Multiwavelength View of the Close-by GRB 190829A Sheds Light on Gamma-Ray Burst Physics

• Published in: Astrophysical journal. Letters 2022-06, Vol.931 (2), p.L19
• Main Authors: Salafia, Om Sharan, Ravasio, Maria Edvige, Yang, Jun, An, Tao, Orienti, Monica, Ghirlanda, Giancarlo, Nava, Lara, Giroletti, Marcello, Mohan, Prashanth, Spinelli, Riccardo, Zhang, Yingkang, Marcote, Benito, Cimò, Giuseppe, Wu, Xuefeng, Li, Zhixuan
• Format: Article
• Language: English
• Subjects: Astrometry; Gamma ray bursts; Gamma rays; Interferometry; Light curve; Magnetic fields; Massive stars; Modelling; Relativistic aberration; Relativistic jets; Shocks; Very long base interferometry; Very long baseline interferometry
• ISSN: 2041-8205; 2041-8213; 2041-8213
• DOI: 10.3847/2041-8213/ac6c28

We monitored the position of the close-by (about 370 Mpc) gamma-ray burst GRB 190829A, which originated from a massive star collapse, through very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array, carrying out a total of nine observations between 9 and 117 days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of a few milliarcseconds. From a state-of-the art analysis of these data, we obtained valuable limits on the source size and expansion rate. The limits are in agreement with the size evolution entailed by a detailed modeling of the multiwavelength light curves with a forward-plus-reverse shock model, which agrees with the observations across almost 18 orders of magnitude in frequency (including the HESS data at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to the multiwavelength, high-cadence coverage of the afterglow, inherent degeneracies in the afterglow model are broken to a large extent, allowing us to capture some unique physical insights; we find a low prompt emission efficiency of ≲10 −3 , a low fraction of relativistic electrons in the forward shock downstream χ e < 13% (90% credible level), and a rapid decay of the magnetic field in the reverse shock downstream after the shock crossing. While our model assumes an on-axis jet, our VLBI astrometry is not sufficiently tight as to exclude any off-axis viewing angle, but we can exclude the line of sight to have been more than ∼2° away from the border of the gamma-ray-producing region based on compactness arguments.