Spectral variations in the low-mass X-ray binary 4U 0614+09

Abstract

We report the X-ray spectra of a binary 4U 0614+09 observed with EXOSAT on five different occasions in 1984 and 1986. In four observations in 1984, the source intensity was "low" compared to its intensity in the 1986 observation. The source was steady on timescales of 10-1000 s in each of the 1984 observations but highly variable during the 1986 variations. The best-fit spectral models for the continuum are a blackbody spectrum plus an emission from a Comptonized region. In addition, a component due to reflection from a cold disk around the source was needed as an absorbiton edge due to cold or ionized iron was seen near 7 keV with good significance in most of the spectra, particularly in the "low" state. The relation of the hardness ratios with intensity follow the behavior seen in the "Atoll" sources. We have studied the behavior of the hardness ratios with the variations in the parameters of the best-fit spectral models. Most of the variability is associated with the blackbody component thought to arise from the boundary layer on the surface of the neutron star. During the "high" state in 1986, the blackbody temperature is higher, its size smaller, and its contribution significantly greater than in the "low" state. The other notable difference between the two states is the lower opacity in the Comptonized region, and a significantly higher absorption in the "high" state. A weak line emission (equivalent width=10-40 eV) is detected during the "high" state. The line energy lies between 6 and 7 keV wh ich implies its origin from ionized iron.