Can We Find the Emission Mechanism Behind the Extremely Bright GRB 230812B?

arXiv:2607.18437 · astro-ph.HE · Submitted 2026-07-20 · Read on arXiv

Utkarsh Pathak, Sameer K. Patil, Hitesh Tanenia, Yashowardhan Rai, Viswajeet Swain, Anuraag Arya, Gaurav Waratkar, Anirudh Salgundi, Mehul Goyal, Varun Bhalerao, Igor Andreoni, Sarah Antier, G. C. Anupama, Sudhanshu Barway, Dipankar Bhattacharya, Tanmoy Chattopadhyay, Michael Coughlin, Anirban Dutta, Christoffer Fremling, Nidhal Guessoum, Mansi Kasliwal, Oliver J. Roberts, Gokul P. Srinivasaragavan, Rishabh Singh Teja, Santosh Vadawale, Peter Veres

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics and Astronomy, University of North Carolina at Chapel Hill, 120 E. Cameron Ave., Chapel Hill, NC 27514, USA · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India · Department of Physics and Astronomy, University of North Carolina at Chapel Hill, 120 E. Cameron Ave., Chapel Hill, NC 27514, USA · IJCLab, Univ Paris-Saclay, CNRS/IN2P3, Orsay, France · Indian Institute of Astrophysics, II Block Koramangala, Bengaluru 560034, India · Indian Institute of Astrophysics, II Block Koramangala, Bengaluru 560034, India · Department of Physics, Ashoka University, Sonepat, Haryana 131029, India · Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA · School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA · Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli 32001, Taiwan · Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA · Physics Department, American University of Sharjah, Sharjah, UAE · Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA · Physics, School of Natural Sciences, University of Galway, University Road, Galway, H91 TK33, Ireland · Department of Astronomy, University of Maryland, College Park, MD 20742, USA · Tsung-Dao Lee Institute, Shanghai Jiao Tong University, 1 Lisuo Road, Shanghai 201210, China · Physical Research Laboratory, Ahmedabad, Gujarat 380009, India · Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35805, USA

astro-ph.HE

Submitted: 2026-07-20

Comments: 16 pages, 7 figures, 3 Tables. Submitted to ApJ

License: http://creativecommons.org/licenses/by/4.0/

The gist: GRB 230812B is a bright long-duration GRB with a luminous, long-lived afterglow and an AstroSat/CZTI polarization measurement during the prompt phase, enabling a joint study of its prompt spectral

Terminology

Abstract

GRB 230812B is a bright long-duration GRB with a luminous, long-lived afterglow and an AstroSat/CZTI polarization measurement during the prompt phase, enabling a joint study of its prompt spectral evolution, polarization, and broadband afterglow. Time-resolved spectroscopy of the prompt emission shows that during the rising phase, the low-energy Band-function index exceeds the synchrotron line of death, favoring the presence of an additional thermal component. At later times, from T 0+2 s to T 0+32 s, the prompt spectra are consistent with predominantly non-thermal emission. Polarization analysis of the prompt emission in the 300 - 600 keV band yields a marginal lower limit on the polarization fraction of 50% at the 1 sigma level. The long X-ray monitoring of the afterglow shows no jet break over the observed baseline. Multiwavelength afterglow modeling favors a wide jet with an inferred half-opening angle of theta j = 15+6-4 degrees observed close to the jet axis with a viewing angle of theta v = 0.9+1.8-0.6 degrees. The inferred circumburst density is low, n 0 = 1.2+0.3-0.1 times10-4, cm-3, and the isotropic-equivalent kinetic energy of the jet is E k, iso = 4.0+1.5-0.8 times 10 53 erg. Taken together, the prompt spectral evolution favors an early phase with a thermal contribution followed by a later phase dominated by non-thermal emission. The polarization constraint in the late prompt phase is consistent with synchrotron emission, although a higher-significance polarization measurement will be required to robustly constrain the magnetic-field geometry and the relative contribution of photospheric emission.

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