Gamma ray burst studies with THESEUS

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.

The Peculiar Short-duration GRB 200826A and Its Supernova*

“…For the spectral lag calculation, we then restricted the following analysis to the time interval [−0.5:4] s and calculated the cross-correlation function (CCF) between the backgroundsubtracted profiles of the two energy bands, using both the original binning of 1 ms and that of 16 ms. A positive lag corresponds to the softer band lagging behind the harder one. The peak of the CCF was found to be about 100 ms. To provide a more accurate estimate, we fitted the CCF around the peak with a third-degree polynomial between −0.4 and 0.6 s and it gave 95 ms. To estimate the uncertainty, we carried out the following simulations: starting from the 16 ms profiles, we obtained smooth versions of these curves by adopting the L1 trend filtering by Politsch et al (2020) properly adapted to modeling GRB light curves (e.g., see Ghirlanda et al 2021). We then obtained 1000 random realizations of both profiles by assuming uncorrelated Poisson noise, assuming the total expected count for each temporal bin (i.e., the interpolated background plus the smoothed GRB signal).…”

Properties of High-redshift Gamma-Ray Bursts

“…Using the intrinsic GRB rate and the corresponding luminosity function for each theoretical model, we estimate the expected Swift/BAT GRB detection rate. For the GRB spectral shape, we adopt a Band function with E peak , α, and β following distributions described in Ghirlanda et al (2015Ghirlanda et al ( , 2021. Note that the E peak from Ghirlanda et al (2021) follows the Amati E peak −L relationship, implying a harder spectrum for a more luminous burst.…”

“…For the GRB spectral shape, we adopt a Band function with E peak , α, and β following distributions described in Ghirlanda et al (2015Ghirlanda et al ( , 2021. Note that the E peak from Ghirlanda et al (2021) follows the Amati E peak −L relationship, implying a harder spectrum for a more luminous burst.…”

Properties of High-Redshift GRBs

“…Using the intrinsic GRB rate and the corresponding luminosity function for each theoretical model, we estimate the expected Swift/BAT GRB detection rate. For GRB spectral shape, we adopt a Band function with E peak , α, and β following distributions described in Ghirlanda et al (2015Ghirlanda et al ( , 2021. Note that the E peak from Ghirlanda et al (2021) follows the Amati E peak − L relationship, implying an harder spectrum for a more luminous burst.…”

“…For GRB spectral shape, we adopt a Band function with E peak , α, and β following distributions described in Ghirlanda et al (2015Ghirlanda et al ( , 2021. Note that the E peak from Ghirlanda et al (2021) follows the Amati E peak − L relationship, implying an harder spectrum for a more luminous burst.…”