Evidence of Energy Injection in the Short and Distant GRB 250221A
Authors
Camila Angulo-Valdez
Rosa L. Becerra
Ramandeep Gill
Noémie Globus
William H. Lee
Diego López-Cámara
Cassidy Mihalenko
Enrique Moreno-Méndez
Roberto Ricci
Karelle Siellez
Alan M. Watson
Muskan Yadav
Yu-han Yang
Dalya Akl
Sarah Antier
Jean-Luc Atteia
Stéphane Basa
Nathaniel R. Butler
Simone Dichiara
Damien Dornic
Jean-Grégoire Ducoin
Francis Fortin
Leonardo García-García
Kin Ocelotl López
Francesco Magnani
Brendan O'Connor
Margarita Pereyra
Ny Avo Rakotondrainibe
Fredd Sánchez-Álvarez
Benjamin Schneider
Eleonora Troja
Antonio de Ugarte Postigo
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 COLIBRÍ, the Harlingten 50 cm Telescope, and the Very Large Telescope. We complement these observations with data from the \textit{Neil Gehrels Swift Observatory} and the \textit{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 $T_0+10$ ks (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 $T_0+0.6$ days, 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 $θ_j=11.5^{\circ}$ 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. This tension over the classification of this burst (short-hard vs. long-soft) as inferred from the prompt and afterglow emissions makes GRB~250221A an unusual event and underscores the limitations of duration-based classifications and the importance of multi-wavelength, time-resolved follow-up observations.
