GRB 220304A: Another Gamma-Ray Burst Dominated by Thermal Radiation

We report a recently detected long-duration gamma-ray burst (GRB) event by Fermi-GBM, GRB 220304A. The spectral analysis of the burst by the Band function shows that both the time-integrated and time-resolved spectra are very narrow, with low-energy spectral index alpha = -0.05 +/- 0.30 and high-energy spectral index beta = -3.53 +/- 0.30. It is reminiscent of GRB 090902B, a special GRB with photosphere radiation characteristics. Then, we perform spectral analysis using the Planck function (blackbody, BB) and the multicolor BB model. It is found that the spectra within -1 similar to 3 s is well fit by the BB model, indicating that the observation within the first 4 s is a pure thermal event. Apart from that most of the spectra can be well modeled as a multicolor BB. We conclude that GRB 220304A is a GRB dominated by thermal radiation. We also find that the spectral widths of the time-resolved spectrum of the burst gradually increase with time. Based on the Amati relation, we infer the redshift to be 0.23, the physical properties of the relativistic outflow, and find that the relationship between the bulk Lorentz factor and the isotropic luminosity Gamma-L iso,52 still exists.