Wide Field X-ray Monitor on board HETE

HETE (High Energy Transient Experiment) project was proposed to solve the questions on gamma-ray bursts (GRBs). One of its prime objectives is to localize GRBs precisely using UV, X-ray, and Gamma-ray instruments. The X-ray instrument named WXM (Wide-field X-ray Monitor) consists of one-dimensional coded apertures in X and Y directions, four identical one-dimensional position-sensitive proportional counters (PSPCs), and electronics. WXM is watching the sky of about 1 steradian and is sensitive from 2 to 25keV. If the X-ray positions on the detectors are binned into X/Y one-dimensional histgrams with a bin size of 2mm (which is the mask element size), the subsequent cross-correlation between the mask pattern and the histogram will give the position resolution of 36' in the sky. This value corresponds to the angle of a single mask element at the distance between the mask and the detector. By reconstructing the one-dimensional histogram of the sky with a bin size of 36'2 (=2mm 2), we found that WXM is expected to detect about 8 GRBs per year in the first 2 s of the bursts onboard and about 40 GRBs by the ground analysis with an accuracy better than 18 arcmin. This characteristic allows ground-based observatories to study GRBs at wide wavelength almost in a real time.