TY - JOUR
T1 - An Unexpectedly Small Emission Region Size Inferred from Strong High-frequency Diffractive Scintillation in GRB 161219B
AU - Alexander, K. D.
AU - Laskar, T.
AU - Berger, E.
AU - Johnson, M. D.
AU - Williams, P. K.G.
AU - Dichiara, S.
AU - Fong, Wen-fai
AU - Gomboc, A.
AU - Kobayashi, S.
AU - Margutti, Raffaella
AU - Mundell, C. G.
N1 - Funding Information:
We thank R. Narayan and D. Frail for useful conversations. K.D.A. and E.B. acknowledge support from NSF grant AST-1714498 and NASA ADA grant NNX15AE50G. K.D.A. additionally acknowledges support provided by NASA through the NASA Hubble Fellowship grant #HST-HF2-51403.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. T.L. is a Jansky Fellow of the National Radio Astronomy Observatory (NRAO). W.F. acknowledges support for Program number HST-HF2-51390.001-A, provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. A.G. acknowledges the financial support from the Slovenian Research Agency (research core funding No. P1-0031 and project grant No. J1-8136) and networking support by the COST Action GWverse CA16104. VLA observations were taken as part of our VLA Large Program 15A-235 (PI: E. Berger). The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved.
PY - 2019/1/10
Y1 - 2019/1/10
N2 - We present Karl G. Jansky Very Large Array radio observations of the long gamma-ray burst GRB 161219B (z = 0.147) spanning 1-37 GHz. The data exhibit unusual behavior, including sharp spectral peaks and minutes-timescale large-amplitude variability centered at 20 GHz and spanning the full frequency range. We attribute this behavior to scattering of the radio emission by the turbulent ionized Galactic interstellar medium (ISM), including both diffractive and refractive scintillation. However, the scintillation is much stronger than predicted by a model of the Galactic electron density distribution (NE2001); from the measured variability timescale and decorrelation bandwidth we infer a scattering measure of SM ≈ (8-70) 10-4 kpc m-20/3 (up to 25 times larger than predicted in NE2001) and a scattering screen distance of d scr ≈ 0.2-3 kpc. We infer an emission region size of μas ( cm) at ≈4 days, and find that prior to 8 days the source size is an order of magnitude smaller than model predictions for a uniformly illuminated disk or limb-brightened ring, indicating a slightly off-axis viewing angle or significant substructure in the emission region. Simultaneous multi-hour broadband radio observations of future GRB afterglows will allow us to characterize the scintillation more completely, and hence to probe the observer viewing angle, the evolution of the jet Lorentz factor, the structure of the afterglow emission regions, and ISM turbulence at high Galactic latitudes.
AB - We present Karl G. Jansky Very Large Array radio observations of the long gamma-ray burst GRB 161219B (z = 0.147) spanning 1-37 GHz. The data exhibit unusual behavior, including sharp spectral peaks and minutes-timescale large-amplitude variability centered at 20 GHz and spanning the full frequency range. We attribute this behavior to scattering of the radio emission by the turbulent ionized Galactic interstellar medium (ISM), including both diffractive and refractive scintillation. However, the scintillation is much stronger than predicted by a model of the Galactic electron density distribution (NE2001); from the measured variability timescale and decorrelation bandwidth we infer a scattering measure of SM ≈ (8-70) 10-4 kpc m-20/3 (up to 25 times larger than predicted in NE2001) and a scattering screen distance of d scr ≈ 0.2-3 kpc. We infer an emission region size of μas ( cm) at ≈4 days, and find that prior to 8 days the source size is an order of magnitude smaller than model predictions for a uniformly illuminated disk or limb-brightened ring, indicating a slightly off-axis viewing angle or significant substructure in the emission region. Simultaneous multi-hour broadband radio observations of future GRB afterglows will allow us to characterize the scintillation more completely, and hence to probe the observer viewing angle, the evolution of the jet Lorentz factor, the structure of the afterglow emission regions, and ISM turbulence at high Galactic latitudes.
KW - gamma-ray burst: general
KW - gamma-ray burst: individual (GRB 161219B)
KW - scattering
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U2 - 10.3847/1538-4357/aaf19d
DO - 10.3847/1538-4357/aaf19d
M3 - Article
AN - SCOPUS:85060237727
SN - 0004-637X
VL - 870
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 67
ER -