay slope (parameter d) with the empirical Kocevski-Ryde-Liang (KRL) function and compare the results with theoretical predictions for the BZ (d approximately 1.67) and the NDAF (d approximately 3.7 to 7.8) mechanisms. We find that the decay slope (parameter d) can differentiate central engine mechanisms, with 15 GRBs consistent with the BZ mechanism and 22 supporting the NDAF mechanism. However, most events exhibit slopes within the range between 2 and 4, suggesting a hybrid of mechanisms, with NDAF being dominant.

Figures (8)

• Figure 1: Simulated long-GRB light curves for two central engine mechanisms: NDAF and BZ. The NDAF model exhibits a steep decay, whereas the BZ model shows a more gradual one. Colours represent variations in initial parameters—(a) black hole spin, (b) accretion rate, (c) accretion disc mass, and (d) black hole mass—illustrated by the colour bars. For each parameter set, the top left panel shows the evolution of the accretion rate over time; the top right panel shows the evolution of the spin parameter; the bottom left panel shows the luminosity evolution, where the NDAF model is plotted as red dashed lines (typical slope $-4.5$) and the BZ model as blue dashed lines (typical slope $-1.5$); and the bottom right panel compares the two mechanisms after alignment.
• Figure 2: Same as Fig. 1 but for simulated short-GRB light curves
• Figure 3: Examples of fitting light curves (1--10 000 keV) to the KRL model (red lines) for GRB 081222, GRB 161218A, GRB 170101A, GRB 180728A, GRB 200922A, GRB 201105A, GRB 210410A, GRB 211225B and GRB 250605A. The dashed vertical lines indicate the pulse start and end times. The observed data (shown in grey) have been corrected for redshift, and the corresponding fitting parameters are listed in Table \ref{['table1']}.
• Figure 4: Comparison between theoretical models and observational data for (a) GRB 120326A and (b) GRB 160131A. Theoretical light curves are shown for the Blandford–Znajek (BZ) mechanism (solid lines) and the neutrino-dominated accretion flow (NDAF) mechanism (dashed lines), following the same convention as in Figures \ref{['fig1']} and \ref{['fig2']}. Observational data are indicated by red circles, with lighter shading showing the error bars. The upper panel exhibits a steep luminosity decay ($L_{\nu\bar{\nu}} \propto t^{-3.72}$--$t^{-7.83}$), consistent with NDAF predictions, whereas the lower panel shows a shallower decay ($L \propto t^{-1.67}$), matching BZ model expectations.
• Figure 5: Distribution of decay slopes $d$ for 85 GRBs, fitted with a three-component Gaussian mixture model (GMM). Three extreme cases (GRB 070306, GRB 140209A and GRB 161004B) with $d > 7.8$ were excluded to avoid skewing the high-$d$ tail due to potential observational or physical anomalies, and their removal does not affect the main results. The best-fit components peak at $d \approx 2.04$, $d \approx 3.56$ and $d \approx 6.04$ with relative weights of 0.50, 0.32 and 0.18, respectively. Most events fall in the range $2 < d < 4$, suggesting hybrid central engines, differing from idealised BZ ($d \approx 1.67$) or NDAF ($d \approx 3.7$--$7.8$) predictions.