Journal
ASTROPHYSICAL JOURNAL
Volume 894, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/ab8799
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Funding
- National Science Foundation via CAREER award [1455090]
- Ramon y Cajal fellowship [RyC-201209975]
- Spanish research project [AYA2017-89384-P]
- State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award [SEV-2017-0709]
- European Research Council [773062]
- NASA MIRO grant [NNX15AP95A]
- NASA/ESA Hubble Space Telescope [GO 14353]
- NASA [NAS 5-26555, NNX10AF62G]
- National Aeronautics and Space Administration through Chandra Award [17500753]
- National Aeronautics Space Administration [NAS8-03060]
- University of Florida
- National Science Foundation [AST-0352664]
- IUCAA
- NSF [AST1009863]
- NASA [800666, NNX15AP95A] Funding Source: Federal RePORTER
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1455090] Funding Source: National Science Foundation
- STFC [ST/S000453/1] Funding Source: UKRI
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We present post-jet-break Hubble Space Telescope, Very Large Array, and Chandra observations of the afterglow of the long gamma-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 is also inconsistent with the f(nu) proportional to 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(nu) proportional to 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 similar to 3 x 10(52) 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(nu) proportional to t(-1)) with no breaks. This requires a highly chromatic jet break and possibly a two-component jet for both bursts.
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