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 (~0.5 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 2 lobster-eye telescopes units, covering a total field of view (FOV) of ~0.5sr 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. |
On-board Instruments
Following the scientific requirements described before, The baseline Instrument suite configuration of THEUS payload includes four lobster-eye modules (F=300mm), a 70cm class IR telescope and two hard X-ray / soft gamma-ray detectors based on Si+CsI(Tl) coupling technology covering the total FOV of the lobster-eye modules. In summary, the scientific payload of THESEUS will be composed by:
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Soft X-ray Imager (SXI): a set of 2 « Lobster-Eye » X-ray (0.3 - 5 keV) telescopes covering a total FOV of 0.5sr field with 0.5-2 arcmin source location accuracy, provided by a UK led consortium (+Belgium and Czech republic);
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InfraRed Telescope (IRT): a 70 cm class near-infrared (0.7 - 1.8 microns) telescope with imaging and moderate spectral capabilities provided by a France led consortium (+ ESA and Switzerland);
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X-Gamma-rays Imaging Spectrometer (XGIS): spectrometer based on 4 detection units based on SDD+CsI(Tl) modules (2 keV – 20 MeV) , covering twice the FOV of the SXI. This instrument will be provided by an Italian led consortium (+Spain, Denmark and Poland);
All the Instrument will be equipped with an Instrument Data Handling Unit (I-DHU) interfacing each of the three instruments with the spacecraft (provided by a German led consortium).
The Soft X-Ray Imager (SXI)
The THESEUS Soft X-ray Imager (SXI) comprises 2 DU. Each DU is a wide field lobster eye telescope using the optical principle first described by Angel (1979) with an optical bench as shown in Figure 2. The optics aperture is formed by an array of 8x8 square pore Micro Channel Plates (MCPs). The MCPs are 40x40 mm2 and are mounted on a spherical frame with radius of curvature 600 mm (2 times the focal length of 300 mm). Table 1 summarizes SXI characteristics. The mass of each SXI camera is about 40 kg.
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| Figure 2. Optical elements of a SXI module. | Table 1. The SXI characteristics. |
The left-hand side of Figure 2 shows the SXI optic assembly. The front surface is spherical with radius of curvature 600 mm giving a focal length of 300 mm. The right-hand panels of Figure 3 shows a schematic of a single plate and a micrograph that reveals the square pore glass structure. The focal plane of each SXI module is a spherical surface of radius of curvature 600 mm situated a distance 300 mm (the focal length) from the optics aperture. The detectors for each module comprise a 2x4 array of CMOS detectors, where each device has an active area of about 40x80 mm2. The detectors are tilted to approximate to the spherical focal surface.
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| Figure 3. Left: The THESEUS SXI optic assembly. Right: MPO plate and micrograph of a typical plate showing the square packet channel structure | |
SXI sensitivity
The ensamble of 2 SXI modules has a total field of view of about 0.5 steradian.
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| Figure 4. The point spread function of the SXI obtained from a ray-tracing simulation at 1 keV. |
The point spread function is shown in Figure 4. The off-axis angle at which the cross arms go to zero is determined by the L/d ratio of the pores and for an optimum performance, we require L/d=60. The central true-focus spot has a FWHM of 5.5 arcminutes and all the true-focus flux is contained by a circular beam of diameter 14.7 arcminutes. Because the angular width of the optics MPO-array is larger than the CMOS-array the field of view is unvignetted at 1 keV and above so the collecting area is constant across the entire field of view. The sensitivity to transient sources as a function of integration time is described on the science requirements web-page.
The X-Gamma ray Imaging Spectrometer (XGIS)
The X-Gamma-ray Imaging Spectrometer (XGIS) comprises two units (telescopes). The two units are pointed at offset directions in such a way that their FOV partially overlap. Each unit (Figure 6) has imaging capabilities in the low energy band (2-150 keV) thanks to the combination of an opaque mask superimposed to a position sensitive detector and the usage of a passive shield placed on the mechanical structure between the mask and the detector plane. Furthermore the detector plane energy range is extended up to 20 MeV without imaging capabilities. The main performances of one XGIS unit are reported in Table 2.
