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# Simultaneous radio and optical polarimetry of GRB 191221B afterglow

## Abstract

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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## 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.

## References

1. Laha, S. et al. GRB 191221B: Swift detection of a burst and a very bright optical candidate. GCN Circ. 26534 (2019).

2. Lipunov, V. et al. GRB 191221B: MASTER OT detection. GCN Circ. 26537 (2019).

3. Buckley, D. A. H. et al. Spectropolarimetry and photometry of the early afterglow of the gamma-ray burst GRB 191221B. Mon. Not. Roy. Astron. Soc. 506, 4621–4631 (2021).

4. Vielfaure, J.-B. et al. GRB 191221B: VLT/X-shooter redshift. GCN Circ. 26553 (2019).

5. Frederiks, D. et al. Konus-Wind observation of GRB 191221B. GCN Circ. 26576 (2019).

6. Sakamoto, T. et al. GRB 191221B: Swift-BAT refined analysis. GCN Circ. 26562 (2019).

7. Cikota, A. et al. Linear spectropolarimetry of polarimetric standard stars with VLT/FORS2. Mon. Not. Roy. Astron. Soc. 464, 4146–4159 (2017).

8. Serkowski, K. Interstellar polarization (review). IAUS 52, 145 (1973).

9. Covino, S. & Gotz, D. Polarization of prompt and afterglow emission of gamma-ray bursts. Astron. Astrophys. Trans. 29, 205–244 (2016).

10. Urata, Y. et al. First detection of radio linear polarization in a gamma-ray burst afterglow. Astrophys. J. Lett. 884, L58 (2019).

11. Sari, R., Piran, T. & Narayan, R. Spectra and light curves of gamma-ray burst afterglows. Astrophys. J. 497, L17–L20 (1998).

12. Zhang, B. & Mészáros, P. Gamma-ray bursts: progress, problems & prospects. Int. J. Mod. Phys. A 19, 2385–2472 (2004).

13. Granot, J., Piran, T. & Sari, R. Images and spectra from the interior of a relativistic fireball. Astrophys. J. 513, 679–689 (1999).

14. Shimoda, J. & Toma, K. Multi-wave band synchrotron polarization of gamma-ray burst afterglows. Astrophys. J. 913, 58 (2021).

15. Panaitescu, A. & Kumar, P. Fundamental physical parameters of collimated gamma-ray burst afterglows. Astrophys. J. Lett. 560, L49–L53 (2001).

16. Rybicki, G. B. & Lightman, A. P. Radiative Processes in Astrophysics (Wiley Interscience, 1979).

17. Medvedev, M. V. & Loeb, A. Generation of magnetic fields in the relativistic shock of gamma-ray burst sources. Astrophys. J. 526, 697–706 (1999).

18. Sironi, L. & Goodman, J. Production of magnetic energy by macroscopic turbulence in GRB afterglows. Astrophys. J. 671, 1858–1867 (2007).

19. Gruzinov, A. & Waxman, E. Gamma-ray burst afterglow: polarization and analytic light curves. Astrophys. J. 511, 852–861 (1999).

20. Sagiv, A., Waxman, E. & Loeb, A. Probing the magnetic field structure in gamma-ray bursts through dispersive plasma effects on the afterglow polarization. Astrophys. J. 615, 366–377 (2004).

21. Toma, K., Ioka, K. & Nakamura, T. Probing the efficiency of electron-proton coupling in relativistic collisionless shocks through the radio polarimetry of gamma-ray burst afterglows. Astrophys. J. 673, L123–L126 (2008).

22. Spitkovsky, A. Particle acceleration in relativistic collisionless shocks: Fermi process at last? Astrophys. J. 682, L5–L8 (2008).

23. Keshet, U., Katz, B., Spitkovky, A. & Waxman, E. Magnetic field evolution in relativistic unmagnetized collisionless shocks. Astrophys. J. 693, L127–L130 (2009).

24. Sari, R. Linear polarization and proper motion in the afterglow of beamed gamma-ray bursts. Astrophys. J. 524, L43–L46 (1999).

25. Ghisellini, G. & Lazzati, D. Polarization light curves and position angle variation of beamed gamma-ray bursts. Mon. Not. Roy. Astron. Soc. 309, L7–L11 (1999).

26. Inoue, T. et al. The origin of radially aligned magnetic fields in young supernova remnants. Astrophys. J. Lett. 772, L20 (2013).

27. Eichler, D. & Waxman, E. The efficiency of electron acceleration in collisionless shocks and gamma-ray burst energetics. Astrophys. J. 627, 861–867 (2005).

28. Sokoloff, D. D., Bykov, A. A. & Shukurov, A. et al. Depolarization and Faraday effects in galaxies. Mon. Not. Roy. Astron. Soc. 299, 189–206 (1998).

29. Murase, K., Ioka, K. & Nagataki, S. et al. High-energy neutrinos and cosmic rays from low-luminosity gamma-ray bursts? Astrophys. J. 651, L5–L8 (2006).

30. Kimura, S. S. et al. High-energy neutrino emission from short gamma-ray bursts: prospects for coincident detection with gravitational waves. Astrophys. J. Lett. 848, L4 (2017).

