ASCA 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 o bserved with ASCA on 1998 September 6 and 26, 100 days after the onset of t he 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, resp ectively. 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 f rom 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 t he 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 colli ding with the environmental matter is probably not the X-ray source, althou gh the emission mechanism is not determined. A possible detection of a K li ne from highly ionized iron is discussed.