Time Variation of X-Rays from GX 5–1

Abstract

Long- and short-term variations of X-rays from GX 5–1 (a neutron-star X-ray binary) were investigated based on the Eastern energy spectral model: a disk-blackbody component plus a blackbody component model. In the horizontal branch (HB), an increase in the X-ray intensity is mainly due to the disk component; this increase is mainly due to an increase in Rin (the inner radius of the accretion disk), although Tin (the temperature of the inner edge of the accretion disk) decreases slightly. A fraction of the blackbody component decreases with the X-ray intensity in HB. In the normal branch (NB), an increase in the X-ray intensity is due to both components. The hardening of the X-ray energy spectrum with the X-ray intensity in NB is mainly due to an increase in the blackbody component: the fraction of the blackbody component increases with the X-ray intensity. The X-ray intensities of the disk component and the blackbody component show different long-term variations from each other, which are similar to the case of the disk component and the power-law component of black-hole candidate X-ray binaries (BHCXBs); GX 339–4 and GS 1124–683. It is possible that the behavior of the Z source from HB to NB is due to hysteretic behavior, such as that shown by the BHCXBs. Short-term variations of X-rays from GX 5–1 were investigated using normalized power spectral density functions (NPSDs), and were compared with those of a BHCXB, GS 1124–683. NPSD of the disk component has a power-law shape, and has an energy dependence: the higher is the X-ray energy, the larger is the variation. NPSD of the blackbody component has a flat-top shape, and has no energy dependence. These are the same as that of the disk component and the power-law energy spectral component of GS 1124–683, in spite of their different energy spectra and different category of X-ray stars. The very-low-frequency noise (VLFN) of GX 5–1 has minimum values in the transition part between HB and NB, where the total X-ray intensity has its maximum value. VLFN seems to be related to a variation of the mass-accretion rate.