Long GRB 250916A: an Off-axis Powerlaw Jet with Thermal Cocoon

Abstract

Some gamma-ray bursts (GRBs) exhibit precursor emission episodes preceding the main emission, with a quiescent period in between. The properties of the precursor emission and the duration of the quiescent interval are related to the central engine activity and jet formation processes, thus providing insights into the physics of GRBs. We present a comprehensive analysis of the prompt emission and multi-wavelength afterglow of GRB 250916A. Using detailed afterglow modeling, we find that the broadband data are best described by a powerlaw structured jet with a relatively narrow core ($θ_c \approx 0.8^\circ$), viewed moderately off-axis at a viewing angle $θ_v \approx 2.7^\circ$. The isotropic-equivalent kinetic energy of the jet ($E_{k,iso} \approx 2.4 \times 10^{54}$ erg) is on the higher side for typical GRBs. The precursor emission is well described by a blackbody spectrum with a temperature of kT $\approx$ 13.2 keV and is separated from the main emission by a long quiescent interval of 150 s. Put together, our results indicate that the precursor is likely to be a shock breakout from a cocoon formed by the interaction of the relativistic jet with the progenitor star. The resulting cocoon pressure and shock collimation naturally lead to the launch of a narrowly collimated jet, consistent with the jet geometry inferred from afterglow observations. The long quiescent interval may imply the central engine turn-off in addition to the effect of the off-axis geometry.

Figures (6)

• Figure 1: The light curve from the brightest NaI detector, $n_9$, in the $9-900$ keV energy range. The green-shaded region marks the precursor emission episode, the cyan-shaded region marks the main emission episode. The red bins represent the background intervals considered. The vertical dashed line shows bins in the main emission episode used for time-resolved analysis.
• Figure 2: Time evolution of the parameters of the best-fit Band function for the main emission episode. $E_p$ shows decline across the main emission episode, $\alpha$ is largely consistent with no evolution.
• Figure 3: GRB global relations based on Band model fit to precursor and main emission episodes. Top left: Amati relation, $E_{p,{\rm z}}$ vs $E_{\gamma,\textrm{iso}}$. Top right: Ghirlanda relation, $E_{p,{\rm z}}$ vs $E_\gamma$. Bottom left: Frail, $E_{\gamma,\textrm{iso}}$ vs $\theta_{j}$. Bottom right: Yonetoku relation, $E_{p,{\rm z}}$ vs - $L_{p,{\rm iso}}$
• Figure 4: Multi-wavelength afterglow light curves of GRB 250916A in X-ray (black), optical ($u$, $g$, $r$, $i$, $R$, $white$, $L$, $VT_R$, $VT_B$, and $J$ bands). The filled markers represent fluxes corrected for galactic extinction while the hollow markers represent uncorrected values. The light curves are described by a broken powerlaw with an initial $\alpha_1 = 1.06 \pm 0.10$ followed by a steep decay $\alpha_2 = 2.07 \pm 0.04$ after the break at $t_{\mathrm{break}} = 53 \pm 3$ hours (green vertical line).
• Figure 5: Model light-curve and posterior constraints for GRB 250916A with the best-fitting Powerlaw jet model. Top: jetsimpy afterglow predictions overlaid on the optical and X-ray observations, downward triangles are upper limits. Bottom: Posterior corner plots obtained with MultiNest.