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Simultaneous radio and optical polarimetry of GRB 191221B afterglow


Gamma-ray bursts (GRBs) are the most luminous transients in the universe and are utilized as probes of early stars, gravitational wave counterparts and collisionless shock physics. In spite of studies on polarimetry of GRBs in individual wavelengths that characterized intriguing properties of prompt emission and afterglow, no coordinated multi-wavelength measurements have yet been performed. Here we report the first coordinated simultaneous polarimetry in the optical and radio bands for the afterglow associated with the typical long GRB 191221B. Our observations successfully caught the radio emission, which is not affected by synchrotron self-absorption, and show that the emission is depolarized in the radio band compared with the optical one. Our simultaneous polarization angle measurement and temporal polarization monitoring indicate the existence of cool electrons that increase the estimate of jet kinetic energy by a factor of more than 4 for this GRB afterglow. Further coordinated multi-wavelength polarimetric campaigns would improve our understanding of the total jet energies and magnetic field configurations in the emission regions of various types of GRBs, which are required to comprehend the mass scales of their progenitor systems and the physics of collisionless shocks.

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Fig. 1: Spectral flux distribution and polarization spectrum of GRB 191221B afterglow at 2.5 days.
Fig. 2: Radio afterglow light curve of GRB 191221BA with the simultaneous optical (R band) polarimetric observation.
Fig. 3: Spectral flux distributions of the GRB 191221B afterglow at 1.5, 2.5, 9.5 and 18.4 days after GRB.

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Data availability

Processed data are presented in the tables and figures in the paper. The ALMA data are available from the ALMA Science Archive. The VLT data are available from the ESO Science Archive Facility.

Code availability

We used standard data reduction tools in Python and CASA31. The theoretical calculation code of the flux and polarization used in this work is not publicly available. Results presented in this work are available from the corresponding author upon reasonable request.


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This work is based on observations collected at the European Southern Observatory (ESO) under ESO programmes 0104.D-0600(C) and 0104.D-0600(A).This paper makes use of the following ALMA data: ADS/JAO.ALMA# 2019.1.01016.T, 2019.1.01484.T. ALMA is a partnership of ESO (representing its member states), National Science Foundation (USA) and National Institutes of Natural Sciences (Japan), together with National Research Council Canada (Canada), Ministry of Science and Technology and Institute of Astronomy and Astrophysics, Academia Sinica (Taiwan), and Korea Astronomy and Space Science Institute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities, Inc./National Radio Astronomy Observatory and National Astronomical Observatory of Japan. This work is supported by the Ministry of Science and Technology of Taiwan grant nos. MOST 105-2112-M-008-013-MY3 (Y.U.) and 106-2119-M-001-027 (K.A.). This work is also supported by Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research No. 18H01245 (K.T.) and No. 20J01086 (J.S.) and by the Graduate Programme on Physics for the Universe (GP-PU), Tohoku University (A.K.). K.W. acknowledges support through a UK Research and Innovation Future Leaders Fellowship awarded to B. Simmons (MR/T044136/1), and support through an Alan Turing Institute Post-Doctoral Enrichment Award. We thank the East Asian ALMA Regional Center, especially P.-Y. Hsieh for support in the ALMA observations.

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Authors and Affiliations



Y.U., K.T., S.C. and K.W. initiated the study. Y.U. and K.T. mainly wrote the texts of this manuscript. Y.U., S.C., K.W., K.H. and S.T. managed the Target of opportunity (ToO) observations and main data analysis. G.P. provided valuable thoughts for establishing efficient ToO managements. K.T., J.S., A.K. and S.N. managed the theoretical interpretations. A.K. played the principal role in numerical modelling. K.A. and H.N. checked the ALMA results. C.-E.C., K.Y. and M.T. provided NH analysis for evaluating the intrinsic absorption using X-ray data. L.I., J.F., A.d.U.P. and M.A. provided VLT/X-shooter data. All of the authors contributed to the data analysis and discussed the results and the texts.

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Correspondence to Yuji Urata.

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Extended data

Extended Data Fig. 1 \({{{{{\rm{E}}}}}^{{{{\rm{src}}}}}}_{{{{\rm{peak}}}}}-{{{{\rm{E}}}}}_{{{{\rm{\gamma }}}},{{{\rm{iso}}}}}\) relation.

