University of Leicester
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A Reverse Shock and Unusual Radio Properties in GRB 160625B

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journal contribution
posted on 2018-01-23, 09:50 authored by K. D. Alexander, T. Laskar, E. Berger, C. Guidorzi, S. Dichiara, W. Fong, A. Gomboc, S. Kobayashi, D. Kopac, C. G. Mundell, N. R. Tanvir, P. K. G. Williams
We present multi-wavelength observations and modeling of the exceptionally bright long γ-ray burst GRB 160625B. The optical and X-ray data are well fit by synchrotron emission from a collimated blastwave with an opening angle of qj » 3.6 and kinetic energy of EK 2 10 » ´ 51 erg, propagating into a low-density (n 5 10 » ´ -5 cm−3 ) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shock, indicating an initial Lorentz factor of G0  100 and an ejecta magnetization of RB » 1 100 – . The second component exhibits peculiar spectral and temporal evolution and is most likely the result of scattering of the radio emission by the turbulent Milky Way interstellar medium (ISM). Such scattering is expected in any sufficiently compact extragalactic source and has been seen in GRBs before, but the large amplitude and long duration of the variability seen here are qualitatively more similar to extreme scattering events previously observed in quasars, rather than normal interstellar scintillation effects. High-cadence, broadband radio observations of future GRBs are needed to fully characterize such effects, which can sensitively probe the properties of the ISM and must be taken into account before variability intrinsic to the GRB can be interpreted correctly.


K.D.A. and E.B. acknowledge support from NSF grant AST-1411763 and NASA ADA grant NNX15AE50G. T.L. is a Jansky Fellow of the National Radio Astronomy Observatory (NRAO). W.F. is supported by NASA through Einstein Postdoctoral Fellowship grant number PF4-150121. VLA observations were taken as part of our VLA Large Program 15A-235 (PI: E. Berger). The VLA is operated by the NRAO, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. Software: CASA (McMullin et al. 2007), pwkit (Williams et al. 2017).



Astrophysical Journal, 2017, 848 (1), 69

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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