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A Reverse Shock and Unusual Radio Properties in GRB 160625B

Alexander, KD and Laskar, T and Berger, E and Guidorzi, C and Dichiara, S and Fong, W and Gomboc, A and Kobayashi, S and Kopac, D and Mundell, CG and Tanvir, NR and Williams, PKG (2017) A Reverse Shock and Unusual Radio Properties in GRB 160625B. Astrophysical Journal, 848 (1). ISSN 0004-637X

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Abstract

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 ${\theta }_{j}\approx 3\buildrel{\circ}\over{.} 6$ and kinetic energy of ${E}_{K}\approx 2\times {10}^{51}$ erg, propagating into a low-density ($n\approx 5\times {10}^{-5}$ 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 ${{\rm{\Gamma }}}_{0}\gtrsim 100$ and an ejecta magnetization of ${R}_{B}\approx 1\mbox{--}100$. 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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences, 0305 Organic Chemistry, 0306 Physical Chemistry (Incl. Structural)
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Q Science > QD Chemistry
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society; IOP Publishing
Related URLs:
Date Deposited: 30 Oct 2017 12:26
Last Modified: 30 Oct 2017 12:26
DOI or Identification number: 10.3847/1538-4357/aa8a76
URI: http://researchonline.ljmu.ac.uk/id/eprint/7445

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