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Long γ-ray bursts are associated with energetic, broad-lined, stripped-envelope supernovae 1 , 2 and as such mark the death of massive stars. The scarcity of such events nearby and the brightness of the γ-ray burst afterglow, which dominates the emission in the first few days after the burst, have so far prevented the study of the very early evolution of supernovae associated with γ-ray bursts 3 . In hydrogen-stripped supernovae that are not associated with γ-ray bursts, an excess of high-velocity (roughly 30,000 kilometres per second) material has been interpreted as a signature of a choked jet, which did not emerge from the progenitor star and instead deposited all of its energy in a thermal cocoon 4 . Here we report multi-epoch spectroscopic observations of the supernova SN 2017iuk, which is associated with the γ-ray burst GRB 171205A. Our spectra display features at extremely high expansion velocities (around 115,000 kilometres per second) within the first day after the burst 5 , 6 . Using spectral synthesis models developed for SN 2017iuk, we show that these features are characterized by chemical abundances that differ from those observed in the ejecta of SN 2017iuk at later times. We further show that the high-velocity features originate from the mildly relativistic hot cocoon that is generated by an ultra-relativistic jet within the γ-ray burst expanding and decelerating into the medium that surrounds the progenitor star 7 , 8 . This cocoon rapidly becomes transparent 9 and is outshone by the supernova emission, which starts to dominate the emission three days after the burst. Multi-epoch observations of a supernova associated with a γ-ray burst reveal spectral features at extremely high expansion velocities within the first day after the burst, indicative of a choked jet.

The immediate optical afterglow of the γ-ray burst GRB 120308A is highly polarized, showing that γ-ray bursts contain magnetized baryonic jets with large-scale uniform fields that can survive long after the initial explosion. First light from a γ-ray burster On 8 March 2012 NASA's Swift satellite identified the γ-ray burster GRB120308A, initially as a single pulse of γ-rays lasting about 100 seconds. The LaPalma-based Liverpool Telescope, responding automatically to the Swift discovery, identified the optical afterglow and was able to track the evolution of polarized light during the crucial early minutes of the outburst. Those results, presented here, provide the tightest constraints yet on the physics of the jets of material ejected in a γ-ray burst explosion. After the initial burst of γ-rays that defines a γ-ray burst (GRB), expanding ejecta collide with the circumburst medium and begin to decelerate at the onset of the afterglow, during which a forward shock travels outwards and a reverse shock propagates backwards into the oncoming collimated flow, or ‘jet’ 1 , 2 . Light from the reverse shock should be highly polarized if the jet’s magnetic field is globally ordered and advected from the central engine 3 , 4 , with a position angle that is predicted to remain stable in magnetized baryonic jet models 5 or vary randomly with time if the field is produced locally by plasma or magnetohydrodynamic instabilities 6 , 7 . Degrees of linear polarization of P  ≈ 10 per cent in the optical band have previously been detected in the early afterglow 6 , 8 , but the lack of temporal measurements prevented definitive tests of competing jet models 9 , 10 , 11 , 12 , 13 , 14 . Hours to days after the γ-ray burst, polarization levels are low ( P  < 4 per cent), when emission from the shocked ambient medium dominates 15 , 16 , 17 . Here we report the detection of P = per cent in the immediate afterglow of Swift γ-ray burst GRB 120308A, four minutes after its discovery in the γ-ray band, decreasing to P = per cent over the subsequent ten minutes. The polarization position angle remains stable, changing by no more than 15 degrees over this time, with a possible trend suggesting gradual rotation and ruling out plasma or magnetohydrodynamic instabilities. Instead, the polarization properties show that GRBs contain magnetized baryonic jets with large-scale uniform fields that can survive long after the initial explosion.

Long-duration gamma-ray bursts (GRBs) are an extremely rare outcome of the collapse of massive stars and are typically found in the distant universe. Because of its intrinsic luminosity (L ~ 3 × 10⁵³ ergs per second) and its relative proximity (z = 0.34), GRB 130427A reached the highest fluence observed in the γ-ray band. Here, we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-meter Liverpool and Faulkes telescopes, and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early universe and over the full range of GRB isotropie energies.

