X-ray Analysis of Gamma-Ray Burst Flares and Underlying Afterglows: Insights into Origin of Flares

TL;DR

This study investigates the origin of X-ray flares in GRB afterglows and their connection to plateau phases using 89 Swift-XRT GRBs modeled with forward-shock synchrotron emission in wind or ISM environments, plus asymmetric Norris-type flares. Flares are found to be highly asymmetric with $t_{\rm decay} \approx 5\, t_{\rm rise}$ and show strong rise–decay correlations, while their properties are largely independent of the presence of plateaus, implying distinct mechanisms for flares and plateaus and favoring a long-lived central-engine origin for flares. The plateau phase appears linked to different environmental or dynamical conditions (e.g., wind environments and late jet behavior) rather than to flare activity, challenging late-time energy-injection models for plateaus. Prompt emission trends indicate that flaring GRBs tend to be longer and more energetic with more complex light curves, whereas plateau-bearing bursts show softer spectra and lower $E_{\rm pk}$, suggesting potential population differences. Overall, the results constrain GRB models by supporting decoupled origins for flares and plateaus and favor central-engine–driven variability as the flare mechanism.

Abstract

Gamma-ray burst (GRB) X-ray light curves exhibit a variety of complex temporal structures, such as flares and plateaus. The origin of flares seen in many GRB early afterglows is still uncertain. Here, we analyze a sample of 89 GRBs, 61 of them with flares, both with and without a "plateau" phase. We fit the Swift-XRT light curves with synchrotron emission from a forward shock propagating into either a constant-density ISM or a stellar wind, and flares on top of that. We find that the flare light curves are not symmetric, with a decay time that is $\sim$five times longer than the rise time. We do not find any differences in flare properties between GRBs with and without a "plateau" phase. Moreover, additional afterglow properties such as the electron power-law index and the end time of the plateau are consistent between bursts with and without flares. These results strongly indicate that flares originate from a mechanism distinct from that producing the plateau and afterglow. When looking at the prompt emission properties, we do find some tendencies: GRBs with flares tend to be brighter and longer lasting than GRBs without flares. We therefore conclude that, unlike plateaus, flares are unlikely to arise from an external origin and are more plausibly associated with prolonged central engine activity that lasts longer than the main episode that produces the prompt phase. As the plateau cannot have the same origin, this result excludes models of late-time energy injection as the source of the GRB plateau.

Figures (10)

• Figure 1: Left: Distributions of the flare asymmetry, $t_{\rm rise}/t_{\rm decay}$ ratio. Right: Distributions of $t_{\rm rise}/t_{\rm pk}$ ratio. The purple bars represent the 42 GRBs (65 flares) with a plateau, while the red bars represent the 19 GRBs (32 flares) without a plateau. In each panel, the right-hand ordinate shows the number of bursts in each bin while the left-hand ordinate shows the value of the kernel density estimation (KDE) drawn by the purple and red solid lines for each sample respectively. The results show that flares exhibit a pronounced asymmetry, regardless of whether a plateau phase is present in the GRB X-ray light curves. We also observe a bimodality in both distributions of $t_{\rm rise}/t_{\rm decay}$ ratio, which is more pronounced for GRBs with a plateau phase. A stronger bimodality is equally evident in both subsamples for the $t_{\rm rise}/t_{\rm pk}$ ratio.
• Figure 2: Relation between the flare rise time ($t_{rise}$) and the flare decay time ($t_{decay}$). The purple points represent the 42 GRBs (65 flares) with plateau phases, the red points represent the 19 GRBs (32 flares) without plateau phases in our subsamples. The errors correspond to a significance of one sigma. The Spearman’s rank correlation coefficient is $r = 0.90~(0.94)$, with a corresponding chance probability of $p \ll 10^{-5} (\ll 10^{-5})$ for GRBs with (without) a plateau phase. This strong correlation between the flare decay and rise times is not changed when separating the first and second flares.
• Figure 3: Distributions of the end time of the steep decay, $T_1$, which marks the beginning of the first afterglow segment (see Appendix A in DB+25) for GRBs with flares (left) and without flares (right), with each panel showing the subsamples with and without plateaus. In the left panel, the 42 GRBs with a plateau phase are shown in purple, and the 19 GRBs without a plateau phase in red. In the right panel, the 15 GRBs with a plateau phase are shown in black, and the 13 GRBs without a plateau in green. In each panel, the right-hand ordinate shows the number of bursts in each bin while the left-hand ordinate shows the value of the kernel density estimation (KDE) drawn by solid lines corresponding to each color. These distributions show no significant difference at $T_1$ across the subsamples.
• Figure 4: Distributions of the electron power-law index, $p$ (see Appendix A in DB+25) for GRBs with flares (left) and without flares (right). The color coding follows that of Figure \ref{['fig:time_after_early_steep_decay']}. The corresponding number of GRBs in each subsample is listed in Table \ref{['tab:afterglow_statistics-1']}. These distributions show no significant difference in the electron power-law index $p$ across the subsamples.
• Figure 5: Distributions of the late afterglow slope obtained using electron power-law index, $p$ in different medium (see Appendix A in DB+25) for GRBs with flares (left) and without flares (right). The color coding follows that of Figure \ref{['fig:time_after_early_steep_decay']}. The corresponding number of GRBs in each subsample is listed in Table \ref{['tab:afterglow_statistics-1']}. These distributions show no significant difference in the late-time slope between the subsamples with and without flares. However, a clear distinction is observed between GRBs with and without a plateau, as confirmed by the K–S test result reported in Table \ref{['tab:afterglow_statistics-2']} having $p < 0.05$.