Abstract
Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe, over–shining for a few seconds all other γ-ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after the core–collapse of massive stars or the merger of compact binaries. THESEUS will open a new window for the use of GRBs as cosmological tools by securing a statistically significant sample of high-z GRBs, as well as by providing a large number of GRBs at low–intermediate redshifts extending the current samples to low luminosities. The wide energy band and unprecedented sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us a new route to unveil the nature of the prompt emission. For the first time, a full characterisation of the prompt emission spectrum from 0.3 keV to 10 MeV with unprecedented large count statistics will be possible revealing the signatures of synchrotron emission. SXI spectra, extending down to 0.3 keV, will constrain the local metal absorption and, for the brightest events, the progenitors’ ejecta composition. Investigation of the nature of the internal energy dissipation mechanisms will be obtained through the systematic study with XGIS of the sub-second variability unexplored so far over such a wide energy range. THESEUS will follow the spectral evolution of the prompt emission down to the soft X–ray band during the early steep decay and through the plateau phase with the unique ability of extending above 10 keV the spectral study of these early afterglow emission phases.
Highlights
Our picture of Gamma Ray Bursts (GRBs) has evolved in the latest 50 years through giant steps forward enabled mainly by outstanding observational discoveries from space or ground based facilities
Through physically motivated population models calibrated with available prompt emission data gathered by Swift and Fermi, we evaluate the properties and redshift distributions of both long and short Gamma-ray Bursts (GRBs) detectable by THESEUS
We showed that THESEUS will enlarge the sample of high-z long GRBs by at least one order of magnitude with respect to what we have
Summary
Our picture of Gamma Ray Bursts (GRBs) has evolved in the latest 50 years through giant steps forward enabled mainly by outstanding observational discoveries from space or ground based facilities. While the observational picture of GRBs was enriched by both the progressively larger sample of GRBs with measured redshifts pinpointed by the Swift satellite and by the study of the emission in the MeV–GeV energy range by the Fermi satellite, the sample of high redshift GRBs still comprises only 6 events at z > 6 [106] These GRBs are crucial to probe the early Universe. The added values are the high sensitivity of the Soft X–ray Imager (SXI), the wide energy range and large effective area of the X–Gamma rays Imaging Spectrometer (XGIS) and the onboard Infra-Red Telescope (IRT) This unique combination of instruments on board THESEUS will allow us to detect high redshift GRBs (and measure their redshifts), and study the prompt and X–ray properties of the full population of detected events.
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