A Broken “α–intensity” Relation Caused by the Evolving Photosphere Emission and the Nature of the Extraordinarily Bright GRB 230307A

Abstract

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.

XMM-Newton and INTEGRAL Observations of the Bright GRB 230307A: Vanishing of the Local Absorption and Limits on the Dust in the Magellanic Bridge

“…A recent example of a GRB with long duration (∼150 s) but most likely originating from a compact binary merger is provided by GRB 230307A (Levan et al 2023a;Dichiara et al 2023;Sun et al 2023;Wang et al 2023). The evidence for a Type I classification comes from the James Webb Space Telescope discovery of an associated kilonova (Levan et al 2023a;Yang et al 2023), similar to that seen for the gravitational-wave merger GW 170817 (Abbott et al 2017a).…”

A Comparative Analysis of Two Peculiar Gamma-Ray Bursts: GRB 230307A and GRB 211211A

“…The redshift reported by Malesani et al (2021) is 0.076. GRB 230307A (trigger 699896651/230307656) triggered the Fermi-GBM at 15:44:06.67 UT on 2023 March 7, and a duration of T 90 ∼ 39.88 s in the 50-300 keV band was estimated (Wang et al 2023). The redshift reported by Gillanders et al (2023) is 0.065.…”

“…Similar correlations between α and flux as well as α and E p also exist but the correlations are weaker than that of E p and flux (see Figures 10 and 11 and Table 2). The weaker correlation might be explained by the fact that the α-F correlation exhibits an obvious broken behavior for GRB 230307A (Wang et al 2023). The correlations between E p and F as well as α and F support the double-tracking mode.…”

Investigation of the Gamma-Ray Bursts Prompt Emission Under the Relativistically Expanding Fireball Scenario

“…The prominence of thermal emission in the GRB prompt phase has been well studied by various authors (Ryde 2004(Ryde , 2005Pe'er 2008;Pe'er & Ryde 2011;Lundman et al 2013) and often supported by observations such as GRB 100724B (Guiriec et al 2013), GRB 101219B (Ghirlanda et al 2013), GRB 081221 (Hou et al 2018), and GRB 220426A (Wang et al 2022). Nevertheless, the prompt emission also carries a significant contribution of nonthermal spectral component (Pe'Er et al 2012;Iyyani et al 2013;Zhang et al 2016;Wang et al 2023). The evolution of the fireball during its coasting phase can deviate the spectral shape of the photospheric thermal emission from the Planck function.…”