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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
GRB 221009A is the brightest Gamma-Ray Burst detected in more than 50 years
of study. Here, we present observations in the X-ray and optical domains
ranging from the prompt emission (optical coverage by all-sky cameras) up to 20
days after the GRB obtained by the GRANDMA Collaboration (which includes
observations from more than 30 professional and amateur telescopes) and the
\textit{Insight}-HXMT Collaboration operating the X-ray telescope HXMT-LE. We
study the optical afterglow both with empirical fitting procedures and
numerical modeling. We find that the GRB afterglow, extinguished by a large
dust column, is most likely behind a combination of a large Milky-Way dust
column combined with moderate low-metallicity dust in the host galaxy. We find
that numerical models describing the synchrotron radiation at the forward shock
of a relativistic top-hat jet propagating through a constant density medium
require extreme parameters to fit the observational data. Based on these
observations, we constrain the isotropic afterglow energy $E_{0} \sim 3.7
\times 10^{54}$ erg, the density of the ambient medium $n_{\mathrm{ism}}
\gtrsim 1~\mathrm{cm}^{-3}$ and the opening angle of the jet core to be
$\gtrsim10.7^\circ$. We do not find evidence (for or against) of jet structure,
a potential jet break and the presence or absence of a SN. Placed in the global
context of GRB optical afterglows, we find the afterglow of GRB 221009A is
luminous but not extraordinarily so, highlighting that some aspects of this GRB
do not deviate from the known sample despite its extreme energetics and the
peculiar afterglow evolution.
