Search

We report our identification of the optical afterglow and host galaxy of the short-duration gamma-ray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0.162, making it one of the lowest redshift short-duration gamma-ray bursts (sGRBs) identified by Swift. Our intensive follow-up campaign using a range of ground-based facilities as well as Hubble Space Telescope, XMM-Newton, and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light curve at X-ray frequencies reveals a narrow jet, deg, that is refreshed at >1 day post-burst by a slower outflow with significantly more energy than the initial outflow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetized shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass M = (1.0 ±0.6) ×10 M and a secular (post-merger) ejecta mass with M = (1.0 ±0.6) ×10 M , consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared data set provides the best-sampled kilonova light curve without a gravitational wave trigger to date.© 2019. The American Astronomical Society. All rights reserved., Partly based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma; and with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos (program 51-504); and with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundacion Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos (program A32TAC_5). The development of CIRCE at GTC was supported by the University of Florida and the National Science Foundation (grant AST-0352664), in collaboration with IUCAA. Based on data from the GTC Public Archive at CAB (INTA-CSIC). N.R.T., A.J.L., K.W., and B.G. have received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246, TEDE, PI Levan). A.J.L. and J.D.L. acknowledge support from STFC via grant ST/P000495/1. N.R.T. and G.P.L. acknowledge support from STFC via grant ST/N000757/1. E.P. acknowledges support from grant ASI/INAF I/088/06/0. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). D.B.M. acknowledges support from the Instrument center for Danish astrophysics (IDA). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. A.C. acknowledges support from the National Science Foundation CAREER award #1455090. A.d.U.P., C.C.T., Z.C., L.I., and D.A.K. acknowledge support from the Spanish research projects AYA2014-58381-P and AYA2017-89384-P, from the State Agency for Research of the Spanish MCIU through the >Center of Excellence Severo Ochoa> award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). A.d.U.P. and C.C.T. acknowledge support from Ramon y Cajal fellowships (RyC-2012-09975, and RyC-2012-09984). Z.C., L.I., and D.A.K. acknowledge support from Juan de la Cierva Incorporacion fellowships (JdCI-2014-21669, IJCI-2016-30940, and IJCI-2015-26153). P.A.E. and K.L.P. acknowledge support from the UK Space Agency. K.E.H. and P.J. acknowledge support by a Project grant (162948-051) from The Icelandic Research Fund. S.R. has been supported by the Swedish Research Council (VR) under grant No. 2016-03657_3, by the Swedish National Space Board under grant No. Dnr 107/16 and by the research environment grant >Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)> funded by the Swedish Research council (VR) under Dnr 2016-06012.