Simulations of a 2 x 1.5D coded aperture camera for X-ray astronomy

Abstract

The concept of two perpendicular one-dimensional coded aperture cameras, necessitated by the imaging capability of the detector, finds its application in the design of the Wide Field Monitor (WFM). This instrument has the future goal to monitor the variable X-ray sky for transient activity. Characteristic of each camera is a fine angular resolution in one direction (typically 5 arcmin) and a coarse one in the other (5 degrees). The coarse perpendicular resolution makes the camera so-called '1.5D'. The WFM has been studied for a number of space-borne X-ray observatory concepts: LOFT, eXTP, Strobe-X, ARCO and now LEM-X. We here report on a study of two decoding algorithms for this instrument and its imaging performance. Detector responses to the X-ray sky are simulated, including the signal processing by the front-end and back-end electronics. The decoding algorithms are the iterative removal of sources (IROS), in combination with cross correlation, and the maximum likelihood method (MLM). IROS is most suited for the determination of the point source configuration of the observed sky and MLM for the optimum determination of the source fluxes. (..) the WFM is a high performance monitoring instrument with straightforward and proven technology that enables the identification of new cosmic X-ray sources, for instance X-ray novae, gamma-ray bursts and electromagnetic counterparts to gravitational wave events from merging compact objects, and the detection of unusual and interesting behavior of persistent cosmic X-ray sources, such as accretion disk state changes.

Figures (14)

• Figure 1: WFM camera shown with the shielding being made slightly transparent. In purple, the 4 SDDs are depicted, on top the mask.
• Figure 2: Par example, a schematic of the WFM configuration foreseen for eXTP. The narrow-field instruments of eXTP are pointed upward, so WFM pair '0' covers the pointing of those instruments at the center of the field of view.
• Figure 3: WFM mask. The cyclic difference set is laid down starting from the bottom left, going first from left to right and then bottom to top.
• Figure 4: Flow diagram of IROS.
• Figure 5: PSF formation along the 'fine' direction. In the top section of the figure, the rays of an off-axis source illuminate the thick mask somewhat sideways and introduce a shadow on one side of the projection of the mask element on the detector (middle blue graph). When cross correlating with the decoding mask (moving the decoding mask from left to right over the detector, see middle blue graph), this introduces (bottom blue graph) 1) a flat top to the PSF which is off centered; 2) a lower PSF peak representative of less sensitive area and 3) a narrower PSF. The red dots indicate where the discrete cross correlation would sample the PSF if the sampling is equal to the mask element size. The sampled cross correlation looks like there is no shadowing but its integral is actually smaller.