[ITAL]ASCA[/ITAL] Observations of the Jet Source XTE J1748−288

XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was observed with ASCA on 1998 September 6 and 26, 100 days after the onset of the radio X-ray outburst. The spectra were modeled with an attenuated power-law model, and the absorbed 2-6 keV flux was 4.6(-0.8)(+1.0) x 10(-11) and 2.2(0.6)(+0.8) x 10(-12) ergs s(-1) cm(-2) on 1998 September 6 and 26, respectively. The light curve shows that the steady exponential decay, with an e-folding time of 14 days, lasted over 100 days and 4 orders of magnitude from the peak of the outburst. The celestial region including the source was observed with ASCA on 1993 October 1 and 1994 September 22, years before the discovery. During those periods, the flux was less than or similar to 10(-13) ergs s(-1) cm(-2), below the ASCA detection limit. The jet blob colliding with the environmental matter is probably not the X-ray source, although the emission mechanism is not determined. A possible detection of a K line from highly ionized iron is discussed.