Orbital Periodicities in the Hard Emission from X-ray Transient - Black Hole Candidates Between Outbursts

Abstract

During long-term observations of the Galactic Centre region in hard X-rays (10-300 keV) in space experiments made on board Prognoz-9 satellite and "Mir" orbital station (GRIF experiment) some periodic sources were revealed. They include periodicities of hour and day range of period: 152 h, 98 h, 82 h (4U1700-37), 69 h, 62 h, 13.3 h, 9.36 h, 8.03h, 8.15 h (Cen X-4), 4.38 h, 4.35h (4U1755-33) and 3.45 h. For all the observed periodic processes the mean phase profiles (light curves) in the different energy ranges were obtained. The mean phase profiles of most of the observed periodic sources differ from both sine and purely eclipse form. Among six sources with day ranges of period at least three were identified with X-ray Novas - black hole candidates: 152 h (H1705-25, Nov.Oph., 1977), 62 h (GRO J 1655-40, Nov. Sco., 1994), 13.3 h (4U1543-47). The results of the GRO J 1655-40 and 4U1543-47 observations in the Prognoz-9 and GRIF "Mir" experiments were compared with the observational data obtained in space observatories CGRO,RXTE, BeppoSax. It should be noted that the orbital periodicity is revealed in the hard emission of the X-ray transient - black hole candidates GRO J 1655-40 and 4U1543-47 even in the epochs between outbursts. Periodic processes with day ranges of periods can be typical for the transient source like X-ray Nova - black-hole candidate. Such periodicity is orbital but its origin is not connected with eclipses of the compact companion of a binary system. Periodic processes in soft and hard X-rays can be associated with orbital motion of binary system companions, but the physical mechanisms and the regions of generation of soft and hard X-rays can be quite different.