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| Table 2. Top: XGS specifications. Bottom: XGIS unit characteristics vs energy range | ||||||||||||||||||||||||||||
The three elements composing each XGIS camera are:
- the detector assembly
- the mask assembly
- the collimator assembly
The detector assembly, or detection plane, of each unit (490x490 mm2) is made of 100 detector modules each one being a matrix of 8x8 detection elements capable of detecting photons in the 2 keV – 20 MeV energy range. For each energy loss in the module, whatever procured by EM radiation or ionizing particle, the energy released, the 3 spatial coordinates and the of the interaction and time of occurrence will be recorded. The detection elements (Figure 5 and 6) are made of a scintillating crystal bar 5x5x30 mm3 in size, covered at the top and bottom extremes with a Silicon Drift Detector (SDD) for the read-out of the scintillation light, while the other sides of the bar are wrapped with a light reflecting material convoying the scintillation light towards the PDs.The size of each SDD is 4.5x4.5x0.45 mm3.
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Figure 5. Left: Principle of operation of the XGS detection units: low-energy X-rays in teract in Silicon, higher energy photons interact in the scintillator, providing an energy range extended to three orders of magnitude. Right: sketch of one XGIS module. A module is made of an array of 8x8 scintillator bars with SDDs at both ends. Both the SDDs and scintillators are used as active detectors. The PDs readout electronics consist of an ASIC pre-amp mounted near each PD’s anode while the rest of the processing chain is placed at the module sides and bottom.
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| Figure 6. Left: detection plane of each XGIS camera. Right: mechanical design of one XGIS camera. |
The operating principle (see Figure 5 left) is the following. The top SDD-PD, facing the X-/gamma-ray entrance window, is operated both as X-ray detector for low energy X-ray photons interacting in Silicon and as a read-out system of the scintillation light resulting from X-/gamma-ray interactions in the scintillator. The bottom SDD-PD at the other extreme of the crystal bar operates only as a read-out system for the scintillations. The discrimination between energy losses in Si and CsI is based on the different shape of charge pulses.
The XGIS coded mask (mask assembly) will spatially modulate with transparent and non-transparent pixel elements the incoming X-ray radiation. The detection plane will detect this mask-modulated signal. The mask pattern (up to now, a random pattern is considered) shadows on the detector plane for a given X-ray source located within the XGIS FOV. The image reconstruction is based on a correlation procedure between the detected image and a decoding array from the mask pattern. The coded mask assembly envelope is 600x600 mm2 and will have a pattern allowing self-support in order to guarantee the maximum transparency of the open elements. The coded mask assembly includes the following parts:
- Mask code
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Mask support structure
- Upper grid
- Lower grid
- Frame
The mask code of each XGIS unit is made of tungsten of 1.0 mm thickness for the non-transparent pixels; the code is placed 630 mm above the detector plane. The mask detector plane distance is intended as the distance from the center of the mask to the top surface of the detector (detector reference plane). The ‘detector reference plane’ is 3.2 mm below the collimator assembly/detector assembly mechanical interface. The coded area of the mask is 561x561mm2 with a 10x10mm square pixel size. A random pattern and 50% open fraction has been considered for this preliminary design. The mask support structure provides mechanical support to the code as well as mechanical interface with the collimator assembly. This support structure has an Al upper grid and an Al lower grid that encapsulate the Tungsten pixels of the coded mask.
The collimator assembly has two main objectives: to mechanically connect the coded mask assembly with the detector assembly and to act as a lateral passive shield for the imaging system. The collimator assembly has two elements:
- Collimator
- Passive shielding
The collimator is made of Al alloy 1 mm thickness and the necessary stiffeners in order to provide enough strength and stiffness to the Al sheet. The collimator also accommodates the passive shielding (0.25mm W) of XGIS imaging system. The passive shielding provides the required opacity to shield the diffuse cosmic ray background. The passive shielding is made of four tungsten slabs all along the collimator height. The combination of the coded area with the collimator aperture in this geometry leads to a 77.0x77.0 deg2 (partially coded) FOV up to 150 keV.
XGIS sensitivity
The following plots show to the imaging sensitivity in the nominal energy ranges for the SDD (2-30 keV) and for the CsI (30-150 keV) detectors. All plots refer to a single XGIS unit. They are for an on-axis source, but in practice the sensitivity is nearly the same in the whole Fully Coded Field of View (i.e. the central region of about 10x10 degrees). For sources at the THESEUS boresight direction (aligned with the IRT) the data of the two units can be combined. This direction is at 20 deg off axis for each unit (where their effective area is 60% of the on-axis value). The combined sensitivity is similar to the on-axis sensitivity of a single unit. Therefore the following plots can be used, as a first approximation, also for the combined sensitivity at the boresight direction. For reference: 1 Crab = 10-8 erg cm-2 s-1 (2-30 keV) = 1.6 10-8 erg cm-2 s-1 (30-150 keV).
