The primary science objectives are: (1) Discover and characterize cosmic X-ray transients, particularly faint, distant and rare X-ray transients, in large numbers. (2) Discover and characterize X-ray outbursts from otherwise normally dormant black holes.  (3) Search for X-ray sources associated with gravitational-wave events and precisely locate them. These populations of cosmic high-energy transients will be characterized over wide time-scales and at high cadences, revealing new insights into a diverse set of systems including dormant black holes, neutron stars, supernova shock breakouts, active galactic nuclei, X-ray binaries, gamma-ray bursts, stellar coronal activity, and electromagnetic-wave sources and gravitational-wave events. Meanwhile, EP will also monitor the variability of various types of X-ray sources in large samples all over the sky. In light of the multi-messenger and multi-wavelength all-sky monitoring capability highly anticipated in the next decade, EP will produce a legacy data set in the X-ray band that are key to characterizing and understanding the nature of cosmic transients and variables, by working in synergy with the sky surveys by other facilities.

The main scientific characteristics and source requirements of this instrument resulting from the design described above are summarized in Table 1. 

                      Table 1. The performance and requirements of FXT.

FXT is a Wolter-I telescope operating in the 0.5-10 keV energy range. It has a narrow field of view (60 arcmin in diameter) and a source localization error of 5-15 arcsec (90% c.l.) depending on the source strength. The FXT is responsible for the quick follow-up observations (within 5 minutes) of the triggered sources from WXT, and will also observe other interested targets during the all sky survey at the rest time.

The design of FXT is shown in Figure 1. The FXT detector is made of two pnCCD modules (the image size 28.8mm × 28.8mm, from MPE). The two pnCCD modules are cooled (-110~-80℃) by two helium pulse tube refrigerators, respectively. The mirror module comprises of 54 nested gold coated nickel shells of Wolter-I type, to be fabricated by Media Lario in Italy The half energy width (HEW) is about 30 arcsec (on axis) at energy of 1keV. The temperature of the mirror module is controlled by several heaters and thermal filters. The X-ray baffle at the entrance of mirror can protect against the X-ray stray-light from single reflections in the Wolter-I type mirror.

The electronic box under the detector performs clock generation, data acquisition, voltage supply and mode switch for FXT.

The scientific observation modes of FXT camera include the Full-Frame Mode (FF), the Partial-Window Mode (PW), and the Timing Mode (TM). The PW and TM are similar to those of XMM-Newton PN. However, the PW or TM modes are newly developed for FXT pnCCD . FF is the normal mode. The advantage of TM is the good time resolution, and the events are readout continuously without position information. PW is a special mode, in which only a small imaging area is active so as to achieve good time resolution. These three operation modes of FXT could be chosen through sending orders from ground.

In front of the pnCCD module, there is a filter wheel with 6 options. Besides the open position, FXT has two kinds of filters mounted on the filter wheel: medium filter, thin filter (Figure 2). The hole position, calibration source position and closed position could be applied in some special scenarios.