2 Wide-field X-ray Monitor (WXM)

The WXM consists of two identical units of one-dimensional position sensitive X-ray detectors (Fig. 1). They are placed in orthogonal directions to each other for measuring the X and Y direction independently. One unit consists of a one-dimensional coded mask and two 1-D position-sensitive proportional counters (PSPCs) placed 187 mm below the mask. The coded mask is made of a plate of aluminum (0.5 mm thickness) and gold (0.025 mm) with a series of slits of randomly varying width (random mask). The location of the GRB is determined by measuring a set of two shift distances of the mask pattern in the X and Y direction. The mask pattern of WXM is selected from 100000 random patterns with the same open fraction and element size based on localization accuracy for GRBs.

Each PSPC has three carbon fiber anode wires with 10 $\mu$m diameter in its upper cells and four tungsten wires in its lower cells, and filled with 1.4 atm Xenon gas with 3% CO₂ as quenching gas. The three upper cells are used for X-ray detection. They have a depth of 25.5 mm and a width of $\sim$27 mm, and are separated by the cathode wires placed at intervals of 3.4 mm. The four lower cells are used for rejecting charged particle events by anti-coincidence, and have a depth of 11.5 mm. The detection area of $120\times83.5$ mm² at the top side is sealed by 100 $\mu$m thick Be windows.

The position of an incoming X-ray is determined by the charge-division method. That is, L/(L+R) has a linear relation with the incident position, and L+R measures photon energy. Here, L and R represent pulse height values measured at left and right ends respectively.

The proportional counters and electronics are provided by RIKEN and Miyazaki University, while the coded aperture and on-board localization software is provided by Los Alamos National Laboratory.

  

Figure 1: Schematic view of the WXM: A set of $X{\rm A}+X{\rm B}$ ($Y{\rm A}+Y{\rm B}$)counters and a coded mask determine $\theta_{x}$ ($\theta_{y}$) that is a zenith angle of the GRB direction on the plane perpendicular to the corresponding mask pattern