A Wide Field Monitor (WFM) for the new generation X-ray missions eXTP (China) and STROBE-X (NASA)

We present the recent development of a Wide Field Monitor (WFM) in the X-ray range from 2 to 50 keV, with about 4 sr field of view capability. Such instrument based on European technology meets the challenge to be part of future X-ray spectral timing space missions currently under study, such as the Chinese enhanced X-ray Timing and Polarimetry mission (eXTP) and the NASA Probe mission ``Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays" (STROBE-X). The WFM design is inherited from the Large Observatory For X-ray Timing (LOFT) mission concept which was proposed and selected by ESA in 2011 as an M3 mission candidate. The WFM is a set of coded mask cameras with solid state-class energy resolution, thanks to the use of Silicon Drift Detectors (SDDs). Since SDDs provide accurate event position measurements in one direction and only coarse positional information along the other direction, pairs of two orthogonal cameras are used to obtain precise 2D source positions. The useful effective field of view of one camera pair is about 28° x 28° (90° x 90° at zero response). A set of 3 or 4 camera pairs can be implemented to provide fully sky coverage. The working principle of the WFM is the classical sky encoding by coded masks, that has been widely used in space borne instruments (e.g. INTEGRAL, RXTE/ASM, Swift/BAT). The coded mask imaging is the most effective technique to observe simultaneously steradian-wide sky regions with arc min angular resolution.