The Late-time Afterglow Evolution of Long Gamma-Ray Bursts GRB 160625B and GRB 160509A

We present post-jet-break Hubble Space Telescope, Very Large Array, 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 is 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 ( ${f}_{\nu }\propto {t}^{-1.96\pm 0.07}$ ) 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, but 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 ( ${t}_{j,\mathrm{radio}}\gt 10\times {t}_{j,\mathrm{optical}}$ ) and possibly a two-component jet for both bursts.