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 100) 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 the soft X-rays (0.5 – 1 keV). 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 – 6 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 (e.g., 5-10°/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 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 – 6 keV): a set of 4 lobster-eye telescopes units, covering a total 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 should include: 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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