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

Tuomas Kangas, Andrew S. Fruchter, S. Bradley Cenko, Alessandra Corsi, Antonio De Ugarte Postigo, Asaf Pe'er, Stuart N. Vogel, Antonino Cucchiara, Benjamin Gompertz, John Graham, Andrew Levan, Kuntal Misra, Daniel A. Perley, Judith Racusin, Nial Tanvir

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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 ({equation presented}) 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 ({equation presented}) and possibly a two-component jet for both bursts.

Original languageEnglish
Article number43
JournalAstrophysical Journal
Issue number1
StatePublished - 1 May 2020

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