2023ApJ...953L...8W

GRB 230307A is one of the brightest gamma-ray bursts detected so far. With the excellent observation of GRB 230307A by the Fermi Gamma-ray Burst Monitor, we can reveal the details of prompt emission evolution. As found in high time-resolution spectral analysis, the early low-energy spectral indices (α) of this burst exceed the limit of synchrotron radiation (α = -2/3) and gradually decreases with the energy flux (F). A tight E_p ∝ F^0.54 correlation holds within the whole duration of the burst, where E_p is the spectral peak energy. Such an evolution pattern of α and E_p with intensity is called “double tracking.” For the α-F relation, we find a log Bayes factor ∼210 in favor of a smoothly broken power-law function over a linear function in log-linear space. We call this particular α-F relation a broken “α-intensity” and interpret it as the evolution of the ratio of thermal and nonthermal components, which is also the evolution of the photosphere. GRB 230307A with a duration of ∼35 s, if indeed at a redshift of z = 0.065, is likely a neutron star merger event (i.e., it is intrinsically “short”). Intriguingly, different from GRB 060614 and GRB 211211A, this long event is not composed of a hard spike followed by a soft tail, suggesting that the properties of the prompt emission light curves are not a good tracer of the astrophysical origins of the bursts. The other possibility of z = 3.87 would point toward a very peculiar nature of both GRB 230307A and its late-time thermal-like emission.