Low State Properties of the Low-Mass X-Ray Binary X1608–522 Observed with Ginga

Abstract

The low-mass X-ray binary X1608–522 was observed with Ginga on three occasions from 1989 to 1991. The source was in the low state throughout the observations. The energy spectrum in the energy range of 2–60 keV can be approximated by a power-law function with a broad absorption-like structure. The structure can be represented either by partial absorption or by a reflection of the power-law spectrum. The ionization degree of the absorbing/reflecting medium increases when the source luminosity increases by a factor of about three, while the percentage of the absorbed/reflected component relative to the total intensity decreases. The power spectrum is characterized by a flat top below a certain frequency, while it decreases approximately as 1/f above that frequency. The turn-over frequency varies according to the source luminosity, and is in a range 0.2 to 1 Hz. Additional structures, which can be represented by a broad Lorentzian peak, also exist in the power spectrum. The physical state of the accretion disk is discussed in conjunction with the low state of black-hole candidates.