# A Reverse Shock and Unusual Radio Properties in GRB 160625B

Kate D. Alexander, Tanmoy Laskar, E. Berger, C. Guidorzi, S. Dichiara, W. F. Fong, A. Gomboc, S. Kobayashi, D. Kopac, C. G. Mundell, N. R. Tanvir

Research output: Contribution to journalArticle

14 Citations (Scopus)

### Abstract

We present multi-wavelength observations and modeling of the exceptionally bright long $\gamma$-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 $\theta_j\approx 3.6^\circ$ and kinetic energy of $E_K\approx 2\times10^{51}$ erg, propagating into a low density ($n\approx 5\times10^{-5}$ 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 $\Gamma_0\gtrsim 100$ and an ejecta magnetization of $R_B\approx 1-100$. 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.
Original language English 69 13 Astrophysical Journal 848 1 Published - 12 Oct 2017

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shock
scattering
ejecta
gamma ray bursts
quasars
scintillation
synchrotrons
temporal evolution
kinetic energy
magnetization
erg
probes
probe
profiles
wavelength
wavelengths

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### Cite this

Alexander, K. D., Laskar, T., Berger, E., Guidorzi, C., Dichiara, S., Fong, W. F., ... Tanvir, N. R. (2017). A Reverse Shock and Unusual Radio Properties in GRB 160625B. Astrophysical Journal, 848(1), [69]. https://doi.org/10.3847/1538-4357/aa8a76, https://doi.org/10.3847/1538-4357/aa8a76

A Reverse Shock and Unusual Radio Properties in GRB 160625B. / Alexander, Kate D.; Laskar, Tanmoy; Berger, E.; Guidorzi, C.; Dichiara, S.; Fong, W. F.; Gomboc, A.; Kobayashi, S.; Kopac, D.; Mundell, C. G.; Tanvir, N. R.

In: Astrophysical Journal, Vol. 848, No. 1, 69, 12.10.2017.

Research output: Contribution to journalArticle

Alexander, KD, Laskar, T, Berger, E, Guidorzi, C, Dichiara, S, Fong, WF, Gomboc, A, Kobayashi, S, Kopac, D, Mundell, CG & Tanvir, NR 2017, 'A Reverse Shock and Unusual Radio Properties in GRB 160625B', Astrophysical Journal, vol. 848, no. 1, 69. https://doi.org/10.3847/1538-4357/aa8a76, https://doi.org/10.3847/1538-4357/aa8a76
Alexander, Kate D. ; Laskar, Tanmoy ; Berger, E. ; Guidorzi, C. ; Dichiara, S. ; Fong, W. F. ; Gomboc, A. ; Kobayashi, S. ; Kopac, D. ; Mundell, C. G. ; Tanvir, N. R. / A Reverse Shock and Unusual Radio Properties in GRB 160625B. In: Astrophysical Journal. 2017 ; Vol. 848, No. 1.
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