SIGMA OBSERVATION OF 2 QUASARS - 3C-273 AND 0241 + 62

The quasar 3C 273 has been observed with the imaging telescope SIGMA on board the GRAN AT satellite on four occasions during 1990 for a total exposure time of 62 hr. In the first two sessions in the summer of 1990 the source was detected, at the expected position, at 4 and 5.4-sigma confidence levels in the 40-120 keV energy band, while a marginal detection (less-than-or-equal-to 2.5-sigma) was obtained 4 months later. Comparison between observations indicates a marginally significant flux variation (3-sigma) of a factor of almost-equal-to 2 in a time scale as short as 41 days; this is the first suggestion of a short time variation at hard X-ray frequencies, and it agrees very well with similar measurements in soft X-rays. The combined data of the first two pointings are well fitted by a power law of photon index-alpha almost-equal-to 1.5 up to about 500 keV with no evidence for a break or steepening in the spectrum. The 2-sigma ul for a broad (40 keV width) positron annihilation line is 1.5 x 10(-3) photons cm-2 s-1. Comparison with previous high-energy measurements and soft X-ray data indicates that 3C 273 is probably characterized from 2 to 500 keV by a simple power-law shape which is significantly flatter than the canonical AGN shape and unusual among QSOs. Two sigma upper limits obtained by SIGMA at the location of the low-luminosity quasar 0241 + 62 suggest a softer X-ray spectrum for this object. We briefly discuss the implications of the SIGMA results with respect to astrophysical processes in quasars and their contribution to the cosmic diffuse back round CDB.