* Astronomy

Title: New insights into the Be/X-ray binary system MXB 0656-072
Authors: E. Nespoli, P. Reig, A. Zezas

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 carry out optical spectroscopy and analyse 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. From optical spectroscopy, we classify the optical counterpart as a O9.5Ve star, confirming its Be nature. From the study of type-I outbursts we unveil a ~100 days periodicity, most likely associated with the orbital period of the system. Balmer lines in emission in the optical spectra, long-term X-ray variability, and the spin period / orbital period relation, are fully consistent with the system being a Be/XRB. The peculiar feature that characterises 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. We explored for the first time the aperiodic X-ray variability of the system, finding a correlation of the central frequency and rms of the main timing component with luminosity, which extends up to a "saturation" flux of 1 x 10^-8 erg cm^2 s^-1 . A correlation between timing and spectral parameters was also found, pointing to an interconnection between the two physical regions responsible for both phenomenologies.

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