Institute for Astronomy and Astrophysics


THESEUS (Transient High Energy Sky and Early Universe Surveyor) is a proposed mission concept for the European Space Agency (ESA). It is one of three missions selected by ESA in 2018 to enter an assessment phase study for a medium-sized mission (M5). THESEUS addresses several themes of ESA’s Cosmic Vision Programme, namely:

  • How did the Universe originate and what is it made of? 
    • The early Universe 
    • The Universe taking shape
    • The evolving violent Universe 
  • The hot and energetic Universe
  • The gravitational wave Universe

With an infrared telescope, four X-ray cameras and three gamma-ray spectrometers, THESEUS will observe a large part of the sky and react immediately to short-term changes in brightness caused by transient high-energy sources such as gamma-ray bursts (GRBs).

Such GRBs can be caused by the merging of two neutron stars or a core collapse supernova, which is the death of a very massive star. Since they are among the brightest events in the sky, GRBs provide information about events from the early Universe. If THESEUS detects a GRB, it quickly determines its coordinates and automatically aligns itself for follow-up observations. Additionally, it passes the coordinates to a ground station within a few seconds for more follow-up observations by other telescopes.

Launch: the planned launch of the selected M5 mission is 2032

Energy Range of the Instruments:

  • Soft X-ray Imager (SXI): 0.3 - 6 keV
  • InfraRed Telescope (IRT): 0.7 – 1.8 μm
  • X-Gamma rays Imaging Spectrometer (XGIS): 2 keV – 20 MeV

Scientific Goals: Explore the physical conditions of the early Universe by localizing and characterizing GRBs and transient X-ray events.

Scientific Instruments

The foreseen instruments onboard THESEUS are:

  1. The Soft X-ray Imager (SXI) consisting of a set of 4 lobster-eye telescope units, covering a total field of view (FOV) of ~1sr with a source location accuracy < 1-2’ at an energy range of 0.3 - 6 keV.
  2. An InfraRed Telescope (IRT), which is a 0.7 m class IR telescope with a 10’x10’ FOV, for fast response, with both imaging and spectroscopy capabilities at wavelengths of 0.7 – 1.8 μm.
  3. The X-Gamma rays Imaging Spectrometer (XGIS), made of 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 ~1.5 sr FOV, a source location accuracy of ~5 arcmin in 2 - 30 keV and an unprecedentedly broad energy band of 2 keV – 20 MeV.


The proposed THESEUS Soft X-ray Imager (SXI) comprises 4 detector units (DUs). Each DU is a wide field lobster eye telescope. The surface of a biological lobster eye is a hemisphere covered with radially oriented, square tubes (see the Figure on the right). When light hits the eye, it bounces off the insides of the reflecting sides and is focused on a smaller hemisphere. A similar design is used for the lobster-eye-telescope to detect X-rays, which only reflect under grazing angles.
The optics aperture is formed by an array of 8×8 square pore Micro Channel Plates (MCPs). The MCPs are 40×40 mm2 in size and are mounted on a spherical frame with radius of curvature 600 mm (2 times the focal length of 300 mm).

MCPs are plates with microscopically small bundled electron multiplier channels. They are made of glass. The upper and lower sides of the MCPs are equipped with metallic electrodes. A typical voltage of 1000 V is applied to them. The inner channel surfaces are coated with semiconductors. A photon hitting the channel surface can trigger a photoelectron (under vacuum). Due to the high field strength in the channel, the electron is accelerated towards the back of the MCP. It will hit the channel wall again and can trigger several electrons there. In the electron multiplier channel, an avalanche of electrons is created, which emerges as an electron cloud from the back of the MCP. In this way, about 10,000 electrons can escape from one channel.


The X-Gamma ray Imaging Spectrometer (XGIS) includes 3 units (telescopes). The three units are pointed at offset directions in such a way that their FOV partially overlap.

The detectors for the X-Gamma rays Imaging Spectrometers (XGIS) are so-called silicon drift detectors (SDDs). The most important advantages of silicon drift detectors are their high energy and time resolution as well as their low weight (~ 1 kg/m2). The interaction of an X-ray photon with the detector material generates an electron cloud. A constant electric field with negative voltage decreasing from the center to the sides of the detector provides a potential gradient to the readout anodes at the edge. The electron cloud now drifts from the point of impact to the anodes within a few microseconds and expands slightly.

