[Paper Review] The Transient High-Energy Sky and Early Universe Surveyor (THESEUS)

L. Amati|arXiv (Cornell University)|Oct 9, 2019

Gamma-ray bursts and supernovae213 references18 citations

TL;DR

THESEUS is a proposed ESA M5 mission concept designed to detect, localize, and redshift gamma-ray bursts (GRBs) across cosmic time, with arcsecond accuracy using a multi-instrument payload. It will enable breakthroughs in early-universe cosmology, multi-messenger astrophysics, and transient source characterization by combining wide-field X-ray and gamma-ray monitoring with rapid near-infrared follow-up, significantly enhancing the scientific return of next-generation facilities like E-ELT, SKA, CTA, and gravitational-wave detectors.

ABSTRACT

THESEUS is a space mission concept currently under Phase A study by ESA as candidate M5 mission, aiming at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. Through an unprecedented combination of X-/gamma-ray monitors, an on-board IR telescope and automated fast slewing capabilities, THESEUS will be a wonderful machine for the detection, characterization and redshift measurement of any kind of GRBs and many classes of X-ray transients. In addition to the full exploitation of high-redshift GRBs for cosmology (pop-III stars, cosmic re-ionization, SFR and metallicity evolution up to the "cosmic dawn"), THESEUS will allow the identification and study of the electromagnetic counterparts to sources of gravitational waves which will be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/aVirgo, LISA, KAGRA, and Einstein Telescope (ET), as well as of most classes of transient sources, thus providing an ideal synergy with the large e.m. facilities of the near future like LSST, ELT, TMT, SKA, CTA, ATHENA.

Motivation & Objective

• Address fundamental gaps in understanding the early Universe, including the formation of first stars (Pop III), reionization, and galaxy evolution at z > 8.
• Overcome limitations of current surveys by detecting and characterizing high-redshift GRBs (up to z ≈ 12) that are inaccessible to current facilities.
• Enable multi-messenger astrophysics by providing real-time, accurate localization of electromagnetic counterparts to gravitational wave and neutrino sources.
• Fill the discovery space for faint, under-luminous, and ultra-long GRBs and other transient phenomena across the high-energy sky.
• Provide a flexible, multi-wavelength follow-up capability for time-domain transients, synergizing with next-generation ground-based and space-based observatories.

Proposed method

• Utilize a dual-instrument approach: Soft X-ray Imagers (SXI) covering 0.3–5 keV with ~1 arcmin localization accuracy and X- and Gamma-ray Imaging Spectrometers (XGIS) covering 2–30 keV with ~1 arcmin accuracy.
• Integrate a 70 cm class near-infrared telescope (IRT) on-board to perform rapid slewing and achieve ~1 arcsec localization of afterglows and kilonovae.
• Employ autonomous spacecraft slewing within minutes of GRB detection to enable timely follow-up observations of transient afterglows.
• Use coded-mask cameras with silicon diodes and CsI scintillators for imaging and spectroscopy in the 2–10 MeV band.
• Leverage the mission’s wide field of view (~1 sr for SXI, several sr for XGIS) to survey the entire high-energy transient sky continuously.
• Enable redshift determination through photometry and moderate-resolution spectroscopy of infrared afterglows and kilonova emission.

Experimental results

Research questions

• RQ1 Can GRBs be used as standard candles to measure the cosmic star formation rate beyond the reach of current galaxy surveys?
• RQ2 What is the detectability and population of high-redshift (z > 8–10) GRBs, and can they reveal the properties of first-generation (Pop III) stars?
• RQ3 How can arcsecond localization of electromagnetic counterparts to gravitational wave events (e.g., NS-NS mergers) improve cosmological distance measurements?
• RQ4 What is the expected detection rate and characterization capability of sub-energetic GRBs, X-ray flashes, and ultra-long GRBs with THESEUS?
• RQ5 To what extent can THESEUS enhance the scientific return of next-generation facilities like E-ELT, SKA, CTA, and ATHENA through coordinated multi-wavelength observations?

Key findings

• THESEUS is projected to detect and localize up to ~100 GRBs per year with redshift measurement, including a significant fraction at z > 8–10, far exceeding current capabilities.
• The mission will achieve ~1 arcsec localization accuracy for near-infrared afterglows and kilonovae, enabling precise redshift determination through photometry and spectroscopy.
• THESEUS is expected to detect short GRBs and kilonova emission from NS-NS and NS-BH mergers with a sensitivity sufficient to cover event rates of up to ~20 per year for third-generation gravitational wave detectors.
• The mission will detect GRBs with isotropic energy Eiso = 10^53 erg up to redshift z = 12, corresponding to the median of the GRB energy distribution.
• THESEUS will provide real-time triggers with ~1 arcmin localization within seconds and ~1 arcsec within minutes, enabling rapid follow-up by optical, radio, and X-ray facilities.
• The combination of simultaneous X-ray and near-infrared observations will allow the study of thousands of X-ray and infrared sources, significantly advancing Observatory Science and transient source characterization.