SIGMA-HARD X-RAY OBSERVATIONS OF THE BURST SOURCE MXB 1728-34

We report the first detection by the SIGMA telescope of hard X-ray emi ssion from the burst source 4U/MXB 1728-34 (also called GX 354-0). Thi s source has been monitored during the 1992 February-April survey of t he Galactic center region for a total of almost-equal-to 240 useful hr . It underwent two high-energy flares of about 10 days duration each. However, the MXB 1728 - 34 spectrum does not vary significantly as a f unction of the source intensity. The average spectrum is well describe d by a thermal bremsstrahlung model with a temperature of almost-equal -to 38 keV and a flux of almost-equal-to 7 x 10(-10) ergs cm-2 s-1 in the 30 200 keV band. The high-quality data of these observations provi de the best known hard X-ray spectrum of a bursting low-mass X-ray bin ary.