Einstein Probe carries a wide-field X-ray monitor WXT and a narrow-field follow-up X-ray telescope FXT. It also has fast downlink telemetry and ToO capability. Once a transient source is detected by WXT, it will be targeted with FXT by slewing the whole satellite for quick follow-up observations in order to measure the X-ray lightcurve and spectrum（as shown in Figure 1）. The transient information is quickly transmitted to the operation center to trigger alerts within a minute or so. The sketch of the mission operation and communication is shown in Figure 2 below. The operational mission life is designed to be three years, which may be extendable to five years.
An orbit with an altitude of about 600 km and an inclination angle less than 30 degree seems to meet the requirements and constraints as listed in Table 1 . This orbit is justified by preliminary analyses of thermal environment, sky coverage and solar power input. A later choice of the orbit has to take fully into account the probability of impact by space debris and micro-meteorites which may endanger the WXT gas counter windows and cause leakage of the gas. The proposed orbit is illustrated in Figure 3.
Table 1 Requirements and constraints on the orbit
|Space charged particles||Avoid radiation belt||Less than 600 km||< 35 degree|
|Lifetime||>= 5 years||>=500-600 km|
|Real-time alert downlink||VHF network coverage or Chinese relay satellites||Low altitude orbit||<=30 degree|
|Proposed launching site||Xichang, Sichuan province, China||>=28 degree|
Figure 3 Illustration of the proposed orbit for EP
There are five operation modes for the EP satellite, as shown in Figure4, including survey mode, follow-up mode, ToO mode, safe mode and calibration mode. The first three modes are explained here.
1) Survey mode: The satellite is slewed to desired directions to bring WXT centering at pre-scheduled fields in the sky, where pointed observations are performed for a fixed period of exposure time (~11minutes in the current design). Upon completion of the exposure the satellite is slewed to the next sky region, and such cycles repeats until the survey mode is interrupted by other modes.
2) Follow-up mode: when a transient is detected by WXT, and is classified and triggered by the processing and alerting system onboard, the satellite is slewed to the position of the source so that it is centered in the FXT FoV. A pointed follow-up observation is performed with FXT for a pre-designed exposure time. This mode is triggered automatically and can be interrupted by new alerts triggered for transient events of a higher priority. In the course of a follow-up mode, WXT continues monitoring observations targeting the sky region centering the position of the transient.
3) Target of opportunity mode: The satellite can be slewed to desired positions to perform pointed observations with FXT on objects triggered as ToO by uplink command. This mode is triggered by ground commands.
Figure 4 Operation modes of EP
WXT and FXT are always pointed to directions away from the Sun with an avoidance angle to the edge of the WXT FoV larger than 90 degrees. In this way, for any given position of the Sun in the sky, only about half of the entire sky—the anti-solar direction—can be monitored. Monitoring the night sky makes it possible to perform follow-up observations of detected transients by ground-based optical/IR telescopes.
In the current design of the observing strategy, there are five consecutive pointings within one orbit, as shown in Figure 5. Each of the pointings lasts for about 11minutes, with ~3 minutes slew time in between. There is an observation time loss of ~27minutes in the orbital daytime, during which the satellite is recharged with its solar panels.
Of three consecutive orbits, one has five pointing directions lying within the orbital plane (sequence of pointings: B-C-D-E-F), while the other two have three pointing directions 60 degrees offset from the orbital plane (B-C1-D1-E1-F). Within three consecutive orbits, half of the entire sky (the dark side) can be covered by the WXT FoV. The pointing directions are shifted by about 1 degree per day to compensate the daily movement of the Sun on the sky. In this way, the entire sky can be covered within half a year.
Figure 5 Illustration of the pointing directions of WXT in one orbit
Upon detection of transients, alerts will be triggered and the positional and flux information data will be sent down to the ground operation center. Currently two possibilities of the fast downlink are under investigation: the French VHF system and text message system via the Chinese satellites network. The science and house-keeping data will be downlinked regularly in X-band everyday with an estimated data rate 5.8Gbit per day.
Given the low weight (150kg) and power consumption (200W) of the payload, the payload can be accommodated with the established platforms of micro- or small-satellite in China. The detailed accommodation studies are currently being carried out independently by two Chinese satellite agencies, the MicroSAT institute of the CAS and CAST (Chinese Administration of Space Technology). The satellite will be three-axis stabilized with an attitude control stability: 0.001°/s (3σ) and a slewing speed 20 degree in 1 minutes. Both, sole/primary passenger and piggyback launches are being considered. A range of launchers, such as the Long-March series CZ-2D, CZ-2C, and CZ-6, may be chosen.