Significant Evolution of Photosphere Emission and a broken “$alpha$-intensity” relation of GRB 230307A. (arXiv:2303.11083v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Wang_Y/0/1/0/all/0/1">Yun Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Xia_Z/0/1/0/all/0/1">Zi-Qing Xia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zheng_T/0/1/0/all/0/1">Tian-Ci Zheng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ren_J/0/1/0/all/0/1">Jia Ren</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fan_Y/0/1/0/all/0/1">Yi-Zhong Fan</a>

GRB~230307A is one of the birghtest gamma-ray bursts detected so far. Its
prompt emission has been analyzed by the high-energy detection data
comprehensive analysis tool ({tt HEtools}) developed by us. With the excellent
observation of GRB~230307A by Fermi-GBM in keV-MeV energy range, we can reveal
the details about the evolution of this relation. As found in the
high-time-resolution spectral analysis, the early low-energy spectral indices
($alpha$) of this burst exceed the limit of synchrotron radiation
($alpha=-2/3$), and gradually decreases with the energy flux ($F$). A tight
$E_{rm p}propto F^{0.44}$ correlation anyhow holds within the whole duration
of the burst, where $E_{rm p}$ is the spectral peak energy. Such evolution
pattern of $alpha$ and $E_{rm p}$ with intensity is called “double
tracking”. In addition, for the relation between the $alpha$ and $F$, we find
a log Bayes factor $sim$ 160 in favor of a smoothly broken power-law function
over a linear function in log-linear space. We call this particular $alpha-F$
relation as broken “$alpha$-intensity”, and interpret it as the evolution of
the ratio of thermal and non-thermal components, which is also the evolution of
the photosphere.

GRB~230307A is one of the birghtest gamma-ray bursts detected so far. Its
prompt emission has been analyzed by the high-energy detection data
comprehensive analysis tool ({tt HEtools}) developed by us. With the excellent
observation of GRB~230307A by Fermi-GBM in keV-MeV energy range, we can reveal
the details about the evolution of this relation. As found in the
high-time-resolution spectral analysis, the early low-energy spectral indices
($alpha$) of this burst exceed the limit of synchrotron radiation
($alpha=-2/3$), and gradually decreases with the energy flux ($F$). A tight
$E_{rm p}propto F^{0.44}$ correlation anyhow holds within the whole duration
of the burst, where $E_{rm p}$ is the spectral peak energy. Such evolution
pattern of $alpha$ and $E_{rm p}$ with intensity is called “double
tracking”. In addition, for the relation between the $alpha$ and $F$, we find
a log Bayes factor $sim$ 160 in favor of a smoothly broken power-law function
over a linear function in log-linear space. We call this particular $alpha-F$
relation as broken “$alpha$-intensity”, and interpret it as the evolution of
the ratio of thermal and non-thermal components, which is also the evolution of
the photosphere.

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