Research Note

Broad-band multicolour observations of the symbiotic binary Z And during quiescence and its activity at the end of 2002 ^*

¹ Institute of Astronomy, National Astronomical Observatory Rozhen, PO Box 136, 4700 Smolyan, Bulgaria e-mail: tomov@astro.bas.bg
² Sternberg Astronomical Institute, Moscow State University, Universitetski pr. 13, Moscow, 119899 Russia

Received: 9 April 2004
Accepted: 28 August 2004

Abstract

Broad-band photometry of the classical symbiotic binary system Z And obtained at the time of the light maximum of its small-amplitude brightening at the end of 2002 as well as at four epochs before and after this brightening is analysed. The parameters of the symbiotic nebula were obtained from its spectral energy distribution at the epochs of the quiescent state of the system using the energy distribution of the stellar components derived in a previous investigation. The continuum analysis based on the data during the active phase shows that the energy distribution of the compact secondary was changed very little – it remained a hot object emitting mostly in the UV, as in the quiescent state. On the basis of this result the brightening is supposed to be due to a slow expansion to about two times its quiescent size. The possibility of hydrogen burning at the surface of the secondary during quiescence is discussed.