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Figure 7. Sensitivity plots for the XGIS. |
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The integrated sensitivity depends on the spectral shape of the source. The following figure shows the sensitivity as a function of the peak energy for GRBs described by a Band spectral function with representative slopes of long and short GRBs.
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| Figure 8. XGIS sensitivity as a function of the peak energy for GRBs |
The InfraRed Telescope (IRT)
The InfraRed Telescope (IRT) on board THESEUS is designed in order to identify, localize and study the transients and especially the afterglows of the GRBs detected by the Soft X-ray Imager (SXI) and the X and Gamma Imaging Spectrometer (XGIS). The telescope associated with the IRT is a Korsch type with a minimal entrance aperture of 0.7 m diameter. The focal plane instrument is composed by a spectral wheel and a filter wheel in which the ZYJH filters, a prism and a volume phase holographic (VPH) grating will be mounted, in order to provide the expected scientific product (imaging, low and high-resolution spectra of GRB afterglows and other transients). The IRT concept block diagram is given below.
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| Figure 9. Left: optical design of the IRT instrument and telescope. Right: concept block diagram of the IRT instrument. |
Specifications of the entire system are given in Table 3. The mechanical instrument support structure holding the internal mirrors, the electronics, the detectors, and the filter wheel is shown in Figure 10. The thermal hardware is composed by a pulse tube cooling the Detector and FEE electronics and a set of thermal straps extracting the heat from the electronic boxes and camera optics coupled to a radiator located on the spacecraft structure. The overall IRT mass is 51 kg and the total power supply is 81 W.
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| Table 3. IRT specifications |
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| Figure 10. Top: IRT instrument structure assembly. Bottom: Accommodation of the IRT camera and telescope on the satellite optical bench. |
IRT observational modes and sequence
The IRT instrument has 3 observation modes:
- The photometric mode for which spectral filters are used;
- The low-resolution (LR) mode for which a dispersive prism is used;
- The high-resolution (HR) mode for which a grism is used.
These components are placed in a filter wheel and are located in the entrance pupil of the instrument. When a gamma-ray burst is identified, the IRT will execure the following observational sequence:
- The IRT will observe in imaging mode as soon as the satellite is stabilized within 1 arc sec in the GRB error box. Three initial frames in the ZJH-bands will be taken (10 s each, goal 19 AB 5 σ sensitivity limit in H) to establish the astrometry and determine the detected sources colours.
- IRT will enter the spectroscopy mode (Low Resolution Spectra, LRS) for a total integration time of 5 minutes (3 σ sensitivity limit per resolution bin expected for an H 18.5 (AB) source).
- Sources with peculiar colours and/or variability (such as GRB afterglows) should have been pinpointed while the low-res spectra were obtained and IRT will take a deeper (20 mag sensitivity limit (AB)) H-band image for a total of 60s. These images will be then added/subtracted on board in order to identify bright variable sources with one of them possibly matching one of the peculiar colour ones. NIR catalogues will also be used in order to exclude known sources from the GRB candidates.
- In case a peculiar colour source or/and bright (< 17.5 H (AB)) variable source is found in the imaging sequence, the IRT computes its redshift (a numerical value if 6<z<10 or an upper limit z<6) from the low-resolution spectra obtained at point 1) and determines its position. Both the position and redshift estimate will be sent to ground for follow-up observations, by larger facilities. The derived position will then be used in order to ask the satellite to slew to it so that the source is places in the in the high-resolution part of the detector plane where the slit-less high-resolution mode spectra are acquired. Following the slew, the IRT enters the High-Resolution Spectra (HRS) mode where it shall acquire at least three spectra of the source (for a total exposure time of 1800s) covering the 0.8-1.6 μm range. Then it can go back to imaging mode (H-band) for at least another 1800s.
- In case that a faint (> 17.5 H (AB)) variable source is found, IRT computes its redshift from the low-resolution spectra, determines its position and sends both information to the ground. In this case too, the IRT asks for a slew to the platform in order to put the source on axis (optimizing the its sensitivity) and takes data in imaging mode for a 3600s time interval to establish the GRB photometric light curve (covering any possible flaring) and leading the light curve to be known with an accuracy of <5 %.
Note that if at point (2) the burst IR counterpart is not identified, the observation will be interrupted and the satellite will switch back to the main survey mode pointing direction.