1 Introduction

Among nine GRB afterglows which displayed optical counterparts, the red-shift of the source is found from the optical spectroscopy for three GRBs, 970508, 971214 and 980703 (Metzger et al. 1997; Kulkarni et al. 1998; Djorgovski et al. 1998), and one, GRB 980329, was implied to be $z\sim5$ by Fruchter (1998) from its fluxes in K-R band. Another noteworthy GRB is GRB 980425 which is suggested to be associated with a SN Ic, SN1998bw, at z=0.0085 (Galama et al. 1998).

In contrast to these events, the burst of 28 August 1997 (GRB 970828) displayed an afterglow only in an X-ray band and accompanied no optical transient down to R=23.8 (Groot et al. 1998). This GRB was detected by ASM/RXTE on August 28.73931, 1997 UT and localized into a small region with $2'\times 5'$ accuracy (Remillard et al. 1997; Smith et al. 1997). A subsequent scan by the PCA/RXTE (Proportional Counter Array) found an X-ray afterglow of 0.5 mCrab (2-10 keV) about four hours after the burst detection (Marshall et al. 1997). Here we report the detection of an afterglow of GRB 970828, and its X-ray properties obtained with ASCA.