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THESEUS Mission Payload and Profile

The scientific goals which come from a full exploration of the early Universe requires the detection of a factor ten more GRBs (about 50-70) in the first billion years of the Universe (z > 6), in the 3 years prime mission life time of THESEUS. Such a requirement is well beyond the capabilities of current and near future GRB detectors (Swift/BAT, the most sensitive one, has detected only very few GRBs above z = 6 in 10 years). As supported by intensive simulations performed by us and other works in the literature, the needed substantial increase of high-z GRBs requires both an increase of ~1 order of magnitude in sensitivity and an extension of the detector passband down to 0.3 keV (soft X-rays). Such capabilities must be provided over a broad field of view (~1 sr) with a source location accuracy < 2’ , in order to allow efficient counterpart detection, on-board spectroscopy and redshift measurement and optical and IR follow-up observations.

Such performances can best be obtained by including in the payload a monitor based on the lobster-eye telescope technology, capable of focusing soft X-rays in the 0.3 – 5 keV energy band over a large FOV. Such instrumentation has been is under development for several years at the University of Leicester, has a high TRL level (e.g., BepiColombo) and can perform all-sky monitoring in the soft X-rays with an unprecedented combination of FOV, source location accuracy (<1-2’) and sensitivity, thus addressing both main science goals of the mission. An onboard infrared telescope of the 0.5-1m class is also needed, together with spacecraft fast slewing capability (< 6°/min), in order to provide prompt identification of the GRB optical/IR counterpart, refinement of the position down to ~arcsec precision (thus enabling follow-up with the largest ground and space observatories), on-board redshift determination and spectroscopy of the counterpart and of the host galaxy. The telescope may also be used for multiple observatory and survey science goals. Finally, the inclusion in the payload of a broad field of view hard X-ray/soft gamma-ray detection system covering the same monitoring FOV as the lobster-eye telescopes and extending the energy band from few keV up to several MeV will increase significantly the capabilities of the mission. As the lobster-eye telescopes can be triggered by several classes of transient phenomena (e.g., flare stars, X-ray bursts, etc), the hard X-ray detection system provides an efficient means to identify true GRBs and detect other transient sources (e.g., short GRBs). The joint data from the three instruments will characterize transients in terms of luminosity, spectra and timing properties over a broad energy band, thus getting fundamental insights into their physics. In summary, the foreseen payload of THESEUS includes the following instrumentation:

  • Soft X-ray Imager (SXI, 0.3 – 5 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 15’x15’ FOV, for fast response, with both imaging and spectroscopy capabilities;
  • X-Gamma rays Imaging Spectrometer (XGIS, 2 keV – 20 MeV): a set of 2 coded-mask cameras using monolithic X-gamma rays detectors based on bars of Silicon diodes coupled with CsI crystal scintillator, granting a ~2sr FOV and a source location accuracy of ~10 arcmin in the 2-150 keV, as well as a >4sr FoV at energies >150 keV.

Each of the three instruments will be equipped with a dedicated Instrument Data Handling Unit (I-DHU) that will serve as their TM/TC and power interface to the spacecraft, as well as:

  • collect, process and store the data stream of the respective instrument;
  • implement the burst trigger algorithm on SXI and XGIS data to identify the gamma-ray bursts and any other relevant transient event;
  • implement the IRT burst follow up observation. 

THESEUS will also be equipped with a Trigger Broadcasting Unit (TBU) to support the prompt transmission of the on-board identified transient event data to the ground within few tens of seconds. The solutions evaluated for an independent and promt burst position broadcast to ground are by the means of VHF on-board transmitters connected with a network of ground VHF receivers inherited from the SVOM mission.

The THESEUS expected spacecraft slewing capability is of at least 6°/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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Figure 1. Orbits traced in 1 day and location of the ground VHF receivers planned for THESEUS.