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1. GRB Polarization: A Unique Probe of GRB Physics

   https://doi.org/10.3390/galaxies9040082  

   Gill, Ramandeep; Kole, Merlin; Granot, Jonathan (December 2021, Galaxies)

   Over half a century from the discovery of gamma-ray bursts (GRBs), the dominant radiation mechanism responsible for their bright and highly variable prompt emission remains poorly understood. Spectral information alone has proven insufficient for understanding the composition and main energy dissipation mechanism in GRB jets. High-sensitivity polarimetric observations from upcoming instruments in this decade may help answer such key questions in GRB physics. This article reviews the current status of prompt GRB polarization measurements and provides comprehensive predictions from theoretical models. A concise overview of the fundamental questions in prompt GRB physics is provided. Important developments in gamma-ray polarimetry including a critical overview of different past instruments are presented. Theoretical predictions for different radiation mechanisms and jet structures are confronted with time-integrated and time-resolved measurements. The current status and capabilities of upcoming instruments regarding the prompt emission are presented. The very complimentary information that can be obtained from polarimetry of X-ray flares as well as reverse-shock and early to late forward-shock (afterglow) emissions are highlighted. Finally, promising directions for overcoming the inherent difficulties in obtaining statistically significant prompt-GRB polarization measurements are discussed, along with prospects for improvements in the theoretical modeling, which may lead to significant advances in the field. 

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2. GRB 180720B: A GRB with Interesting Spectral Characteristics

   https://doi.org/10.3847/1538-4357/ac14b8  

   Chen, Jia-Ming; Peng, Zhao-Yang; Du, Tan-Tan; Yin, Yue; Wu, Hui (October 2021, The Astrophysical Journal)
3. Self-similarities and Power Laws in the Time-resolved Spectra of GRB 190114C, GRB 130427A, GRB 160509A, and GRB 160625B

   https://doi.org/10.3847/1538-4357/acb20b  

   Li, Liang; Rueda, J. A.; Moradi, R.; Wang, Y.; Xue, S. S.; Ruffini, R. (March 2023, The Astrophysical Journal)

   Abstract Binary-driven hypernova (BdHN) models have been adopted to explain the observed properties of long gamma-ray bursts (GRBs). Here, we perform a comprehensive data analysis (temporal and spectral analysis, GeV emission, and afterglow) on GRB 130427A, GRB 160509A, and GRB 160625B. We identify three specific episodes characterized by different observational signatures and show that these episodes can be explained and predicted to occur within the framework of the BdHNe I model, as first observed in GRB 190114C and reported in an accompanying paper. Episode 1 includes the “SN-rise” with the characteristic cutoff power-law spectrum; Episode 2 is initiated by the moment of formation of the black hole, coincident with the onset of the GeV emission and the ultrarelativistic prompt emission phase, and is characterized by a cutoff power law and blackbody spectra; Episode 3 is the “cavity,” with its characteristic featureless spectrum. 

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4. A Reverse Shock in GRB 181201A

   https://doi.org/10.3847/1538-4357/ab40ce  

   Laskar, Tanmoy; Eerten, Hendrik van; Schady, Patricia; Mundell, C. G.; Alexander, Kate D.; Duran, Rodolfo Barniol; Berger, Edo; Bolmer, J.; Chornock, Ryan; Coppejans, Deanne L.; et al (October 2019, The Astrophysical Journal)
5. A Millimeter Rebrightening in GRB 210702A

   https://doi.org/10.3847/1538-4357/ad77bb  

   de_Wet, Simon; Laskar, Tanmoy; Groot, Paul J; Barniol_Duran, Rodolfo; Berger, Edo; Bhandari, Shivani; Eftekhari, Tarraneh; Guidorzi, Cristiano; Kobayashi, Shiho; Perley, Daniel A; et al (October 2024, The Astrophysical Journal)

   Abstract We present X-ray to radio frequency observations of the bright long gamma-ray burst GRB 210702A. Our Atacama Large Millimeter/submillimeter Array 97.5 GHz observations show a significant rebrightening by a factor of ≈2 beginning at 8.2 days post-burst and rising to peak brightness at 18.1 days before declining again. This is the first such rebrightening seen in a millimeter afterglow light curve. A standard forward shock model in a stellar wind circumburst medium can explain most of our X-ray, optical, and millimeter observations prior to the rebrightening, but significantly overpredicts the self-absorbed radio emission, and cannot explain the millimeter rebrightening. We investigate possible explanations for the millimeter rebrightening, and find that energy injection or a reverse shock from a late-time shell collision are plausible causes. Similar to other bursts, our radio data may require alternative scenarios such as a thermal electron population or a structured jet to explain the data. Our observations demonstrate that millimeter light curves can exhibit some of the rich features more commonly seen in optical and X-ray afterglow light curves, motivating further millimeter wavelength studies of GRB afterglows. 

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