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GRB 240205B: A Reverse Shock Detected in Rapid Response Radio Observations

S. I. Chastain, G. E. Anderson, A. J. van der Horst, L. Rhodes, C. Morley, A. Gulati, J. K. Leung, T. D. Russel, S. D. Ryder

Abstract

Here we present broadband radio modeling of GRB 240205B, using observations with the Australia Telescope Compact Array (ATCA) and the South African MeerKAT radio telescope. Our observations include an automatically triggered early-time ATCA observation that began approximately 13 minutes after the gamma-ray signal and continued for 12 hours, resulting in the earliest detected GRB radio afterglow to date at about 35 minutes post-burst. Following this initial detection, we conducted an extensive radio follow-up campaign for more than 5 months. Although the observations beyond one day post-burst are well described by a standard forward shock model, the observation before one day post-bust reveals an additional synchrotron component, which can be explained as the reverse shock. This component would have been missed without the automated ATCA rapid-response trigger. We find that a combined reverse and forward shock model in a stellar wind medium best describes the radio afterglow. We constrain the spectral breaks due to synchrotron self-absorption and the minimum electron energy, and we use the light-curve peaks to constrain the microphysical parameters. We put GRB 240205B in the context of the growing sample of GRBs with radio detections in the first hours after the gamma-ray trigger. Using our rapid response observation, we estimate the highest model independent constraint on a GRB minimum bulk Lorentz factor of around 100 at about 35 minutes post burst. We also discuss future prospects of detecting similar long GRBs at centimeter wavelengths, as well as potential improvements to future strategies for targeting their radio afterglows.

GRB 240205B: A Reverse Shock Detected in Rapid Response Radio Observations

Abstract

Here we present broadband radio modeling of GRB 240205B, using observations with the Australia Telescope Compact Array (ATCA) and the South African MeerKAT radio telescope. Our observations include an automatically triggered early-time ATCA observation that began approximately 13 minutes after the gamma-ray signal and continued for 12 hours, resulting in the earliest detected GRB radio afterglow to date at about 35 minutes post-burst. Following this initial detection, we conducted an extensive radio follow-up campaign for more than 5 months. Although the observations beyond one day post-burst are well described by a standard forward shock model, the observation before one day post-bust reveals an additional synchrotron component, which can be explained as the reverse shock. This component would have been missed without the automated ATCA rapid-response trigger. We find that a combined reverse and forward shock model in a stellar wind medium best describes the radio afterglow. We constrain the spectral breaks due to synchrotron self-absorption and the minimum electron energy, and we use the light-curve peaks to constrain the microphysical parameters. We put GRB 240205B in the context of the growing sample of GRBs with radio detections in the first hours after the gamma-ray trigger. Using our rapid response observation, we estimate the highest model independent constraint on a GRB minimum bulk Lorentz factor of around 100 at about 35 minutes post burst. We also discuss future prospects of detecting similar long GRBs at centimeter wavelengths, as well as potential improvements to future strategies for targeting their radio afterglows.
Paper Structure (13 sections, 4 equations, 4 figures, 4 tables)

This paper contains 13 sections, 4 equations, 4 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. ATCA
  4. MeerKAT
  5. Modeling
  6. Modeling the Reverse Shock
  7. Full Modeling Results
  8. Discussion
  9. Low Frequency Observations
  10. Constraining Physical Parameters
  11. Minimum Bulk Lorentz Factor
  12. Conclusions
  13. Acknowledgments

Figures (4)

  • Figure 1: GRB 240205B afterglow light curves for each radio observing frequency. The open circles show data from the uv-fitting of the initial 9 GHz observation. The solid lines show the best fit for a combined forward plus reverse shock model for a stellar wind density profile, with a reduced $\chi^2$ of 3.0 with 36 degrees of freedom (DOF); the dashed lines show a fit using the forward shock only, which has a reduced $\chi^2$ of 6.2 with 38 DOF. 68% confidence intervals for the combined fits are indicated by the shaded regions around the lines. The dotted lines indicate the reverse shock component of the combined fit and the dash-dotted lines indicate the forward shock component of the combined fit. The vertical shaded regions indicate the duration of the observation. The statistical error of each flux measurement is added in quadrature with the RMS noise to give the errors shown by the error bars.
  • Figure 2: Same as for Figure \ref{['fig:lc']} except assuming a uniform density profile with the combined forward plus reverse shock model resulting in a reduced $\chi^2$ of 5.4 with 36 DOF; the dashed lines show a fit using the forward shock only, which has a reduced $\chi^2$ of 7.6 with 38 DOF.
  • Figure 3: These plots show our models at three frequencies that will be observable with the SKA and compares the sensitivity of current instruments to the Future SKA.
  • Figure 4: Minimum bulk Lorentz factor $\Gamma_{\rm{min}}$ as a function of cosmological rest frame time for GRBs with detections within the first 12 hours (in observer time) after the gamma-ray trigger 2003AJ....125.2299F2006ApJ...652..490C2014MNRAS.440.2059A2023MNRAS.523.4992A2024ApJ...975L..13A2025arXiv250814650A2015ApJ...815..102F2021ApJ...906..127F2015ApJ...810...31Vlaskar16grb160509a2018ApJ...859..134Llaskar19grb190114c2023NatAs...7..986B. For times at which there are simultaneous observations in multiple observing bands, only the most-constraining (highest) value for $\Gamma_{\rm{min}}$ is shown. The dashed line indicates the time dependence of the $\Gamma_{\rm{min}}$ calculation to guide the eye and is not a fit.