XTE J1748 - 288 is a new X-ray transient with a one-sided radio jet .
It was observed with ASCA on 1998/09/06 and 1998/09/26 , 100 days after the onset of the radio-X-ray outburst .
The spectra were fitted with an attenuated power-law model , and the 2–6-keV flux was 4.6 ^ { +1.0 } _ { -0.8 } \times 10 ^ { -11 } erg s ^ { -1 } cm ^ { -2 } and 2.2 ^ { +0.8 } _ { -0.6 } \times 10 ^ { -12 } erg s ^ { -1 } cm ^ { -2 } on 09/06 and 09/26 , respectively .
The light curve showed 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 had been observed with ASCA on 1993/10/01 and 1994/09/22 , years before the discovery .
In those period , the flux was \stackrel { < } { \sim } 10 ^ { -13 } erg s ^ { -1 } cm ^ { -2 } , below ASCA ’ s detection limit .
The jet blob colliding to the environmental matter was supposedly not the X-ray source , although the emission mechanism has not been determined .
A possible detection of a K line from highly ionized iron is discussed .