Precise measurements of self-absorbed rising reverse shock emission from gamma-ray burst 221009A

Abstract

The deaths of massive stars are sometimes accompanied by the launch of highly relativistic and collimated jets. If the jet is pointed towards Earth, we observe a ‘prompt’ gamma-ray burst due to internal shocks or magnetic reconnection events within the jet, followed by a long-lived broadband synchrotron afterglow as the jet interacts with the circumburst material. While there is solid observational evidence that emission from multiple shocks contributes to the afterglow signature, detailed studies of the reverse shock, which travels back into the explosion ejecta, are hampered by a lack of early-time observations, particularly in the radio band. We present rapid follow-up radio observations of the exceptionally bright gamma-ray burst GRB 221009A that reveal in detail, both temporally and in frequency space, an optically thick rising component from the reverse shock. From this, we are able to constrain the size, Lorentz factor and internal energy of the outflow while providing accurate predictions for the location of the peak frequency of the reverse shock in the first few hours after the burst. These observations challenge standard gamma-ray burst models describing reverse shock emission.

Jet Structure and Burst Environment of GRB 221009A

“…The inferred X-ray LC is extend from (2023). We interpolated data from Bright et al (2023) to convert to 0.25 day. Zhang et al 2023a).…”

“…Our study suggests that GRB 221009A possesses a core +wing structure, where E k,iso (θ) shows a smooth transition, but Γ 0 (θ) displays a clear interface stratification between the core and wing. We neglected the ejecta cocoon component in our modeling, but the underestimation of the late-time low-frequency radio flux suggests its possible existence, and future observations of GRB 221009A will help answer this question (Bright et al 2023;Laskar et al 2023). The structure of the jet in GRB 221009A is different from that of the short GRB 170817A, possibly due to differences in the jet magnetization and the envelope properties of the progenitor.…”

“…For simplicity, we have disregarded the RS of the core component and focused solely on the radiation from the FS, as there is a lack of very early radio-to-optical-band observations to say whether the RS exists or not. In contrast, we considered both FS and RS radiation in the wing of the jet, as early radio observations exhibited signs of RS emission (Bright et al 2023). In our modeling, the electron acceleration efficiency ξ e,r,2 = 1 of the RS is fixed, due to the lack of sufficient data to constrain this parameter effectively.…”

“…We considered an appropriate extinction correction to be (Pei 1992) on this basis, but did not consider the subtraction of late supernova and host galaxy contributions (e.g., Blanchard et al 2023;Fulton et al 2023;Levan et al 2023;Shrestha et al 2023;Srinivasaragavan et al 2023), so the late optical data may exceed the model curves. We took radio data from three papers: Bright et al (2023), Laskar et al (2023), andO'Connor et al (2023). The radio LCs used the data from the first two papers (Figure 3) and radio spectra from all of them (Figure 5).…”

“…LHAASO reported their observations and analytical conclusions in the 0.2-7 TeV energy range (LHAASO Collaboration et al 2023). Thanks to the extremely high brightness of GRB 221009A, a wealth of afterglow data was collected through multiwavelength observations (e.g., Bright et al 2023;Kann et al 2023;Laskar et al 2023;LHAASO Collaboration et al 2023;Ren et al 2023). Combining a rich assortment of multiwavelength afterglow data together with valuable insights obtained from the newly accessible VHE energy regime, the study of GRB 221009A has the potential to significantly enhance our understanding of GRB phenomena.…”