Adv. Space Res. Vol. 13, No. 12. pp. (12)139—(12)148, 1993 0273—1177/93 $6.00 + 0.00 Printed in Great Britain. All rights reserved. Copyright © 1993 COSPAR SIGMA OBSERVATIONS OF HARD X-RAY AND SOFT GAMMA-RAY EMISSION FROM X-RAY BINARIES P. Laurent,* A. Claret,* F. Lebrun,* J. Pau1,’~ M. Dennis,** D. Barret,** L. Bouchet,** P. Mandrou,** R. Sunyaev,*** E. Churazov,*** M. Gilfanov,*** N. Khavenson,*** A. Dyachkov,*** B. Novikov,*** R. Kremnev*** and V. Kovtunenko*** * Service d’Astrophysique, CEN Saclay, 91191 G~f-sur-Yvette Cedex; France ** Centre d’Etude Spaliale des Rayonnements, 9 Avenue du Colonel Roche, BP 4346, 31029 Toulouse Cedex, France ~“ Space Research Institute, Profsoyouznaya 84/32, Moscow 117296, Russia ABSTRACT After more than two years of operation, the imaging v-ray SIGMA telescope has accumulated several days of observation toward well known X-ray binaries. Four bright sources falling in this category have been detected so far: The pulsar GX 1+4 near the center of our galaxy, the stellar wind accreting system 4U 1700-377, and the black hole candidates Cygnus X- 1 and OX 339-4. Moreover, SIGMA have observed three transients sources, which turned out to be also hard X-ray sources : The burster KS 1731-260, Tra X-1, and the Musca Nova. The properties of these systems in the SIGMA domain will be reviewed and a spectral distinction between black holes and neutron stars will be sketched. THE BRIGHT SOURCES The Neutron Stars The stellar wind accreting system 4U 1700-377. The 3.4 day eclipsing system 4U 1700-377 was discovered by Uhuru in 1970 /1/. The optical companion is the 07f star HD 153919 whose distance ranges from 1.2 to 2.2 kpc. The light curve is dominated by X-ray flaring activity /2/. The X-ray spectrum and the optical mass function show that the X-ray source is a neutron star, although no X-ray pulsations have been observed so far /3/. The SIGMA telescope /4,5,6/ has observed 4U 1700-377 in 1990-1991 /7/. Here are presented some of the main results obtained: The temporal variability of the source on timescales of minutes to hours is shown on Figure 1. Each bin represents the count rate of the experiment on all the detection surface during 120 s in the 40-74 KeV energy band versus the orbital phase of the source. Several bursts are visible, the stronger is at phase 0.24. Its luminosity has reached 3.8 x 10~’ergs/s during 20 minutes. It was the strongest burst ever observed from this source at hard X-ray energy. The flaring activity of the source makes the search for periodicities difficult. A search made on each observation when the source was visible in the images and when the flaring activity was not too important has been made by phase folding and fast Fourier transform methods. No significant periodicity has been found between 10 and 6000 s, nor at the previously reported periods of 67.4 s /8/, 24 q~in /9/, and 97 mm /10/. ,1A~1 13:12-K (12)139