Detection of the relativistic cocoon structure around the ultra-relativistic Jet

(ASIAA Science Highlight posted on 4th. March, 2020)

Using the combined power of the Submillimeter Array (SMA) and NASA’s and JAXA’s satellite missions, a team led by Mr. Wei Ju Chen, Prof. Urata and Dr. Asada (ASIAA) in Taiwan has confirmed the shocked jet cocoon afterglow of a Gamma-Ray Burst ⁽¹⁾ through the observation of an energetic GRB, GRB160623A. The team utilizes multi-frequency observations including long-term monitoring in a submillimeter range to characterize the two components of jets. For both the two populations of the GRBs, short and long GRBs, understanding of the jet and its structure is essential. Although the structured jet of short GRB has not yet observationally confirmed, numerous theoretical models are trying to use the off-axis viewing of the short GRB jet including its structures to explain the unusual weak short GRB170817 associated with the gravitational wave transient GW170817 caused by binary neutron star merger. Therefore, this result would be feedback to the multi-messenger astrophysics.

GRB is believed to be stellar explosions accompanied with relativistic outflows and narrowly-collimated jets. Since direct imaging of GRB jets is impossible unlike AGN jets, studying the GRB jet collimation and its structure have been made by the multi-frequency monitoring observations. Based on the past observations, the typical value of GRB jet opening-angles is 3.5 deg, which is in the same order with that of AGN jets (1.5 deg). In this time, the team made use of the submillimeter-observations using SMA, which has been playing an essential role in revealing new insights of the GRB afterglows ⁽³⁾ and relativistic transients ⁽⁴⁾. Their observations revealed that the temporal and spectral evolutions of the radio afterglow agree with those expected from a synchrotron radiation modelling with typical physical parameters except for the fact that the observed wide jet opening angle (~30 deg) for the submillimeter emission is significantly larger than the theoretical maximum opening angle. By contrast, the opening angle (<4.5 deg) of the X-ray afterglow is consistent with the typical value of GRB jets. Since the theory of the relativistic cocoon afterglow emission is similar to that of the regular afterglow with the jet opening angle wider than that of regular afterglow, the observed radio emission can be interpreted as the shocked jet cocoon emission. This result therefore indicates that the two components of the jets observed in the GRB 160623A afterglow is caused by the jet and the shocked jet cocoon afterglows.

Finally, the research team deeply thanks for the staffs of SMA for various supports for this observing project.