The eXTP Payload
The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV. Key elements of the payload are:
- the Spectroscopic Focusing Array (SFA): a set of 11 X-ray optics operating in the 0.5-10 keV energy band with a field-of-view (FoV) of 12 arcmin each and a total effective area of ∼0.9 m2 and 0.6 m2 at 2 keV and 6 keV respectively. The telescopes are equipped with Silicon Drift Detectors offering <180 eV spectral resolution.
- the Large Area Detector (LAD): a deployable set of 640 Silicon Drift Detectors, achieving a total effective area of ∼3.4 m2 between 6 and 10 keV. The operational energy range is 2-30 keV and the achievable spectral resolution better than 250 eV. This is a non-imaging instrument, with the FoV limited to <1° FWHM by the usage of compact capillary plates.
- the Polarimetry Focusing Array (PFA): a set of 2 X-ray telescope, achieving a total effective area of 250 cm2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters. The FoV of each telescope is 12 arcmin and the operating energy range is 2-10 keV.
- the Wide Field Monitor (WFM): a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, covering in total a FoV of 3.7 sr and operating in the energy range 2-50 keV.
The Spectroscopic Focusing Array
The SFA is an array of 11 identical X-ray telescopes covering the energy range 0.5-10 keV and featuring a total effective area larger than 0.6 m2 at 6 keV, and ∼0.9 m2 between 1 and 2 keV. For each telescope, the requirement on the angular resolution is better than 1′ (HEW) while the Field of View (FoV) is expected to be about 12′ (FWHM). The telescopes are based on slumped glass optics (SGO) technology and are currently developed at the Institute of Precision Optical Engineering (IPOE), of the Tongji University, in China. Different type of coatings - with and without multilayer - are being studied to maximize the total area at 6 keV. Different solutions, which meet the requirements of 0.6 m2 at 6 keV and 0.9 m2 between 1 and 2 keV, have been obtained. In the current baseline the “without multilayer” option, consisting of only several layers of carbon, nickel and platinum or iridium, has been adopted. According to the current configuration the telescope focal length is 4.5 m for an aperture diameter of 450 mm.
The baseline detector for the SFA is a 7 pixel Silicon Drift Detector (SDD). Studies on the SFA detector are led by the Max-Planck-Institut für extraterrestrische Physik, Garching (Germany). The pixel size is required to be smaller than 3′. The energy resolution (FWHM) will be better than 180 eV at 6 keV, while the time resolution of the instrument is 10μs. Currently a trade-off study is being performed to understand whether ASICs are a mandatory choice for the detector Front End Electronics or whether a solution based on discrete elements electronics can be implemented.
The Large Area Detector
The LAD onboard eXTP uses the same design and technology of the LOFT mission. The eXTP LAD effective area reaches ∼3.4 m2 at 6 keV. The nominal energy range of the LAD is 2 -30 keV, but the dynamic range can be extended up to 80 keV to record bright events (e.g., GRBs or magnetar flares) shining through the collimator. The energy resolution is 250 eV at 6 keV. The FoV is limited to <1° FWHM by compact capillary plate (CP) collimators. The absolute time accuracy is 1μs while the dead time is less than 1% at 1 Crab flux level. The LAD reaches a sensitivity of 0.01mCrab for an exposure time of 104 s.
The instrument consists of 40 modules of the same type developed for the LOFT mission. Each module in fact consists of a set of 4 x 4 detectors and 4 x 4 collimators, supported by two grid-like frames. The module hosts the read-out electronics, and the power supplies, organized in the Front-End Electronics (FEE) and Module Back-End Electronics (MBEE). The eXTP LAD modules are organized in large deployable panels, which host the Panel Back-end electronics (PBEE). The detectors are large-area SDDs originally developed for the ALICE/LHC experiment at CERN and optimized for the LOFT mission. The typical size is 11 x 7 cm2 and 450 μm thickness. Each detector is segmented in two halves with 112 channels each (970 μm pitch anodes). The rather compact capillary plate collimator is based on micro-channel plate technology. For the LAD, it consists of a 5-mm thick sheet of lead-glass (>40% Pb mass fraction) with the same size as the SDD detector, perforated by thousands of round micro-pores with 83μm diameter. The baseline CP collimator is based on Hamamatsu’s round-pore technology, offering large open area ratio (75%) and a large Pb mass fraction. Alternative design options are currently being studied in China by Night Vision Technology Co, as well as in Europe, by Photonis (France) Ltd. According to the current working scheme, the eXTP LAD will be procured by the institutions and countries already participating to the LOFT Consortium. The development of the instrument is led by IAPS-INAF (Italy) and MSSL (UK).
