Evidence of Energy Injection in the Short and Distant GRB 250221A in a High Density Environment

TL;DR

This paper analyzes short GRB 250221A, revealing evidence of energy injection in a dense environment, with detailed multi-wavelength observations supporting a complex afterglow behavior and suggesting a possible merger origin in a high-density medium.

Contribution

It provides the first direct afterglow spectroscopy of GRB 250221A, determines its redshift, and models the afterglow to reveal energy injection and a dense circumburst medium, challenging typical progenitor assumptions.

Findings

Evidence of energy injection at ~5×10^4 s in X-ray and optical bands.

Dense circumburst medium with n≈80 cm^-3.

Host galaxy has a star-formation rate of ~3 M_sun/yr.

Abstract

We present the photometric and spectroscopic analysis of the short-duration GRB 250221A ($T_{90}=1.80\pm0.32$ s), using a data set from the optical facilities COLIBR\'I, the Harlingten 50~cm Telescope, and the Very Large Telescope. We complement these observations with data from the Neil Gehrels Swift Observatory and the Einstein Probe, as well as radio observations from the Very Large Array. GRB 250221A is among the few short GRBs with direct afterglow spectroscopy, which gives a secure redshift determination of $z=0.768$ and allows the unambiguous identification of the host as a galaxy with a star-formation rate of $\sim3\,M_\odot\,{\rm yr}^{-1}$. The X-ray and optical light curves up to $T_0+3\times 10^4$ s (where $T_0$ refers to the GRB trigger time) are well described by forward-shock synchrotron emission in the slow-cooling regime within the standard fireball framework. However, at $T_0 \sim 5\times 10^4$ s, both the X-ray and optical bands exhibit an excess over the same interval, which we interpret as evidence of energy injection into a jet with a half-opening angle of $\theta_j=11.5^{\circ}$ through a refreshed shock powered by late central engine activity or a radially stratified ejecta. The burst properties (duration, spectral hardness, peak energy, and location in the Amati plane) all favour a compact binary merger origin. However, our modelling of the afterglow suggests a dense circumburst medium ($n\sim80$ cm$^{-3}$), which is more typical of a collapsar environment.