With the launch of the Gaia satellite, detection of many different types of transient sources will be possible, with one of them being optical afterglows of gamma-ray bursts (GRBs). Using the knowledge of the satellitea[tm]s dynamics and properties of GRB optical afterglows, we performed a simulation in order to estimate an average GRB detection rate with Gaia. Here, we present the simulation results for two types of GRB optical afterglows, differing in the observera[tm]s line of sight compared with a GRB jet axis: regular (on-axis) and orphan afterglows. Results show that for on-axis GRBs, less than 10 detections in 5A yr of foreseen Gaia operational time are expected. The orphan afterglow simulation results are more promising, giving a more optimistic number of several tens of detections in 5A yr.

After the initial burst of γ-rays that defines a γ-ray burst (GRB), expanding ejecta collide with the circumburst medium and begin to decelerate at the onset of the afterglow, during which a forward shock travels outwards and a reverse shock propagates backwards into the oncoming collimated flow, or 'jet' (1,2). Light from the reverse shock should be highly polarized if the jet's magnetic field is globally ordered and advected from the central engine (3,4), with a position angle that is predicted to remain stable in magnetized baryonic jet models (5) or vary randomly with time if the field is produced locally by plasma or magnetohydrodynamic instabilities (6,7). Degrees of linear polarization of P ≅ 10 per cent in the optical band have previously been detected in the early afterglow (6,8), but the lack of temporal measurements prevented definitive tests of competing jet models (9-14). Hours to days after the γ-ray burst, polarization levels are low (P < 4 per cent), when emission from the shocked ambient medium dominates (15-17). Here we report the detection of P = [28.sup.+4.sub.-4] per cent in the immediate afterglow of Swift γ-ray burst GRB 120308A, four minutes after its discovery in the γ-ray band, decreasing to P = [16.sup.+5.sub.-4] per cent over the subsequent ten minutes. The polarization position angle remains stable, changing by no more than 15 degrees over this time, with a possible trend suggesting gradual rotation and ruling out plasma or magnetohydrodynamic instabilities. Instead, the polarization properties show that GRBs contain magnetized baryonic jets with large-scale uniform fields that can survive long after the initial explosion.

With the launch of theGaiasatellite, detection of many different types of transient sources will be possible, with one of them being optical afterglows of gamma-ray bursts (GRBs). Using the knowledge of the satellite’s dynamics and properties of GRB optical afterglows, we performed a simulation in order to estimate an average GRB detection rate withGaia. Here, we present the simulation results for two types of GRB optical afterglows, differing in the observer’s line of sight compared with a GRB jet axis: regular (on-axis) and orphan afterglows. Results show that for on-axis GRBs, less than 10 detections in 5 yr of foreseenGaiaoperational time are expected. The orphan afterglow simulation results are more promising, giving a more optimistic number of several tens of detections in 5 yr.

Long-duration [gamma]-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterized by an initial phase of bright and highly variable radiation in the kiloelectronvolt-to-megaelectronvolt band, which is probably produced within the jet and lasts from milliseconds to minutes, known as the prompt emission.sup.1,2. Subsequently, the interaction of the jet with the surrounding medium generates shock waves that are responsible for the afterglow emission, which lasts from days to months and occurs over a broad energy range from the radio to the gigaelectronvolt bands.sup.1-6. The afterglow emission is generally well explained as synchrotron radiation emitted by electrons accelerated by the external shock.sup.7-9. Recently, intense long-lasting emission between 0.2 and 1 teraelectronvolts was observed from GRB 190114C.sup.10,11. Here we report multi-frequency observations of GRB 190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from 5 × 10.sup.-6 to 10.sup.12 electronvolts. We find that the broadband spectral energy distribution is double-peaked, with the teraelectronvolt emission constituting a distinct spectral component with power comparable to the synchrotron component. This component is associated with the afterglow and is satisfactorily explained by inverse Compton up-scattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed teraelectronvolt component are typical for GRBs, supporting the possibility that inverse Compton emission is commonly produced in GRBs.