Optical and X-ray observations of 4U 2129+47/V1727 CYG in an extended quiescent state.

Abstract

Observations of the 5.^h2 h X-ray binary 4U2129 + 47/V1727 Cyg for more than one orbital cycle on September 29 and October 4, 1983 with the ESA X-ray satellite EXOSAT failed to detect the source in the low and medium X-ray energy band. Quasi-simultaneous UBV photometry using the 2.2 m telescope at the German-Spanish Astronomical Center, Calar Alto, did not show the large amplitude light curve reported in earlier observations but showed the optical companion in a low intensity state. The large amplitude light curve has been interpreted as due to X-ray heating of the optical star by the X-ray source similar to the system Her X-1/HZ Her. Our optical observations indicate that the heating X-ray source has been shut off and this directly explains why EXOSAT did not detect the source. During optical follow up observations in September 1984 and December 1984 and June 1985 the source still remained in the low state. This behaviour resembles the optical quiescent state of Her X-1, seen for instance from 1949 to 1956 using archival plates.