Swedish participation in THESEUS

THESEUS - the Transient High-Energy Sky and Early Universe Surveyor - is a proposed ESA/M7 mission that currently is in a Phase A study, with a possible launch in around 2037.The THESEUS mission will study short-lived transients, such as gamma-ray bursts (GRBs) using multiple detectors on an autonomous, rapid-slewing satellite.

It will provide an essential step forward in the study of GRBs phenomenon, by providing a wide-area detection and characterisation of GRBs over a very broad energy band (0.3 keV to 10 MeV), including on-board near-infrared imaging and spectroscopy.

It grew out of the recommendation following the ESA L2/L3 selection process: “The SSC strongly endorses the need to continue pursuing in the future the discovery of GRBs”. The key scientific goals include(i) Investigating the first billion years of the Universe through high-redshift GRBs.

This will shed light on main open issues in modern cosmology, like the population of primordial low mass and luminosity galaxies, sources and evolution of cosmic re-ionization, and the star formation rate and metallicity evolution up to the "cosmic dawn" and across Pop-III stars.(ii) Identifying electromagnetic counterparts to sources of gravitational waves and neutrinos.

With the advent of multi-messenger astronomy with the historical observation of a kilonova both in gravitational waves and across the whole electromagnetic spectrum, THESEUS will also add huge value to the currently planned new photon and multi-messenger astrophysics infrastructures coming on-line in the 2020s and 2030s.(iii) Study of all classes of GRB sources.

With the unique capabilities of THESEUS instrumentation, the long-standing unanswered quesitons regarding the radiation process during the prompt phase can be fully addressed.

The GRB group at KTH has a long experience and expertise in modelling and analysis of the prompt phase in GRBs which is one of the main, highlighted areas of research for THESEUS.The intention of the THESEUS consortium is invite our group at KTH to the consortium in order for us to provide our expertise on the physics of GRBs.

The consortium has also identified a useful Swedish contribution to the hardware of the satellite though the intersatellite communication system (ICS).

This system will be important to be able to transmit trigger and localistation information to ground in order for follow-up observations of other satellites and ground facitlites.

Since GRBs are short transients an such an efficient system is essential for the science return.The current funding application is for covering the Swedish efforts during the Phase A study, 2025-2026.

These will include (i) Study of the science requiremnents, in particular in light of spectral modelling of GRBs that we are experts in, comparing photospheric and synchrotron emission models. (ii) Involvement in working groups, such as coordination roles. (iii)  Investigation of the detailed budget and cost share among member states for the proposed intersatellite communication system (ICS) in order to let us explore possible Swedish contributions to such a system. (iv) Investigation of Swedish industry connection in the development, delivery and running of such a system.The THESEUS mission will be a very important step forward for GRB science, as well as for multimessenger astronomy, including followup observations of gravitaional wave detections.