Correlated X-ray spectral and timing variability of the Be/X-ray binary V0332+53/BQ Camelopardalis during a type II outburst

Abstract

Context. After more than 15 years of quiescence the Be/X-ray binary V0332+53 underwent a giant outburst in December 2004. Aims. We have investigated the timing properties of the source in correlation with its spectral states as defined by different positions in the colour-colour diagram. Methods. We have used RXTE and INTEGRAL light curves to obtain colour-colour diagrams, power spectra and phase-lag spectra. The power spectra were fitted with a multi-Lorentzian function. Results. The source shows two distinct branches in the colour-colour diagram that resemble those of the Z sources. The hard branch (similar to the horizontal branch of Z sources) is characterised by a low-amplitude change of the hard colour compared to the change in the soft colour. In the soft branch (analogue to the normal branch) the amplitude of variability of the hard colour is about three times larger than that of the soft colour. As the count rate decreases the source moves up gradually through the soft to the hard branch. The aperiodic variability (excluding the pulse noise) consists of band-limited noise (represented by three broad Lorentzian components) and two QPOs at 0.05 Hz and 0.22 Hz. The strength of the lower frequency QPO increases as the source approaches the hard branch (similar to HBOs in Z sources). The higher frequency QPO reaches maximum significance when the source is in the middle of the branch (like NBOs). We have performed the first measurements of phase lags in the band limited noise below 8 Hz in an accreting X-ray pulsar and found that soft lags dominate at high frequencies. Above the pulse frequency (0.23 Hz), the amplitude of the lag increases as the X-ray flux increases. Conclusions.The Z topology appears to be a signature of the neutron star binaries as it is present in all types of neutron-star binaries (Z, atoll and, as we show here, in accreting pulsars as well). However, the motion along this track, the time scales through the different branches of the diagram and the aperiodic variability associated with portions of the Z track differ for each subclass of neutron-star binary.

Activity from the Be/X-ray binary system V0332+53 during its intermediate-luminosity outburst in 2008

“…It was accounted for with a Gaussian absorption line (gabs). The use of this model component and/or the obtained line parameters are consistent with previous studies Tsygankov et al 2006;Reig et al 2006;Reig & Nespoli 2013;Lutovinov et al 2015). The parameters were left free in the HEXTE data and tied to those in the XRT and PCA data.…”

“…The rms strength of this QPO also amounts to a few per cent in that work. The 0.22 Hz QPO was observed around the spin frequency in both the INTEGRAL (Mowlavi et al 2006) and RXTE data during the 2005 outburst decline Reig et al 2006). In addition, Takeshima et al (1994) discovered another QPO with a centroid frequency of ≈0.05 Hz and a relative rms amplitude of ≈5% in the 2.3−37.2 keV data from the 1989 outburst of V 0332+53.…”

High variability in Vela X-1: giant flares and off states

“…V 0332+53 shows an interesting evolution (Reig et al 2006). We derived a CCD for Vela X-1 but no interesting behavior was found.…”

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

“…A similar analysis to the one presented here has already been published in Reig et al (2006). For the sake of clarity and in order to apply the same reduction procedure to all sources, we have re-analysed the observations in Reig et al (2006).…”

“…For the sake of clarity and in order to apply the same reduction procedure to all sources, we have re-analysed the observations in Reig et al (2006). Note also that the present work includes data during the rise of the outburst that were not previously analysed and that the frequency range of the power spectra is two orders of magnitude larger than in Reig et al (2006).…”