Rapid spectral and timing variability of Be/X-ray binaries during type II outbursts

Abstract

X-ray colour–colour (CD) and colour–intensity (HID) diagrams are powerful tools that allow investigation of spectral variability without assuming any spectral model. These diagrams have been used extensively for low-mass X-ray binaries and black-hole candidates, but very few applications have been found for high-mass X-ray binaries. We investigated the spectral and timing variability of four accreting X-ray pulsars with Be-type companions during major X-ray outbursts. The aim is to define source states based on the properties (noise components) of the aperiodic variability in correlation with the position in the colour–colour diagram. Different spectral states were defined according to the value of the X-ray colours and flux. Transient Be/X-ray binaries exhibit two branches in their colour–colour and colour-intensity diagrams: i) the horizontal branch corresponds to a low-intensity state and shows the highest fractional rms, similar to the the island state in atolls and horizontal branch in Z sources; ii) the diagonal branch corresponds to a high-intensity state, in which the source stays for about 75% of the total duration of the outburst. Despite the complexity of the power spectra due to the peaks of the pulse period and its harmonics, the aperiodic variability of Be/X-ray binaries can be described with a relatively low number of Lorentzian components. Some of these components can be associated with the same type of noise as seen in low-mass X-ray binaries, although the characteristic frequencies are about one order of magnitude lower. The analysis of the CD/HID and power spectra results in two different types of Be/X. While in 4U 0115+63, KS 1947+300 and EXO 2030+375 the hard colour decreases as the count rate decreases, it increases in V0332+53. The pattern traced by V0332+53 then results in a Z-shaped track, similar to the low-mass Z sources, without the flaring branch. In contrast, the horizontal branch in 4U 0115+63, KS 1947+300 and EXO 2030+375 corresponds to a low/soft state, which is not seen in other types of X-ray binaries. The noise at very low frequencies follows a power law in V0332+53 (like in LMXB Z), and it is flat-topped in 4U 0115+63, KS 1947+300, and EXO 2030+375 (like in LMXB atoll). V0332+53 shows a noise component coupled with the periodic variability that it is not seen in any of the other three sources.