The Polarimetry Focusing array (PFA)
The Polarimetry Focusing Array consists of two identical telescopes with angular resolution better than 30′′ (with a goal of 15′′) and total effective area of about 250 cm2 at 2 keV. According to the actual baseline, optics are based on the Nickel technology, but a solution based on SGO is also being considered. The telescope features a focal length of 4.5 m with an aperture diameter of 450 mm. The Field of View is 12′.
The focal assembly consists of two identical photoelectric X-ray polarimeters based on the Gas Pixel Detector concept (GPD). The GPD is able to measure the linear polarization of photo-absorbed photons by reconstructing the emission direction of the ejected photoelectrons. The GPD comprises a gas cell with a 50 μm thick Beryllium entrance window, a Gas electron multiplier (GEM) and a pixelated charge collection plane, directly connected to the analog readout electronics. The GEM amplifies the charge of the electron tracks generated in the drift gap, without changing the track shape, and providing the energy and time information. Below the GEM, at less than a few hundreds micron, the top layer of the ASIC is covered by metal pads with a high filling factor distributed on a hexagonal pattern. Each pad is connected with an independent analog electronic channel. The ASIC, a development of INFN-Pisa, has 105600 pixels at 50 μm pitch, and it is at its third generation of development. A prototype of the detector has been recently assembled and tested at Tsinghua University following a design by the INFN-Pisa group. Thanks to an improved design a very careful manufacturing a good uniformity of the electric field has been obtained. The energy resolution (FWHM) is about 18% at 6 keV. The measured modulation factor very well agrees with the one predicted by simulations and reaches 0.6 above 6 keV. The systematic error for polarization measurement is less than 1% (at a confidence level of 99%). The energy range of the PFA is 2-10 keV, and the time resolution is 500 μs. The sensitivity is better than 1% MDP for 1 Crab flux with an exposure time of 104 s. The procurement of the PFA is led by the Chinese Team.
The Wide Field Monitor (WFM)
The WFM consists of three pairs of coded mask cameras covering 3.7 sr of the sky at any time with a sensitivity of 4 mCrab for an exposure time of 1 d in the 2-50 keV energy range. The sensitivity, combining 1 yr of observations, reaches 0.2 mCrab outside the Galactic plane. The effective FoV of each camera pair is ~70°x70° (90°x90° at zero response). The energy resolution is ~300 eV at 6 keV, and the absolute time accuracy is 1 μs.
The same SDDs as the LAD are implemented in the WFM but in a slightly modified geometry. In fact SDDs can provide accurate positions in one dimension and only rough position information along the second dimension. Therefore, pairs of two orthogonal cameras are combined to obtain rather precise 2D positions of the monitored sources. ASICs and Front End Electronics share a design similar to the one of the LAD-SDDs. To obtain the required position resolution, the WFM-SDD anode pitch is reduced to 145 μm (vs. 970 μm of the LAD SDDs). The required number of ASICs per SDD is higher (28x IDeF-X HD ASICs, with a smaller pitch than LAD, and 2x OWB-1 ASICs). The location accuracy is better than 1′, while the angular resolution is better than 5′. A 25 μm thick Beryllium window above each SDD protects against micrometeoroid impacts. The WFM uses the same BEE and PSU of the LAD, but with additional capability to determine photon positions. The ICU controls each of the six cameras independently and interfaces the PDHU, performing onboard computation to locate bright transient events in real time.
A schematic view of the three pairs of the WFM cameras.
|Cameras of each pairs are mounted orthogonally to obtain accurate 2D positions of the monitored sources.|