Evidence of Energy Injection in the Short and Distant GRB 250221A

TL;DR

GRB 250221A challenges simple short/long classifications by exhibiting an achromatic rebrightening at ~0.6 days, best explained by energy injection into the forward shock (a refreshed shock) within a dense ISM. The study combines rapid optical spectroscopy with broad multi-wavelength photometry and spectroscopy of the host to constrain redshift ($z=0.768$) and host star formation, and to model the afterglow dynamics. The results indicate a high initial Lorentz factor and a jet opening angle around $\theta_j\approx11.5^{\circ}$, with an energy budget $E_{k,iso}\gtrsim10^{52}$ erg and injected energy $E_{inj}\approx4.5\,E_{k,iso}$ during the rebrightening; the inferred circumburst density $n\gtrsim80\ \mathrm{cm}^{-3}$ leans toward a Collapsar-like environment, while prompt properties hint at a compact binary merger, highlighting the limitations of duration-based GRB classification. Overall, GRB 250221A demonstrates the critical role of time-resolved, multi-wavelength follow-up and afterglow spectroscopy for disentangling progenitor channels and energy-injection physics in GRBs.

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.

Figures (13)

• Figure 1: Top: X-ray (0.3--10.0 keV, at 1 keV, black circles), optical (yellow diamonds for i, red hexagons for r, green stars for g) and radio (at 9 GHz from ATCA 39501 and 10 GHz from VLA, brown pentagons) observations of GRB 250221A. The dashed lines show the power-law fits to the afterglow for X-rays (fitted before $T \lesssim 2 \times 10^{4}$ s and after $T > 10^{5}$ s) and optical (fitted for $T < 10^{4}$ s for the i band and at $T > 10^{5}$ s for the late r band). Bottom: Optical-to-X-rays spectral indices for three different epochs, the first from $T_0+100$ s to $T_0+10^{4}$ s, with a median arrival time of the X-rays photons at $T_0+1800$ s (see Figure \ref{['fig:sed']}) (purple), second at $T_0+ 9\times 10^{4}$ s (blue), and third at $T_0+1.8\times 10^{5}$ s (green). We also illustrate the optical light curves of SN1998bw Galama1998Clocchiatti2011 and SN2011kl Greiner2015 (placed at $z = 0.768$) to highlight that the excess is similar in brightness (in the optical) but differs in timescale.
• Figure 2: Field of GRB 250221A. We show the R filter image obtained with the VLT/FORS2 after $T_0+0.9$ days (panel a) and $T_0+11.9$ days (panel b). We also show the contribution of the afterglow after image subtraction (panel c). We highlight the presence of two galaxies at $photo-z=0.343\pm0.077$Dey2019 and at $z=0.768$ (G2, magenta circle).
• Figure 3: VLT/X-shooter optical spectrum of GRB 250221A (black line) and its corresponding 1$\sigma$ error (gray line), obtained at $T_0 + 21.4$hours. We identify emission lines of [OII], [NeIII], H$\gamma$, H$\beta$, and [OIII] along with ISM absorption lines from MgII, FeII, all consistent with a redshift of $z = 0.768$ (magenta). We also zoom in on the H$\alpha$ emission line (which, for readability, is not shown in the main figure) using the same y-axis interval adopted for the estimation of the star formation rate of G2 (see Section \ref{['sec:redshift']}). We also illustrate the telluric lines (gray shaded regions).
• Figure 4: Masked-weighted Swift/BAT (15--350 keV) light curve of GRB 250221A with 1 s binning produced with the HEASoft/FTOOLs software (black solid line). The background count rate is shown in gray. The $T_{90}$ duration of $1.80 \pm 0.32$ s is indicated by the shaded magenta region.
• Figure 5: HR versus $T_{90}$ diagram for GRB 250221A (crimson star), plotted alongside a sample of Swift/BAT GRBs. Division between LGRBs (orange) and SGRBs (blue) is done automatically by Swift/BAT based on $T_{90}$ duration of 2 seconds. Hybrid GRB events (black diamonds) are poorly classified using a purely duration based classification scheme, resulting in ambiguity in progenitor assessment. Both GRB 250221A and GRB 170817A (black star) lie in the ambiguous region between the GRB classes in this space, and within the wide spread of the hybrid GRBs.