In XGIS, the basic element of a module (figure below, left) is a 5×5×30 mm3 bar made of scintillating crystal (CsI(Tl)). Each extreme of the bar is covered with a Photo Diode (PD) to read out the scintillation light. The PD works under the above described SDD principle and has a size of 5x5 mm2 to match the scintillator module. The SDD and PD detect X- and gamma-rays. When such a ray enters the detector (see figure below, right) it either interacts with the Silicon (low energy X-ray photons) or with the scintillator material, which in turn emits scintillation light. The discrimination between the type of photons (X- and gamma-rays) is based on the different shape of charge pulses. The interaction of X-rays in Silicon generates a fast signal, while the scintillation light is dominated by the fluorescent states de-excitation time, which is much slower. Therefore, pulse shape analysis (PSA) is used to discriminate between the signals.


The InfraRed Telescope (IRT) foreseen for THESEUS is designed to identify, localize and study the transients and afterglows of the GRBs detected by SXI and XGIS. It is a 0.7m aperture Cassegrain NIR telescope with a 0.23m secondary mirror and a 10x10 arcmin imaging field of view.

In order to achieve the desired performances, the telescope needs to be cooled at 240±3 K, which can be achieved by passive means. For the IRT camera, the optics box needs to be cooled to 190±5 K and the IR detector itself to 95±10 K: this allows the detector dark current to be kept at an acceptable level. The cooling of the detector at these low temperatures cannot be achieved with a passive system in a low Earth orbit such as the one foreseen for THESEUS, due to the irradiation of the radiators of the infrared flux by the Earth atmosphere. Therefore a Miniature Pulse Tube Cooler (MPTC) will be used.


  SXI detector XGIS detector IRT
Energy range 0.3 - 5 keV 2 keV - 20 MeV

0.7 - 1.8 μm

Type Lobster eye

Low-energy detector (2 - 30 keV): SDD

High-energy detector (> 30 keV): CsI(Tl)

FOV ~ 1 sr ~ 1.5 sr 10x10 arcmin


Scientific Goals

THESEUS will address multiple fundamental questions of modern cosmology and astrophysics. Within ESA’s Cosmic Vision Programme, it will address the themes “How did the Universe originate and what is it made of?”, “The gravitational wave Universe” and “The hot and energetic Universe”. It will do so by exploring the early Universe (the cosmic dawn and re-ionization era) by studying the GRB population in the first billion years and by performing an unprecedented deep monitoring of the soft X-ray transient Universe. Thus, THESEUS will also cooperate notably with next-generation gravitational waves and neutrino detectors within the so called multi-messenger astrophysics, as well as the large electromagnetic facilities of the next decade, such as ATHENA, E-ELT, SKA, CTA, LSST, etc.

A. Exploring the Early Universe

The question of what happened during the first billion years in the Universe is one of the fundamental questions of astrophysics and cosmology. It asks what happened with the first structures in the universe, such as Population III (Pop III) stars, black holes and galaxies. Pop III stars are hypothesized, extremely massive and hot stars with next to no metals.

In the figure on the left is a timeline of the Universe from the big bang to the present. After the so-called “recombination” and “decoupling”, the Universe was transparent and cool enough for light to travel. However, there were no light-producing structures such as stars and galaxies. These started to form only after the “Dark ages”, at an age of about 400 million years, roughly coinciding with the onset of the “Epoch of Reionization”. During this epoch, the intergalactic medium (IGM) was re-ionized and became richer in metals. Both the re-ionization and the build up of metals in the universe are not well understood and need further research. Therefore the question remains whether it was predominantly radiation from massive stars that brought about and sustained these changes, or whether more exotic, unknown, mechanisms were responsible.

In order to work towards answering these questions, two subsidiary issues can be addressed: first, how much massive star formation was occurring as a function of redshift (as redshift corresponds to time, see also the figure), and second, on average, what proportion of the ionizing radiation produced by these massive stars escaped the galaxies. GRBs and their host galaxies can be powerful tools to answer these questions, potentially investigating star formation, metal enrichment and galaxy evolution possibly before re-ionization.

