The Jets Of Gamma-ray Bursts “Cool Down” Like This (Planetary Science)

Published on Nature Communications a study led by Samuele Ronchini, PhD student at Gssi and associate Infn, which sheds new light on the cooling processes in the jets of gamma-ray bursts, also suggesting that protons, and not electrons, emit radiation through synchrotron process

What are the processes that determine the cooling of relativistic particles in gamma-ray bursts, giving rise to the typical rapid decrease in flux that accompanies the initial explosion? To give a convincing answer to this question comes the work of a group of researchers from the Gran Sasso Science Institute (GSSI), in collaboration with colleagues from the National Institute of Astrophysics (Inaf) in which a report is presented that unites several gamma-ray bursts. (or Grb, Gamma-Ray Burst in English) during a transition phase of the phenomenon called “steep decay”. The uniqueness of this relationship led the group to go in search of a universal process that could give a correct interpretation. The studywas published today in the journal Nature Communications .

Gamma-ray bursts are considered to be among the most catastrophic and energetic events in the Universe as they can release in fractions of a second the entire energy emitted by a star such as the Sun throughout its life. The discovery of supernovae and gravitational waves associated with gamma bursts confirmed that some of these phenomena are the product of the collapse of a massive star, and others of the merger of two neutron stars. Known since the 1960s, their nature remained debated until the advent of the Neil Gehrels “Swift” Observatory, an observatory in orbit since 2004 dedicated to the detection, localization and characterization of Grb. After years of observing in different bands of the electromagnetic spectrum, from radio waves to gamma rays, we now know that Grbs are produced by jets of energy and matter launched from a black hole that expand at close to the speed of light. The particles carried by the jet are accelerated through shock waves or in magnetic reconnection regions, converting the kinetic (or magnetic) energy into electromagnetic radiation, which typically peaks in gamma rays. Subsequently the jet emits radiation at lower frequencies in the encounter and interaction with the interstellar medium, giving rise to the so-called afterglow phase, which can last from minutes, hours to months after the initial gamma-ray burst. The steep decay marks the boundary between the end of the initial impulsive phase, where most of the high-energy radiation is released, and the beginning of the afterglow . Its characteristic rapidity of decay in flow is generally associated with the curved geometry of the surface of the jet inside which the particles are accelerated and then dissipate their energy in the form of radiation.

However, this scenario, long accepted by the scientific community to interpret steep decay , fails to reproduce the spectral relationship found by the research team. The observational evidence therefore imposed a paradigm shift for the theoretical interpretation and the solution was found by assuming adiabatic cooling of the particles. By cooling we mean the process by which the particles lose their initial energy, while the term adiabatic refers to the expansion of the volume that contains the particles, which does not exchange heat with the external environment.

“I began to notice that we were observing something unique when I kept finding this strong spectral evolution in data analysis, independent of the characteristics of gamma-ray bursts. From there we began to understand the potential of the result ”, says Samuele Ronchini , a third-year doctoral student at GSSI. «It is certainly not easy to question well-established models in the scientific community and carrying out this work was a significant responsibility. However, the perseverance, competence and support of the entire research group allowed us to go all the way and obtain the desired results ». This result has a profound impact on the understanding of the emission and cooling processes of accelerated particles in Grb.

“The dominance of the adiabatic process indicates that the particles are unable to efficiently dissipate their energy, giving valuable information on the physical properties of the relativistic jet, such as the evolution of the magnetic field in the acceleration site, as well as on the nature of the particles themselves”, he explains. Gor Oganesyan , postdoc researcher at GSSI and second author of the article. “In particular, we understood that a mild decay of the magnetic field is required to reproduce the observed data, together with adiabatic cooling.”

Furthermore, the inefficient energy dissipation of the accelerated particles could suggest that it is protons, not electrons, that emit radiation via the synchrotron process. «Grb are particularly difficult to study», concludes Om Sharan Salafia of INAF in Milan, co-author of the study, «because, despite having some distinctive characteristics, they are extremely different from each other. Finding common and universal traits, as in this case, is the key to unifying them and understanding the physical processes that produce them ».

Future wide-field X-band observers able to monitor the entire evolution of the Grb from the very first moments after the explosion will be crucial to fully understand the result found by extending the work to a larger sample and in other bands of the spectrum. . “Understanding the mechanisms underlying these catastrophic events is of primary importance also in the context of multi-message astronomy”, underlines Marica Branchesi, full professor at GSSI and co-author of the work. “The combined study of gravitational waves and electromagnetic radiation associated with GRBs may in the future clarify many open questions related to the nature of the compact objects that generate them, their distribution on cosmological scales, as well as the physics of jets and the acceleration of particles at their indoor”.

Featured image: Artist’s impression of the cooling of gamma-ray bursts (Grb). Credits: Samuele Ronchini Gran Sasso Science Institute

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