Short-duration GRB 250221A Afterglow Driven by Two-Component Jets from merger of compact star

Abstract

GRB 250221A is a short gamma-ray burst (GRB) at redshift $z=0.768$, with a duration of 1.8 s and no extended emission in either Swift/BAT or Konus-Wind bands. A remarkable re-brightening feature in both optical and X-ray bands was observed at $\sim 0.6$ days after the burst trigger, but no supernova or kilonova signature was detected. The burst properties and empirical correlations or distributions (e.g., duration, spectral hardness, location in the Amati correlation, $\varepsilon-$value, $f_{\rm eff}$ parameter, and physical offset) favor a compact binary merger origin. However, a dense circumburst medium with $n\sim 80\rm~cm^{-3}$, obtained by adopting the energy injection into a jet to interpret the late-time re-brightening is inconsistent with the compact binary merger origin. In this paper, we propose a two-component jet model to explain the multiwavelength afterglow observations of GRB 250221A, in which the relativistic narrow jet ($\rm θ_{c} \sim 3.8^\circ$) produces the prompt and the early decay afterglow emission, while the mildly relativistic wide jet ($\rm θ_{w} \sim 4.4^\circ$) dominates at later times, resulting in the observed re-brightening feature. If this is the case, one can obtain a lower medium density with $n\sim 0.72\rm~cm^{-3}$ which is a little bit higher than that of short GRBs in merger environments, but falls into the reasonable and acceptable range. Finally, a possible kilonova emission is also discussed within the scenario of compact star merger origin of GRB 250221A.

Figures (5)

• Figure 1: (a) $E_{\rm p}-E_{\rm \gamma,iso}$ correlation plane. Gray diamonds and black filled circles represent Type II and Type I GRBs, respectively. (b) 1D and 2D distributions in the $T_{90}-\varepsilon$ plane. The dashed line marks the empirical boundary $\varepsilon = 0.03$. (c) The $f_{\rm eff}-T_{\rm 90}$ distribution. The vertical solid line denotes the traditional classification boundary at $T_{90}$ = 2 s. The data adopted above are taken from 2002AA...390...81A2009ApJ...703.1696Z2010ApJ...725.1965L2014MNRAS.442.1922L.
• Figure 2: Left: Physical offset distributions of long (gray, 2016ApJ...817..144B) and short (blue, 2010ApJ...708....9F) GRBs. Right: Distribution of star formation rates of host galaxies for long (gray, 2025ApJ...993...20D) and short (blue, 2016ApJS..227....7L and 2022ApJ...940...57N) GRBs. The dotted line is the best Gaussian fits, and the red vertical line is GRB 250221A.
• Figure 3: Left: Best-fit multiwavelength afterglow light curves with the two-component jet model (solid lines). The blue circles represent the X-ray data (at 1 keV), while the optical (orange) and radio (green) observational data are taken from 2026MNRAS.tmp..179A. Right: Distribution of circumburst medium density for short GRBs 2015ApJ...815..102F. The red vertical dotted line is the inferred circumburst density (e.g., $n\sim 0.72~\rm cm^{-3}$) of GRB 250221A.
• Figure 4: Corner diagram of the posterior distributions with the MCMC method by adopting two-component jet model.
• Figure 5: Kilonova emission in $i$ band (red) and $r$ band (blue). The parameters are adopted as ejecta masses $M_{\rm ej} = 10^{-3}$--$10^{-1}~M_{\odot}$, ejecta velocity $\beta = 0.1$, and opacity $\kappa = 1~\rm cm^2~g^{-1}$. The red circles and blue diamonds are the observational data in the optical $i$ band and $r$ band, respectively.