The radio afterglow of the ultra-long GRB 220627A

TL;DR

This study presents the radio afterglow campaign of GRB 220627A, the most distant ultra-long GRB to date at $z=3.084$, and uses VLBI to test the gravitational-lensing hypothesis. A re-analysis of the Fermi/GBM data shows two emission episodes with distinct spectra, disfavoring lensing and confirming the ultra-long classification. The radio light curve exhibits an initial $F_ u\propto t^{-2}$ decay transitioning to $F_ u\propto t^{-1/2}$ around $\sim$20 d, which is explained by three scenarios: energy injection from slower ejecta, a steep external-density profile $n\propto r^{-k}$ with $k\approx8/3$, or an additional wide, cocoon-like ejecta component; a wide-ejecta/cocoon interpretation aligns with a blue supergiant progenitor. The VLBI experiment yielded a non-detection, highlighting limitations in sensitivity and trigger speed, and underscores the need for real-time correlation capabilities and higher-sensitivity arrays (e.g., SKA/ngVLA-era) to confirm lensed GRBs and enable time-delay cosmography with milli-lenses. Overall, the work demonstrates the potential of high-resolution radio astronomy to test fundamental physics, constrain GRB energetics and environments, and pave the way for future gravitational-lensing studies in the era of next-generation facilities.

Abstract

We present the discovery of the radio afterglow of the most distant ultra-long gamma-ray burst (GRB) detected to date, GRB~220627A at redshift $z=3.084$. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting ${\sim} 15$\,min, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the \textit{Fermi}/GBM spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456\,d post-burst: an initial, steep decay ($F_ν \propto t^{-2}$) is followed by a shallower decline ($F_ν \propto t^{-1/2}$) after ${\sim} 20$\,d. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as $n \propto r^{-8/3}$; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via Very Long Baseline Interferometry (VLBI) observations at ${\sim} 12$\,d post-burst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.

Figures (7)

• Figure 1: Background subtracted light curve of GRB 220627A from the Fermi/GBM NaI#3 and BGO#0 detectors showing the count rate obtained integrating the signal in the $10-900\,$keV and $300-30\,000\,$keV energy range, respectively. The shaded pink and green regions show the time intervals selected for the spectral analysis.
• Figure 2: Posterior distributions of the parameters of the spectral fit to the two time intervals (violet and green for interval 1 and 2 respectively, see Table \ref{['tab:gbm']}). The median and 95% confidence intervals are marked by the dashed vertical lines in the histograms.
• Figure 3: Spectra energy distribution of the prompt emission spectrum. The two time intervals analysed are shown. The shaded regions represent the 68% confidence interval on the best fit model (solid lines).
• Figure 4: Left: Radio light curves for GRB 220627A. Radio detections are represented with circular markers. $3\sigma$ upper limits, represented using downward-pointing triangular markers, are used for all non-detections, with the exception of the VLBA observation ($t=11.7$ d, $\nu=15.2$ GHz), where a $5\sigma$ upper limit is used instead. The grey dotted and dashed lines show a $t^{-2}$ and a $t^{-1/2}$ slope, respectively, for reference. Right: Radio spectra for GRB 220627A. The plot shows different spectral snapshots centred around 3 different times as indicated in the legend. Markers carry the same meaning as those in the light curves plot. The grey dotted and dashed lines show a $\nu^{2}$ and a $\nu^{1/3}$ slope, respectively, for reference.
• Figure 5: Left: Multi-wavelength light curves for GRB 220627A and forward shock synchrotron model from deWet2023. The radio data points are represented with circular markers, the near-infrared and optical data points with star markers, and the X-ray data points with square markers. $3\sigma$ upper limits, represented using downward-pointing triangular markers, are used for all non-detections, with the exception of the VLBA observation (highlighted with a red square), where a $5\sigma$ upper limit is used instead. The dashed lines represent the $5\sigma$ sensitivity thresholds expected for the VLBA+HSA and SKA-VLBI/ngVLA-LBA arrays (in 10 and 1 h of integration, respectively). Right: Multi-wavelength SED snapshots for GRB 220627A, from radio frequencies through to X-ray energies. The plot shows different spectral snapshots of the afterglow centred around 6 different times as indicated in the legend. The temporal window for data points included in each SED snapshot is indicated by the shaded vertical bands in the light curve plot. Markers in the SED snapshots plot carry the same meaning as those in the light curves plot. The residual plots on the bottom, showing the difference between the data and the model for all detections in logarithmic space, highlight the discrepancy between the shallow radio evolution after 15 d post-burst and the synchrotron model from deWet2023.