Evidence for an intrinsic luminosity-decay correlation in GRB radio afterglows

Shilling, S P R and Oates, S R and Kann, D A and Patel, J and Racusin, J L and Cenko, B and Gupta, R and Smith, M and Rhodes, L and Hinds, K R and Nicholl, M and Breeveld, A and Page, M and De Pasquale, M and Gompertz, B (2025) Evidence for an intrinsic luminosity-decay correlation in GRB radio afterglows. Monthly Notices of the Royal Astronomical Society, 542 (3). pp. 2421-2430. ISSN 0035-8711

Abstract

We present the discovery of a correlation, in a sample of 16 gamma-ray burst 8.5 GHz radio afterglows, between the intrinsic luminosity measured at 10 d in the rest frame, $L_{\mathrm{Radio,10\,d}}$, and the average rate of decay past this time, $\alpha _{{>}10\,\mathrm{ d}}$. The correlation has a Spearman’s rank coefficient of $-0.70 \pm 0.13$ at a significance of ${>}3\sigma$ and a linear regression fit of $\alpha _{>10\,\mathrm{ d}} = -0.29^{+0.19}_{-0.28} \log \left(L_{\mathrm{Radio,10\,d}} \right) + 8.12^{+ 8.86}_{-5.88}$. This finding suggests that more luminous radio afterglows have higher average rates of decay than less luminous ones. We use a Monte Carlo simulation to show the correlation is not produced by chance or selection effects at a confidence level of ${>}3\sigma$. Previous studies found this relation in optical/UV, X-ray, and GeV afterglow light curves, and we have now extended it to radio light curves. The Spearman’s rank coefficients and the linear regression slopes for the correlation in each waveband are all consistent within $1\sigma$. We discuss how these new results in the radio band support the effects of observer viewing geometry, and time-varying microphysical parameters, as possible causes of the correlation as suggested in previous works.