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

Kangas, T, Fruchter, AS, Cenko, SB, Corsi, A, de Ugarte Postigo, A, Pe'er, A, Vogel, SN, Cucchiara, A, Gompertz, B, Graham, J, Levan, A, Misra, K, Perley, DA, Racusin, J and Tanvir, N (2020) The Late-time Afterglow Evolution of Long Gamma-Ray Bursts GRB 160625B and GRB 160509A. Astrophysical Journal, 894 (1). ISSN 0004-637X

 Text The Late-time Afterglow Evolution of Long Gamma-Ray Bursts GRB 160625B and GRB 160509A.pdf - Accepted Version Restricted to Repository staff only until 5 May 2021. Download (1MB)

## Abstract

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.

Item Type: Article 0201 Astronomical and Space Sciences, 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics, 0306 Physical Chemistry (incl. Structural) Q Science > QB AstronomyQ Science > QC Physics Astrophysics Research Institute American Astronomical Society; IOP Publishing Author 06 Aug 2020 08:34 09 Sep 2020 12:00 10.3847/1538-4357/ab8799 http://researchonline.ljmu.ac.uk/id/eprint/13451

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