The THESEUS mission

THESEUS is a mission concept proposed in response to the ESA call for medium-size mission (M5) within the Cosmic Vision Programme. A few M5 mission candidates will be selected by ESA toward the end of 2017 to enter an assessment study phase.

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THESEUS in context.

The mission is designed to vastly increase the discovery space of the high energy transient phenomena over the entirety of cosmic history. Its primary scientific goals will address the Early Universe ESA Cosmic Vision themes “How did the Universe originate and what is made of?” (4.1, 4.2 and 4.3) and will also impact on “The gravitational wave Universe” (3.2) and “The hot and energetic Universe” themes. This is achieved via a unique payload providing an unprecedented combination of: 1) wide and deep sky monitoring in a broad energy band (0.3keV - 20 MeV); 2) focusing capabilities in the soft X-ray band providing large grasp and high angular resolution; and 3) on board near-IR capabilities for immediate transient identification and redshift determination.


The foreseen payload of THESEUS includes the following instrumentation:

  • Soft X-ray Imager (SXI, 0.3 – 6 keV): a set of 4 lobster-eye telescopes units, covering a total field of view (FOV) of ~1sr with source location accuracy < 1-2’;
  • InfraRed Telescope (IRT, 0.7 – 1.8 μm): a 0.7m class IR telescope with 10’x10’ FOV, for fast response, with both imaging and spectroscopy capabilities;
  • X-Gamma rays Imaging Spectrometer (XGIS, 2 keV – 20 MeV): a set of coded-mask cameras using monolithic X-gamma rays detectors based on bars of Silicon diodes coupled with CsI crystal scintillator, granting a ~1.5sr FOV, a source location accuracy of ~5 arcmin in 2-30 keV and an unprecedently broad energy band.

The mission profile includes: an onboard data handling units (DHUs) system capable of detecting, identifying and localizing likely transients in the SXI and XGIS FOV; the capability of promptly (within a few tens of seconds at most) transmitting to ground the trigger time and position of GRBs (and other transients of interest); and a spacecraft slewing capability of ~10-20°/min). The baseline launcher / orbit configuration is a launch with Vega-C to a low inclination low Earth orbit (LEO, ~600 km, <5°), which has the unique advantages of granting a low and stable background level in the high-energy instruments, allowing the exploitation of the Earth’s magnetic field for spacecraft fast slewing and facilitating the prompt transmission of transient triggers and positions to the ground.

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A sketch of the THESEUS satellite showing the instruments accommodation is shown on the left panel. The middle panel shows the GRASP (field of view versus effective area) of the THESEUS/SXI in the soft X-ray energy band compared to XMM-Newtin and eROSITA . The GRASP of X-ray monitrs on-board MAXI and ASTROSAT are also show for completeness, even though these are not focusing and their sensitivity for a given eff. area is substantially sworse than that of focusing teelscopes. The leap in monitoring / syervey of the soft X-ray sky allowed by THESEUS/SXI is outstanding. The panel on the right shows the cumulative distribution of GRBs with redshift determination as a function of the redshift for Swift (in 10 yr) and the prediction for THESEUS (in 3 yr).


The main scientific goals of the proposed mission are to:

  1. Explore the Early Universe (cosmic dawn and reionization era) by unveiling a complete census of the Gamma-Ray Burst (GRB) population in the first billion years. Specifically to:

  • Perform unprecedented studies of the global star formation history of the Universe up to z ~ 10 and possibly beyond;

  • Detect and study the primordial (pop III) star population: when did the first stars form and how did the earliest pop III and pop II stars influence their environments?

  • Investigate the re-ionization epoch, the interstellar medium (ISM) and the intergalactic medium (IGM) up to z ~ 8 - 10: how did re-ionization proceed as a function of environment, and was radiation from massive stars its primary driver? How did cosmic chemical evolution proceed as a function of time and environment?

  • Investigate the properties of the early galaxies and determine their star formation properties in the re-ionization era.

  1. Perform an unprecedented deep monitoring of the X-ray transient Universe in order to:

  • Locate and identify the electromagnetic counterparts to sources of gravitational radiation and neutrinos, which may be routinely detected in the late ‘20s / early ‘30s by next generation facilities like aLIGO/aVirgo, eLISA, ET, or Km3NET;

  • Provide real-time triggers and accurate (~1 arcmin within a few seconds; ~1’’ within a few minutes) locations of (long/short) GRBs and high-energy transients for follow-up with next-generation optical-NIR (E-ELT, JWST if still operating), radio (SKA), X-rays (ATHENA), TeV (CTA) telescopes;

  • Provide a fundamental step forward in the comprehension of the physics of various classes of Galactic and extra-Galactic transients, e.g.: tidal disruption events (TDE), magnetars /SGRs, SN shock break-outs, Soft X-ray Transients SFXTS, thermonuclear bursts from accreting neutron stars, Novae, dwarf novae, stellar flares, AGNs and Blazars;

  • Provide unprecedented insights into the physics and progenitors of GRBs and their connection with peculiar core-collapse SNe and substantially increase the detection rate and characterization of sub-energetic GRBs and X-Ray Flashes;

  • Fill the present gap in the discovery space of new classes of high-energy transient events, thus providing unexpected phenomena and discoveries.

By satisfying the requirements coming from the above main science drivers, the THESEUS payload will also automatically enable excellent observatory science opportunities, including, e.g., performing IR observatory science, especially providing capability for response to external triggers, thus allowing strong community involvement. We remark that THESEUS has survey capabilities for high-energy transient phenomena complementary to the Large Synoptic Survey Telescope (LSST) in the optical. Their joint availability in the next decade would enable a remarkable scientific synergy between them.