Núria Jordana-Mitjans
Gamma-ray Bursts (GRBs) are the most powerful explosions in the Universe. When a massive star in a distant galaxy collapses into a black hole, the material is accelerated to ultra-high speeds along the narrow beam of a jet. As this jet continues to travel outwards, it collides with external material surrounding the dying star, producing fading light called the “afterglow” that can be seen across the entire electromagnetic spectrum –from the most energetic gamma-rays to radio wavelengths.
But how can such material be accelerated and focused into narrow beams? The internal shock model proposes that repeated violent collisions between material blasted out during the explosion can produce the gamma-ray flash and the subsequent fading afterglow. The competing magnetic model credits primordial large-scale ordered magnetic fields with collimating and accelerating the relativistic outflows. To distinguish between these models and ultimately determine the power source for these prodigious cosmic explosions, our team studies the polarization of the light during the first minutes after the explosion –using novel instruments on fully autonomous telescopes around the globe– to probe directly the magnetic field properties in these distant GRB jets.
Using this technology, our team made the first detection of highly polarised optical light –confirming the presence of mildly magnetized jets with large-scale primordial magnetic fields (e.g., GRB 120308A; Mundell et al. 2013). Our most recent observations of the most energetic and first GRB detected at very high TeV energies (GRB 190114C) opens a new frontier in GRB magnetic field studies –suggesting that some jets can be launched highly magnetized (Jordana-Mitjans et al. 2020) and that the collapse and destruction of these fields at very early times may have powered the explosion itself.
When the GRB jets point towards Earth, they are identified by dedicated satellites orbiting above the Earth as intense, short-lived flashes of high-energy gamma-rays. No one can predict where or when one will appear, so autonomous rapid-response robotic telescopes are crucial to catch the fast-fading light of the afterglow in the first minutes after the burst –when the underlying physics of the ejected material is still encoded in the emitted light. Currently, there are two competing jet models: baryonic vs magnetized. The baryonic jet model states that repeated violent collisions between material blasted out during the explosion produce the gamma-ray flash. In contrast, the magnetic jet model postulates that a primordial magnetic field advected from the black hole in the dying star collapses within seconds of the initial explosion, releasing energy to power the prodigious blast. To distinguish between the two models, we use polarization studies during the first hundreds of seconds after the explosion to determine the structure of the magnetic field and therefore, the degree of order. The GRBs emission starts with the bright and variable gamma-ray prompt lasting seconds, followed by a fast-fading reverse shock emission which is also short-lived. Both types of emission are sensitive to primordial magnetic fields and we expect no polarization for a baryonic jet and polarized emission for a magnetized jet.
Currently, highly polarized reverse shock emission from GRBs favours a mildly magnetic jet model (e.g., P=28%; Mundell et al. 2013), in which the gamma-ray prompt is understood as in the baryonic jet model but the magnetic fields still play a key role in driving the explosion. However, the polarization observations of the most energetic GRB ever detected (named GRB 190114C; Jordana-Mitjans et al. 2020) showed surprisingly low polarization (P=2%) for reverse shock emission and it was only significantly higher at prompt emission timescales (P=7.7%). Our modelling of the fading brightness of the light tells us that the jet in GRB 190114C was magnetized and the polarization observations indicate that the magnetic fields collapsed catastrophically straight after the explosion (e.g., via reconnection), releasing their energy and powering the bright light detected across the electromagnetic spectrum. This scenario suggests that the jet was launched highly magnetized.
This work was done thanks to the support from C.G. Mundell (University of Bath), S. Kobayashi (Liverpool John Moores University), R.J. Smith (Liverpool John Moores University), C. Guidorzi (University of Ferrara), I.A. Steele (Liverpool John Moores University), M. Shrestha (Liverpool John Moores University), A. Gomboc (University of Nova Gorica), M. Marongiu (University of Ferrara), R. Martone (University of Ferrara), V. Lipunov (Lomonosov Moscow State University), E.S. Gorbovskoy (Lomonosov Moscow State University), D.A.H. Buckley (South African Astronomical Observatory), R. Rebolo (Instituto de Astrofísica de Canarias) and N.M. Budnev (Irkutsk State University).