New insights into the Be/X-ray binary system MXB 0656-072

Abstract

The X-ray transient MXB 0656-072 is a poorly studied member of high-mass X-ray binaries. Based on the transient nature of the X-ray emission, the detection of pulsations, and the early-type companion, it has been classified as a Be X-ray binary (Be/XRB). However, the flaring activity covering a large fraction of a giant outburst is somehow peculiar. Our goal is to investigate the multiwavelength variability of the high-mass X-ray binary MXB 0656-072. We carried out optical spectroscopy and analysed all RXTE archive data, performing a detailed X-ray-colour, spectral, and timing analysis of both normal (type-I) and giant (type-II) outbursts from MXB 0656-072 This is the first detailed analysis of the optical counterpart in the classification region (4000-5000 A). From the strength and ratio of the elements and ions, we derive an O9.5Ve spectral type, in agreement with previous classification. This confirms its Be nature. The characterisation of the Be/XRB system relies on Balmer lines in emission in the optical spectra, long-term X-ray variability, and the orbital period vs. spin period and EW(H\alpha) relation. The peculiar feature that distinguishes the type-II outburst is flaring activity, which occurs during the whole outburst peak, before a smoother decay. We interpret it in terms of magneto-hydrodynamic instability. Colour and spectral analysis reveal a hardening of the spectrum as the flux increases. We explored the aperiodic X-ray variability of the system for the first time, finding a correlation of the central frequency and rms of the main timing component with luminosity, which extends up to a "saturation" flux of 1E-8 erg/cm^2/s. A correlation between timing and spectral parameters was also found, pointing to an interconnection between the two physical regions responsible for both phenomenologies.