31. McMullin, J. P. et al. CASA architecture and applications. ASP Conf. Series 376, 127 (2007).

32. Goldoni, P. et al. Data reduction software of the X-shooter spectrograph. SPIE Conf. Series 6269, 2 (2006).

33. Modigliani, A. et al. The X-shooter pipeline. SPIE Conf. Series 7737, 28 (2010).

34. Selsing, J. et al. The X-shooter GRB afterglow legacy sample (XS-GRB). Astron. Astrophys. 623, 92 (2019).

35. Carnall, A.C. SpectRes: a fast spectral resampling tool in Python. Preprint at arXiv:1705.05165 (2017).

36. Gordon, K. D. et al. A quantitative comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way ultraviolet to near-infrared extinction curves. Astrophys. J. 594, 279–293 (2003).

37. Gordon, K. D. et al. FUSE measurements of far-ultraviolet extinction. III. The dependence on R(V) and discrete feature limits from 75 galactic sightlines. Astrophys. J. 705, 1320–1335 (2009).

38. Covino, S. et al. Dust extinctions for an unbiased sample of gamma-ray burst afterglows. Mon. Not. Roy. Astron. Soc. 432, 1231–1244 (2013).

39. Patat, F. et al. Properties of extragalactic dust inferred from linear polarimetry of Type Ia Supernovae. Astron. Astrophys. 557, 53 (2015).

40. Covino, S., Malesani, D. & Ghisellini, G. et al. Polarization evolution of the GRB 020405 afterglow. Astron. Astrophys. 400, L9–L12 (2003).

41. Wiersema, K. et al. Detailed optical and near-infrared polarimetry, spectroscopy and broad-band photometry of the afterglow of GRB 091018: polarization evolution. Mon. Not. Roy. Astron. Soc. 426, 2–22 (2012).

42. Uehara, T. et al. GRB 091208B: first detection of the optical polarization in early forward shock emission of a gamma-ray burst afterglow. Astrophys. J. Lett. 752, L6 (2012).

43. Gorosabel, J. et al. GRB 110205A: detection of optical linear polarization from CAHA. GCN Circ. 11696 (2011).

44. Wiersema, K. et al. Circular polarization in the optical afterglow of GRB 121024A. Nature 509, 201–204 (2014).

45. King, O. G. et al. Early-time polarized optical light curve of GRB 131030A. Mon. Not. Roy. Astron. Soc. 445, L114–L118 (2014).

46. Jordana-Mitjans, N. et al. Lowly polarized light from a highly magnetized jet of GRB 190114C. Astrophys. J. 892, 97 (2020).

47. Evans, P. A. et al. An online repository of Swift/XRT light curves of γ-ray bursts. Astron. Astrophys. 469, 379–385 (2007).

48. Evans, P. A. et al. Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs. Mon. Not. Roy. Astron. Soc. 397, 1177–1201 (2009).

49. Willingale, R. et al. Calibration of X-ray absorption in our Galaxy. Mon. Not. Roy. Astron. Soc. 431, 394–404 (2013).

50. Wilms, J., Allen, A. & McCray, R. On the absorption of X-rays in the interstellar medium. Astrophys. J. 542, 914–924 (2000).

51. Schady, P. et al. Dust and metal column densities in gamma-ray burst host galaxies. Mon. Not. Roy. Astron. Soc. 401, 2773–2792 (2010).

52. Zhang, B. & Mészáros, P. Gamma-ray burst afterglow with continuous energy injection: signature of a highly magnetized millisecond pulsar. Astrophys. J. 552, L35–L38 (2001).

53. Rees, M. J. & Mészáros, P. Refreshed shocks and afterglow longevity in gamma-ray bursts. Astrophys. J. 496, L1–L4 (1998).

54. Zhang, W. & MacFadyen, A. The dynamics and afterglow radiation of gamma-ray bursts. I. Constant density medium. Astrophys. J. 698, 1261–1272 (2009).

55. van Eerten, H. J. & MacFadyen, A. I. Observational implications of gamma-ray burst afterglow jet simulations and numerical light curve calculations. Astrophys. J. 751, 155 (2012).

56. Gao, H. et al. A complete reference of the analytical synchrotron external shock models of gamma-ray bursts. New Astron. Rev. 57, 141 (2013).

57. Sari, R. & Mészáros, P. Impulsive and varying injection in gamma-ray burst afterglows. Astrophys. J. 535, L33–L37 (2000).

58. Yost, S. A., Harrison, F. A., Sari, R. & Frail, D. A. A study of the afterglows of four gamma-ray bursts: constraining the explosion and fireball model. Astrophys. J. 597, 459–473 (2003).

59. Cenko, S. B. et al. The collimation and energetics of the brightest Swift gamma-ray bursts. Astrophys. J. 711, 641–654 (2010).

60. Rossi, E. M., Lazzati, D., Salmonson, J. D. & Ghisellini, G. The polarization of afterglow emission reveals γ-ray bursts jet structure. Mon. Not. Roy. Astron. Soc. 354, 86–100 (2004).

61. Sironi, L. & Spitkovsky, A. Particle acceleration in relativistic magnetized collisionless electron-ion shocks. Astrophys. J. 726, 75 (2011).

62. Amati, L. et al. Intrinsic spectra and energetics of BeppoSAX gamma-ray bursts with known redshifts. Astron. Astrophys. 390, 81–89 (2002).

63. Yamaoka, K. et al. Suzaku Wide-band All-sky Monitor (WAM) observations of GRBs and SGRs. Publ. Astron. Soc. Japan 69, R2 (2017).

## Acknowledgements

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.

## Author information

Authors

### Contributions

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.

### Corresponding author

Correspondence to Yuji Urata.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review

### Peer review information

Nature Astronomy thanks Shuang-Nan Zhang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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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.

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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). https://doi.org/10.1038/s41550-022-01832-7

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• DOI: https://doi.org/10.1038/s41550-022-01832-7