Line of research overview

Gamma-Ray Bursts as multimessenger and fundamental physics probes

Principal Investigator: Annalisa Celotti

Abstract: The project proposes a multi–messenger investigation of long and short Gamma-Ray Bursts (GRBs), involving three fundamental, interconnected aspects: 1) the study of the electromagnetic and Gravitational Wave (GW) event rates in a self-consistent cosmological context, 2) the characterization and interpretation of the electromagnetic outputs, 3) the use of GRB radiation as a tool to set constrains on fundamental physics.

Starting from the assumption that the progenitors of short and long GRBs are, respectively, compact binary mergers and massive star collapses, their rates will be estimated in the context of models for galaxy evolution with consistent star formation and chemical enrichment histories in both spheroidal and disk galaxies. The predicted rates will then be compared to detected rates of GRBs and GW events, to set constraints on model parameters. At the same time galaxy evolution models can give informations on the environment where GRBs take place. This fact can be very useful to give better constraints on GRB progenitor models.

The phenomenology related to the ultra-relativistic flows (“jets”) launched in binary mergers and in core-collapses of massive stars is still quite uncertain. The second part of the project is then aimed at reaching a better understanding of the electromagnetic output, including a study of the jet dynamics, composition, and emission mechanisms.

Finally, the possibility of setting constraints on fundamental physics will be investigated. The short variability timescales and the extremely high energy photons characterising GRB radiation are promising features for the study of Lorentz invariance violation, especially in view of new facilities that will allow better measurements of variability and high-energy radiation.

Status of project and perspectives: Gamma-Ray Bursts (GRBs) are non-repeating cosmological sources characterized by short timescale gamma-ray emission. The phenomenological properties, in terms of emission duration and spectra, appears to be bimodal, suggesting the existence of two different classes of GRBs: short-hard and long-soft. The two different classes are believed to be the outcome of different progenitors: compact binary mergers for short GRBs and massive star core collapses for long ones. The core-collapse hypothesis for long GRBs has found immediate support in the associated detection of Supernova (SN) features. The validity of a compact binary merger scenario for short GRBs has instead been confirmed only recently, with the detection of gravitational waves from a NS-NS merger, in association with a short GRB.

Within this context, GW event rates, GRB rates and characterisation of the host galaxy properties via multi-wavelength observations can yield astrophysical constraints on GRB origin and stellar binary evolution (SN kicks, common envelope effects, mass transfers), on galaxy formation and evolution scenarios (chemical evolution, star formation histories, initial mass function) and even on cosmology at large.

Rates in the electromagnetic channel can be calibrated from large samples collected from past and present dedicated gamma-ray missions. Moreover, the number of simultaneous electromagnetic and GW events – currently available for only one source – will soon increase thanks to the upcoming observational runs of GW detectors with improved sensitivities.

A full exploitation of the electromagnetic signal from GRBs requires a good understanding of its physical origin. Part of the project is then aimed at improving our still poor understanding on how the radiation is produced. In particular, the composition of the jet (baryonic or magnetic), the location of the region where the prompt gamma-ray emission is generated, the dissipation mechanism and the nature of the radiative process are still unknown. Recent progresses on the characterisation of the “prompt” spectra suggest that the prompt dissipating region is located at distances much larger than those usually considered. Larger distances in turn favour a magnetically (rather than matter) dominated outflow where the dissipation proceed through magnetic reconnection.

In this respect, variability of high energy emission can provide a complementary information. The team envisages that a mission, such as HERMES, currently under development, could be a key in such studies. We are involved in the development of a first mini-costellation of cubesats hosting high performances X-ray and gamma-ray detectors, the HERMES Scientific Pathfinder (HSP), which will include 7 units. HSP will provide position of long and short GRB with arcmin to deg accuracy, depending on the GRB brightness and temporal structure, using the delay time of the arrival of the GRB signal to different detectors. HSP payload has two main characteristics, which make it unique among GRB monitors: 1) extremely broad band from a few keV to a few MeV; and 2) extremely good timing capabilities, down to 300 ns, about 7 times better than the best instruments flown so far. These characteristics will allow both broad band studies to constrain GRB emission mechanisms and high resolution timing studies, to study the GRB inner engine.

Further constraints could be provided by the detection of an extremely high energy (TeV band) component. While the existence of emission from GRBs in the TeV range was highly questioned, very recently TeV radiation has been detected for the first time, by the MAGIC Cherenkov Telescope. This important detection sets a very promising path for the upcoming, next generation of Cherenkov Telescope Array (CTA). Team members are involved in the MAGIC, CTA and Fermi-LAT consortia.

Good timing, broad band spectral information and detection of high-energy gamma-rays will also allow us to use GRB as beacons to investigate fundamental physics, such as the quantum nature of space-time, and set limits on Lorentz invariance violation.

Regarding the IFPU activities, we are organising the Fifth National Congress on Gamma-Ray Bursts to be held at IFPU from 12 to 16 of September 2022. The focus will be on the major unsolved issues about the physics of GRBs, from both the theoretical and observational perspectives, with the contribution of Italian GRB experts on the subject.