THESEUS is predicted to detect between 30 and 80 GRBs at z > 6 during a three year mission, 10-25 of these at z > 8 and several at z > 10, see the figures below. With the on-board follow-up instrumentation, redshifts will be estimated for most of these and next generation telescopes can make additional follow-up observations (e.g. with JWST, E-ELT, ATHENA, etc.).

B. GW sources and multi-messenger astrophysics

Both gravitational wave (GW) and neutrino astronomy have advanced greatly in recent years and second and third generation detectors for GWs and neutrinos will be built in the next decades and likely revolutionize astrophysics in both areas. THESEUS will operate at the same time as these detectors, see the figure below.

Several of the most powerful transient sources of GWs (e.g. binary neutron star (NS-NS) or NS-black hole (BH) mergers) are expected to produce bright electromagnetic (EM) signals across the entire EM spectrum, including the X-ray and gamma-ray ranges, as well as neutrinos. Both GW and neutrino detectors have shortcomings when it comes to the localization of sources. Thus, in order to maximize the science return of the multi-messenger studies, it is essential to have a complementary in-orbit trigger and search facility that can localize detected sources with higher accuracy. This is fulfilled by THESEUS, which is able to observe the sky within the GW/neutrino error boxes and localize sources or vice versa send candidates to earth that can be followed-up by GW detectors.

The detection of the EM counterparts in X-rays of GW or neutrinos allows to localize the source with an accuracy good enough for optical follow-up and therefore its redshift and luminosity may be measured. Not finding an EM counterpart on the other hand will constrain merger types (e.g. BH-BH mergers), emission mechanisms, astrophysical conditions at the time of merger, and total energetics.

Moreover, cooperation with currently planned facilities like James Webb Space Telescope (JWST), WFIRST, ATHENA, Einstein Probe, E-ELT, TMT, GMT, SKA, CTA zPTF and LSST telescopes would be desirable in order to be able to extend the multi-messenger observations across the whole EM spectrum.

The science goals of THESEUS and the corresponding instrument and spacecraft requirements are summarized in the figure below.

IAAT Participation

The Institute for Astronomy and Astrophysics in Tübingen (IAAT) participates in the instrument data handling units (I-DHUs, instrument computers).

The observing strategy of THESEUS is as follows: New transient events are localized with SXI and XGIS. The SXI can be triggered by many classes of phenomena (e.g. flare stars, X-ray bursts, GRBs etc.). XGIS can then help identify high energy transients (GRBs, Soft Gamma-ray Repeaters etc.). Therefore, parallel to the selection by the SXI-DHU, additional trigger conditions from the XGIS-DHU are looked at. On top of that, there are autonomous XGIS-GRB trigger conditions based on data rate and/or images. The figure on the right shows an overview of the I-DHU operation modes and transitions.

Once the trigger conditions are satisfied, the corresponding data is sent to the on-board data handling (OBDH), whose logic then selects the events to be followed–up by the IRT. These can be events that have been recognized to be unknown transients with SXI or events imaged with the XGIS that satisfy all the XGIS trigger conditions.

Each of the three instruments will have a dedicated I-DHU that will be its telecommand (TC), telemetry (TM) and power interface to the spacecraft. The mechanical and electrical design, operating system and basic software that is running on the Processor Board of the I-DHUs will be the same for all instruments. Additionally there will be an instrument-specific data processing software with, for example, the trigger algorithms and event detection codes. The I-DHU consists of two main boards that are mounted inside an aluminum case.

The main functions of the I-DHU on-board software are: instrument control, health monitoring and science data processing, formatting. At the heart of the I-DHU design is the Processor Board. It hosts the central CPU, the mass memory, time synchronization and distribution circuits and the house keeping (HK)/health monitoring acquisition chain.

The Processor Board will be developed by the IAAT in Tübingen, Germany. The Power Board within the I-DHU will be developed by the Centrum Badan Kosmicznych, Poland. It will generate the voltages for the Processor Board and distribute the power to the instrument.

National and International Collaborations

  • Istituto Nazionale di Astrofisica (INAF), Italy
  • DTU Space, Denmark
  • Centrum Badan Kosmicznych, Poland
  • Ferrara University, Italy
  • University of Leicester, UK
  • University of Geneva, Switzerland
  • CEA Saclay

Last Update 01/2019: Inga Saathoff, Chris Tenzer