First results from the PanRadio GRB Collaboration: the 400-day afterglow of GRB~230815A

TL;DR

PanRadio launches an unbiased, multi-year radio survey of southern Swift GRBs to map jet structure, environment, and microphysics from early to late times. The first results for GRB 230815A combine ATCA radio data with VLT/HAWK-I near-IR and Swift/XRT X-ray measurements, revealing an early X-ray jet break that cannot be explained by radio evolution under a single-component jet; a two-component jet, with a narrow core ($\theta_j \approx 2.1^\circ$) and a wide sheath ($\theta_{j,w} \gtrsim 35^\circ$), reconciles the observations and explains the persistent radio emission out to ~400 days. The radio data favor an ISM-like environment and exhibit a chromatic turnover best described by evolving synchrotron break frequencies $\nu_m$ and $\nu_a$, with $\nu_m(t) \propto t^{-2.1\pm0.3}$ and $\nu_a(t) \propto t^{0.5}$ before crossing, while the inferred electron-index $p \approx 2.57^{+0.41}_{-0.33}$ aligns with broad afterglow populations. Overall, the study demonstrates the power of comprehensive, multi-frequency radio monitoring to constrain jet geometry, circumburst environments, and microphysical parameters, highlighting the need for larger, unbiased PanRadio samples to achieve population-level insights and robust calorimetry.

Abstract

We introduce the PanRadio Gamma-ray Burst (GRB) program carried out on the Australia Telescope Compact Array: a systematic, multi-year, radio survey of all southern \textit{Swift} GRB events, comprehensively following the multi-frequency evolution of their afterglows from within an hour to years post-burst. We present the results of the 400-day observing campaign following the afterglow of long-duration (collapsar) GRB~230815A, the first one detected through this program. Typically, GRB~230815A would not otherwise receive traditional radio follow-up, given it has no known redshift and lacks comprehensive multi-wavelength follow-up due to its high line-of-sight extinction with $A_V = 2.3$. We found its early X-ray jet break at ${\sim}0.1$ days post-burst to be at odds with the evolution of the multi-frequency radio light curves that were traced over an unusually long duration of $400$ days. The radio light curves approximately evolved (with minor deviations) according to the standard self-similar expansion for a relativistic blast wave in a homogeneous environment prior to jet break, showing no evidence for evolution in the microphysical parameters describing the electron acceleration processes. We reconcile these features by proposing a two-component jet: the early X-ray break originates from a narrow component with a half-opening angle ${\sim}2.1^{\circ}$, while the evolution of the radio afterglow stems from a wider component with a half-opening angle $\gtrapprox 35^{\circ}$. The PanRadio GRB program will establish a sample of comprehensively followed GRBs, where a rigorous inspection of their microphysical and dynamical parameters can be performed, thereby revealing the diversity of features in their outflows and environments.

Figures (12)

• Figure 1: X-ray light curve for GRB 230815A. Data points obtained by the Swift/XRT in the WT mode are represented by blue error bars while those obtained in PC mode are represented by orange error bars. The data points used in the fits to the X-ray light curve, obtained from binning the Swift/XRT data points, are represented by red square markers. Fits to a smoothly broken power law and a power law are shown by the solid and dot-dashed lines, respectively. The slopes of the fits are annotated beside the lines.
• Figure 2: Broadband SED of GRB 230815A, showing two temporal snapshots at approximately 0.5 (red) and 1.5 (blue) days post-burst. The light gray points represent data from later epochs after 1.5 days post-burst. Radio detections are shown with circular markers, near-infrared detections with star markers, and all upper limits are shown with inverted triangular markers. The X-ray spectrum (dark red filled) is extrapolated to the near-infrared part of the spectrum (light red filled) using a photon index of $\Gamma = 1.94^{+0.11}_{-0.10}$, where the shaded region corresponds to a 68% confidence interval. The slopes between various data points in each spectral snapshot are annotated next to the corresponding dashed guide lines.
• Figure 3: Left: Multi-wavelength light curves for GRB 230815A. The radio data points are represented with circular markers, the near-infrared data points with star markers, and the X-ray data points with square markers. All upper limits are shown with inverted triangular markers. The fits describing the evolution of the X-ray and near-infrared light curves are shown using dashed lines with the corresponding temporal slopes annotated next to them. The radio data points surrounded by the red rectangle are shown in more detail on the right panel. The coloured vertical strips indicate the temporal windows used for constructing the SEDs presented in Figures \ref{['fig:bbsed']} and \ref{['fig:radioseds']}, where the colour of each strip here corresponds to an SED of the same colour in those Figures. Right: Radio light curves for GRB 230815A at 5.5, 9, 16.7, and 21.2 GHz (top to bottom). Each detection is represented by a circular marker and upper limit with an inverted triangular marker. At each frequency, the solid blue line represents the smoothly broken power-law fit (model 1; independent $\delta_1$ and $\delta_2$ per frequency) to the light curve using the best-fit parameters estimated from nested sampling, while the 50 lines with weaker line intensity are random posterior samples used to illustrate the fit uncertainties.
• Figure 4: Radio SEDs of GRB 230815A at the 12 different epochs constructed from the temporal windows shown in Figure \ref{['fig:mwlc']}. The epoch is labelled in the legend for each panel. The dot-dashed lines are guide lines showing different spectral slopes. These spectral slopes are labelled in SED 1 (0.5 days post-burst) with $p=2.2$ adopted here (see text). Detections are shown in filled circular markers while $3\sigma$ upper limits for non-detections are shown using inverted triangular markers. The thin lines in each panel show model spectra from 100 random posterior samples from the nested sampling procedure performed to constrain our evolving synchrotron model in Subsection \ref{['ssec:radiosed_new']}.
• Figure 5: Time evolution of the injection ($\nu_\mathrm{m}$, red lines) and self-absorption ($\nu_\mathrm{a}$, blue lines) frequencies from 100 random posterior samples of the evolving synchrotron spectrum model of Subsection \ref{['ssec:radiosed_new']}. The grey dash-dotted lines show the expected evolution of the two frequencies in the self-similar phase of a Blandford1976 blast wave (valid for an homogenous ISM environment), as given in Granot2002, assuming $p=2.6$. The dotted line shows $t^{1/2}$, which approximates the initial evolution of $\nu_\mathrm{a}$.