The enhanced X-ray Timing and Polarimetry (eXTP) is a space science mission designed to study fundamental physics under extreme conditions of matter density, gravity, and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring the effects of quantum electro-dynamics, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, the eXTP mission is poised to become a leading observatory for time-domain and multi-messenger astronomy in the 2030s, as well as providing observations of unprecedented quality on a variety of galactic and extragalactic objects.

 

The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments for the first time, enabling simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range of 0.5-10 keV. In the new baseline design (Figure 1), the scientific payload of eXTP consists of two main instruments: the Spectroscopic Focusing Array (SFA) and the Polarimetry Focusing Array (PFA). The SFA comprises six identical X-ray focusing telescopes with five using silicon drift detectors (SDDs) as the focal plane detectors and one employing the pnCCD (charge coupled device) camera, referred to as SFA-T and SFA-I, respectively. The PFA features three identical telescopes equipped with Gas Pixel Detectors (GPDs) to supply polarimetry ability. As shown in Figure 2, the field of views of SFA-T, SFA-I, and PFA are designed as 18′ (diameter), 18′ × 18′, and 9.8′ × 9.8′, respectively.

 

• Energy range: 0.5-10 keV

• Payloads: SFA-T, SFA-I, and PFA 

• Features: large area spectroscopy, timing, and polarimetry

• Orbit: low Earth orbit, 20 deg inclination

• Telemetry limit: a 15 Crab source up to 300 min; no limit for a 1 Crab source

 

Figure 1. Illustration of the eXTP satellite.

 

 

Figure 2. Comparison between the FoVs of SFA-T (grey color), SFA-I (blue color) and PFA (blue color in the center) cameras. Circles denote the angular scale in 2′ increments from 2′ to 10′.

Optics

 

Nine X-ray grazing-incidence Wolter I (parabola + hyperbola) optics modules with focal length 5.25 m will be implemented onboard eXTP as shown in Figure 1. The mirror technique implemented is nickel replication, which has already been successfully used for high throughput X-ray telescopes with good angular resolution.

 

For SFA, the gold film is selected with 100 nm thickness as the reflective material considering that it exhibits stable reflectivity characteristics in the 0.2-10 keV energy range and the gold-plating process offers high maturity and reliability.

 

For PFA, nickel is directly employed as the reflective material to fulfill its requirements for effective area and quality factor.

 

Figure 1. The schematic structure of one of the SFA telescopes.

 

Silicon Drift Detector (SDD)

 

The focal plane detector of each SFA-T telescope consists of an SDD sensor arranged in 19 hexagonal cells (see Figure 2), with readout provided by custom ASICs, three of which are needed for each 19-cell SDD. The detector geometry ensures that the vast majority of source photons are focused onto the inner 7 cells of the array, whereas the cells of the outermost ring are used to accurately determine the background. Thus, the side length of a hexagonal cell has been determined to be 3.2 mm, given the expected angular resolution of the optics, a tradeoff between the number of pixels and some sampling of the mirror’s response. The sensor has a 450 µm sensitive thickness, which delivers excellent quantum efficiency throughout the SFA energy range. The signal processing chain of each SDD cell includes a charge-sensitive preamplifier, a fast shaper, and a slow shaper with sample-and-hold circuit. A field programmable gate array (FPGA) based back-end electronics (BEE) module is implemented to control the frontend electronics (FEE) and read the digitized data from the analog-to-digital converters (ADCs).

 

• Energy range: 0.5-10 keV

• Time resolution: 10 μs

 

Figure 2. The schematic drawing of the SDD.

 

P-N Junction Charge Coupled Device (pnCCD)

 

The SFA-I telescope employs a pnCCD as its focal plane detector, offering three specialized observation modes to accommodate diverse scientific objectives. The key technical specifications include a 28.8×28.8 mm² detector active area with 75×75 µm² pixels, where pileup constraints dictate mode selection for bright source observations. The SFA mirror module with the best angular resolution will be selected for SFA-I, to better match the ~3″ pixel size of the pnCCD camera. The full frame mode serves wide-field surveys and multiobject spectroscopy, while the windowed mode facilitates time resolved studies of medium-brightness sources and the timing mode specializes in millisecond variability analysis of bright transients as illustrated in Figure 3.

 

• Energy range: 0.5-10 keV

• Time resolution: 50 ms (full frame), 3 ms (windowed mode), 240 μs (timing mode)

 

Figure 3. Left: pnCCD windowed mode (61×128 pixels). Right: pnCCD timing mode (384×128 pixels fast readout).

 

Gas Pixel Detector (GPD)

 

The focal plane detector of each PFA telescope is GPD, which is a gas chamber sealed by a 50 µm thick Beryllium entrance window as shown in Figure 4. The working gas is pure dimethyl ether (DME), which absorbs the incident X-rays and converts them to photoelectrons. A drift field of about 2 kV cm^-1 is applied in the chamber to drive the secondary electrons ionized by the photoelectron to move towards the anode. To enable measurements with a sufficient signal-to-noise ratio, a gas electron multiplier (GEM) is mounted above the anode to multiply the number of electrons by a gain factor of a few hundred, which can be adjusted by the high voltage across the top and bottom layers of the GEM. The readout plane is mounted underneath the GEM, which is an ASIC chip pixelated with a pitch of 50 µm, responsible for collection and measurement of the charges after multiplication. The ASIC has a dimension 1.5 cm × 1.5 cm, which defines the sensitive region of the detector. The readout noise is around 50 e^-. The BEE is designed to control and operate the ASIC, drive the analog-to-digital conversion, organize and store the data, and communicate with the satellite.

 

• Energy range: 2-8 keV

• Time resolution: 10 μs

• Minimum detectable polarization: 3% (1 mCrab, 1Ms)

 

Figure 4. A schematic drawing of the GPD.