Wide-Field MAXI (WF-MAXI: Wide-Field Monitor of All-sky X-ray Image) is a proposed mission to detect and localize X-ray transients including electro-magnetic counterparts of gravitational-wave events such as gamma-ray bursts and supernovae etc., whic
h are expected to be directly detected for the first time in late 2010s by the next generation gravitational telescopes such as Advanced LIGO and KAGRA. The most distinguishing characteristics of WF-MAXI are a wide energy range from 0.7 keV to 1 MeV and a large field of view (~25 % of the entire sky), which are realized by two main instruments: (i) Soft X-ray Large Solid Angle Camera (SLC) which consists of four pairs of crisscross coded aperture cameras using CCDs as one-dimensional fast-readout detectors covering 0.7 - 12 keV and (ii) Hard X-ray Monitor (HXM) which is a multi-channel array of crystal scintillators coupled with avalanche photo-diodes covering 20 keV - 1 MeV.
Using a pulse-fit method, we investigate the spectral lags between the traditional gamma-ray band (50-400 keV) and the X-ray band (6-25 keV) for 8 GRBs with known redshifts (GRB 010921, GRB 020124, GRB 020127, GRB 021211, GRB 030528, GRB 040924, GRB
041006, GRB 050408) detected with the WXM and FREGATE instruments aboard the HETE-2 satellite. We find several relations for the individual GRB pulses between the spectral lag and other observables, such as the luminosity, pulse duration, and peak energy (Epeak). The obtained results are consistent with those for BATSE, indicating that the BATSE correlations are still valid at lower energies (6-25 keV). Furthermore, we find that the photon energy dependence for the spectral lags can reconcile the simple curvature effect model. We discuss the implication of these results from various points of view.
