Multi-epoch afterglow rebrightenings in GRB 250129A: Evidence for successive shock interactions

Abstract

Most long gamma-ray bursts (GRBs) exhibit afterglows broadly consistent with external forward-shock emission, typically described by smooth broken power-law decays in the multiband light curve. However, a minority of well-sampled GRBs deviate from this behavior, including GRB 250129A. This object shows multiple late-time rebrightenings at X-ray and optical wavelengths. Rebrightenings are often attributed to energy injection from prolonged central engine activity, refreshed shocks from delayed shell collisions, density jumps in the ambient medium, or angular jet structure and viewing-angle effects. After analysing the prompt emission observed in gamma-rays and the near-infrared, we investigate the origin of X-ray and optical flaring episodes in GRB 250129A. Physical processes in the afterglow light curves were investigated using methods ranging from empirical fitting to Bayesian inference. The well-sampled flares and the connection between the prompt and afterglow emission allow us to test the consistency of the fireball model and alternative scenarios. Conducting the prompt and time-resolved analyses, we obtained an isotropic-equivalent energy of E_iso,gamma = (1.35 +/- 0.12) x 10^53 erg. By modeling the afterglow using an agnostic Bayesian framework (NMMA), we rule out both a single external-shock evolution and a one-time energy-injection scenario. Numerical calculations show that the rebrightening episodes are consistent with refreshed shocks from delayed collisions between relativistic shells. Based on the consistency between our analyses of the prompt and afterglow GRB 250129A data, we find that two statistically significant rebrightening episodes occur within 1.1 days post trigger and can be explained by a sequence of refreshed shocks. Temporally and spectrally rich GRB datasets such as the one presented in this work, provide a powerful means to test current modeling frameworks.

Figures (9)

• Figure 1: Top: BAT mask-weighted count-rate light curve with Bayesian block edges. Middle: BAT (10 keV) and TAROT $R$-band flux light curves. Bottom: Best-fit spectral indices.
• Figure 2: BAT 15--150 keV time-resolved spectra and best-fit models for GRB 250129A. For clarity, the fluxes of successive intervals are scaled by a constant factor of 0.1 relative to the previous interval; the horizontal dashed lines indicate the reference flux levels of the first interval. The fit results are reported with 90% confidence intervals. "0" corresponds to the time of the trigger $T_0$.
• Figure 3: TAROT-TCH telescope image of GRB 250129A taken at $T-T_0 = 0.14$ d. The insets show the zoomed-in region of three images from the KAIT, Abastumani-T70, and Euler telescopes centered on the GRB location, taken at phases of $T-T_0 = 0.22$, $T-T_0 = 1.64$, and $T-T_0 = 10.14$ d, respectively, relative to the Swift GRB trigger (MJD 60704.198).
• Figure 4: Observations from this work. Apparent magnitudes corrected for Milky Way extinction with the calibration of the Milky Way dust maps from Schlafly_2011. Here, $t_{\rm inj,1}$, $t_{\rm inj,2}$, and $t_{\rm inj,3}$ correspond to the time of the first ($T-T_0 = 0.129^{+0.011}_{-0.013}$), second ($T-T_0 = 0.926^{+0.074}_{-0.042}$), and third ($T-T_0 = 2.55^{+0.301}_{-0.05}$) rebrightenings in days (see Section \ref{['tab:posterior_nmma_analysis']}). For clarity, XRT flux densities are scaled by a factor of 100, and the AB-magnitude axis reflects the corresponding scaled flux densities Oke83Secondary The dashed lines represent the power-law fits to the afterglow, and the corresponding temporal slope values are listed in Table \ref{['tab:empirical_slopes']}. The solid curve shows the smooth broken power-law (SBPL) fit to the early $R$-band evolution. The $R$-band temporal decay fits are shown with a +0.35 dex vertical offset to aid visualization.
• Figure 5: X-ray-to-optical SED of GRB 250129A at $T-T_\mathrm{0} \approx 7.04$ days using the MW (red), LMC (black), and SMC (cyan) extinction curves. Dashed lines: intrinsic simple power-law model of the afterglow. Solid lines: Best fits to the data, including the X-ray absorption and the optical extinction. The three curves overlap because the negligible best-fit host extinction makes the models indistinguishable.