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The spectroscopy focusing array is one of the four main scientific instruments of the enhanced X-ray Timing and Polarimetry mission, tasked with spectral and timing observation in the energy range 0.5-10 keV. An engineering model of the spectroscopy focusing array with a 4 mirror shells assembly and a focal plane detector using commercial detectors has now been developed. To evaluate the performance, the spectral and timing calibration of the engineering model has been held in the 100-m X-ray Test Facility. A multi-target X-ray source with multiple emission lines is used to calibrate the spectral performance. A timing X-ray source based on a grid controlled X-ray tube has been utilized for the timing calibration. The timing X-ray source can generate X-ray pulses to measure the response time distribution, and can also simulate the pulsar lightcurves to examine the detection ability for pulsars. The energy-channel relation and energy resolution are determined through spectral calibration. The energy resolution at 5.95 keV is 142 eV, now. According to the timing calibration, the mean response time of the engineering model is 1.55 μ s, the full width at half maximum of the response time distribution is 0.45 μ s, and the engineering model has sufficient ability to detect the profile of millisecond pulsars.

The Einstein Probe (EP) satellite is designed for X-ray time-domain astronomy. The Follow-up X-ray Telescope (FXT) is one of the scientific payloads onboard EP. It will mainly be used for the follow-up X-ray observation, and it will also be used for the sky survey and Target of Opportunity (ToO) observation. The focal plane detector of FXT provided by the Max Planck Institute for Extraterrestrial Physics (MPE) adopts a PNCCD sensor. For detector cooling, a helium pulse tube refrigerator is used, provided by the Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS), to keep the detector working at a temperature of −90 ± 0.5 °C. The PNCCD driving and data acquisition electronics are developed by the Institute of High Energy Physics (IHEP), CAS. To observe different celestial sources, we designed six filter wheel positions and three scientific operating modes for the PNCCD detector: the full-frame mode, the partial-window mode, and the timing mode. In the full-frame mode, the system frame rate is 20 frame/s and the energy resolution of the whole system reaches 92 eV @ 1.49 keV (FWHM). The frame rate of partial-window mode is 500 frame/s. In the timing mode, the time resolution is about 94 μs. This paper mainly introduces the design and test results of the focal plane camera.

We report the discovery of a peculiar X-ray transient, EP240408a, by Einstein Probe (EP) and follow-up studies made with EP , Swift , NICER , GROND, ATCA and other ground-based multiwavelength telescopes. The new transient was first detected with Wide-field X-ray Telescope (WXT) on board EP on April 8th, 2024, manifested in an intense yet brief X-ray flare lasting for 12 s. The flare reached a peak flux of 3.9 × 10 −9 erg cm −2 s −1 in 0.5–4 keV, ∼300 times brighter than the underlying X-ray emission detected throughout the observation. Rapid and more precise follow-up observations by EP /FXT, Swift and NICER confirmed the finding of this new transient. Its X-ray spectrum is non-thermal in 0.5–10 keV, with apower-law photon index varying within 1.8–2.5. The X-ray light curve shows a plateau lasting for ∼4 d, followed by a steep decay till becoming undetectable ∼10 d after the initial detection. Based on its temporal property and constraints from previous EP observations, an unusual timescale in the range of 7–23 d is found for EP240408a, which is intermediate between the commonly found fast and long-term transients. No counterparts have been found in optical and near-infrared, with the earliest observation at 17 h after the initial X-ray detection, suggestive of intrinsically weak emission in these bands. We demonstrate that the remarkable properties of EP240408a are inconsistent with any of the transient types known so far, by comparison with, in particular, jetted tidal disruption events, gamma-ray bursts, X-ray binaries and fast blue optical transients. The nature of EP240408a thus remains an enigma. We suggest that EP240408a may represent a new type of transients with intermediate timescales of the order of ∼10 d. The detection and follow-ups of more of such objects are essential for revealing their origin.

We report the discovery of a peculiar X-ray transient, EP240408a, by Einstein Probe (EP) and follow-up studies made with EP, Swift, NICER, GROND, ATCA and other ground-based multi-wavelength telescopes. The new transient was first detected with Wide-field X-ray Telescope (WXT) on board EP on April 8th, 2024, manifested in an intense yet brief X-ray flare lasting for 12 seconds. The flare reached a peak flux of 3.9x10^(-9) erg/cm2/s in 0.5-4 keV, about 300 times brighter than the underlying X-ray emission detected throughout the observation. Rapid and more precise follow-up observations by EP/FXT, Swift and NICER confirmed the finding of this new transient. Its X-ray spectrum is non-thermal in 0.5-10 keV, with a power-law photon index varying within 1.8-2.5. The X-ray light curve shows a plateau lasting for about 4 days, followed by a steep decay till becoming undetectable about 10 days after the initial detection. Based on its temporal property and constraints from previous EP observations, an unusual timescale in the range of 7-23 days is found for EP240408a, which is intermediate between the commonly found fast and long-term transients. No counterparts have been found in optical and near-infrared, with the earliest observation at 17 hours after the initial X-ray detection, suggestive of intrinsically weak emission in these bands. We demonstrate that the remarkable properties of EP240408a are inconsistent with any of the transient types known so far, by comparison with, in particular, jetted tidal disruption events, gamma-ray bursts, X-ray binaries and fast blue optical transients. The nature of EP240408a thus remains an enigma. We suggest that EP240408a may represent a new type of transients with intermediate timescales of the order of about 10 days. The detection and follow-ups of more of such objects are essential for revealing their origin.