A spectral classification of QPO sources

Abstract

The spectral and power-spectral similarities between different modes of the QPO sources Cyg X-2 and Sco X-1 are emphasized. It is shown that low-mass X-ray binaries in general can be found in three different states, forming a Z-shaped spectral correlation diagram. There is a direct relation between the spectral states and three different QPO modes. The formation of the spectral modes is a problem that has to be solved simultaneously with the QPO models. The most interesting open question is, how the mass accretion rate changes along the spectral correlation diagram.