\({{{{{\rm{E}}}}}^{{{{\rm{src}}}}}}_{{{{\rm{peak}}}}}-{{{{\rm{E}}}}}_{{{{\rm{\gamma }}}},{{{\rm{iso}}}}}\) relation (Amati et al. 2002, Yamaoka et al. 2017)62,63. GRB 191221B, marked with the red box point, obeys the relation. All error bars represent 1 − σ uncertainties.

Extended Data Fig. 2 Fit by a simple power-law and SMC (Gordon et al. 2003) extinction curve for the ultraviolet arm of the VLT-X-shooter.

Fit by a simple power-law and SMC (Gordon et al. 2003) extinction curve for the ultraviolet arm of the VLT-X-shooter. The black solid line is the best-fit and the dashed line shows the unexstinguished afterglow spectrum. Marginalizing the power-law index and normalization, it turns out that the amount of rest-frame extinction is below 0.038 mag (95% upper limit).

Extended Data Fig. 3 Stokes raw Q and U measurements for field stars and the GRB afterglow.

Stokes raw Q and U measurements for field stars and the GRB afterglow. Green and blue circles indicate field stars and the afterglow, respectively. The orange box shows the weighted average of the field stars.

Extended Data Fig. 4 X-ray spectrum for GRB191221B (a) and GRB190114C (b) afterglows.

X-ray spectrum for GRB191221B (a) and GRB190114C (b) afterglows. a, GRB191221B X-ray afterglow spectrum described by a single power law modified with intrinsic and Galactic adsorptions, the latter of which is fixed at NH = 8.6 × 1020 cm−2. The derived best-fitting values of the intrinsic absorption column density and spectral index are \({{{{\rm{N}}}}}_{{{{\rm{H}}}}}=({1.{6}^{+0.9}}_{-0.8})\times 1{0}^{21}\,{{{{\rm{cm}}}}}^{-2}\) and β = − 0.96 ± 0.06, respectively, with reduced χ2/dof = 0.88/246. b, GRB190114C X-ray afterglow spectrum. The highly obscured spectrum with the intrinsic absorption column density of NH = (8. 5+0.3−0.3) × 1022 cm−2 is reasonable for the dust induced origin of polarization observed in the optical afterglow reported by Jordana-Mitjans et al. (2020)46 error bars represent 1 − σ uncertainties.

Extended Data Fig. 5 Stokes I (a), Q (b), and U (c) maps of the GRB191221B afterglow taken on 2019 December 24 (2.5 days after the burst).

Stokes I (a), Q (b), and U (c) maps of the GRB191221B afterglow taken on 2019 December 24 (2.5 days after the burst). The ALMA beam size is shown in the open circles. The map null detection on the Stokes-Q map constrained the range of PA of 37.7-52.3 deg with 1 − σ uncertainties.

Extended Data Fig. 6 GRB 191221B afterglow light curves in X-ray, optical, and radio bands together with optical and radio polarization variabilities.

GRB 191221B afterglow light curves in X-ray, optical, and radio bands together with optical and radio polarization variabilities. a, Multi-frequency light curves. Dotted lines indicate the model light curves fitted to radio, optical, and X-ray data observed by ALMA and VLT. Differences in the early optical afterglow (green small circles) and its wiggles may be caused by the magnitude-to-flux conversion of optical observations made by the very broad-band clear filter. b, PD temporal evolution in optical R band (purple stars and lines) and radio 97.5 GHz band (3 − σ upper limits with red symbols and lines). The solid and dashed lines indicate the plasma-scale turbulent magnetic field model with ξ2 = 0.56 (solid) and ξ2 = 0.81 (dashed). c, PA in the optical R band. The purple dashed-dot lines indicate the plasma-scale turbulent magnetic fields model with any ξ2. All error bars represent 1 − σ uncertainties. The upper limits are at the 3 − σ level.

Extended Data Table 1 Summary of Optical Polarization. Summary of Optical Polarization. Measurements with no special notation are summarized with 1 − σ errors. *The values for 0.121 and 0.417 days were derived in the wavelength range of R band based on data reported by Buckley et al (2021)3
Extended Data Table 2 X-ray spectral properties for 8 known-z GRBs with optical polarimetry. X-ray spectral properties for 8 known-z GRBs with optical polarimetry
Extended Data Table 3 Photometric Observing Log. Photometric Observing Log. Measurements are summarized with 1 − σ errors and statistical errors only. *The optical flux at 2.385 days was derived using the acquisition images taken as part of the acquisition sequence of the VLT imaging polarimetry

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Urata, Y., Toma, K., Covino, S. et al. Simultaneous radio and optical polarimetry of GRB 191221B afterglow. Nat Astron 7, 80–87 (2023).

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