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

Research output: Contribution to journalArticlepeer-review

Abstract

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 t3/4 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 (fν ∝ t1.96±0.07) is also inconsistent with the fν ∝ tp 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 t3/4 steepening and with fν ∝ 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 fν ∝ t1) with no breaks. This requires a highly chromatic jet break (tj,radio > 10 × tj,optical) and possibly a two-component jet for both bursts.

Original languageEnglish
JournalUnknown Journal
StatePublished - Jun 8 2019

Keywords

  • Gamma-ray burst: general
  • Gamma-ray burst: individual (GRB 160625B; GRB 160509A)
  • Relativistic processes

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