The late-time afterglow evolution of long gamma-ray bursts GRB 160625B and GRB 160509A

We present post-jet-break HST, VLA and Chandra observations of the afterglow of the long γ-ray bursts GRB 160625B (between 69 and 209 days) and GRB 160509A (between 35 and 80 days). We calculate the post-jet-break decline rates of the light curves, and find the afterglow of GRB 160625B inconsistent with a simple t⁻³/⁴ steepening over the break, expected from the geometric effect of the jet edge entering our line of sight. However, the favored optical post-break decline (f_ν ∝ t^{−1.96±0.07}) is also inconsistent with the f_ν ∝ t⁻p decline (where p ≈ 2.3 from the pre-break light curve), which is expected from exponential lateral expansion of the jet; perhaps suggesting lateral expansion that only affects a fraction of the jet. The post-break decline of GRB 160509A is consistent with both the t⁻³/⁴ steepening and with f_ν ∝ t^−p. We also use boxfit to fit afterglow models to both light curves and find both to be energetically consistent with a millisecond magnetar central engine, although the magnetar parameters need to be extreme (i.e. E ∼ 3 × 10⁵² erg). Finally, the late-time radio light curves of both afterglows are not reproduced well by boxfit and are inconsistent with predictions from the standard jet model; instead both are well represented by a single power law decline (roughly f_ν ∝ t⁻¹) with no breaks. This requires a highly chromatic jet break (t_j,radio > 10 × t_j,optical) and possibly a two-component jet for both bursts.