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We have performed a timing and spectral analysis of the X-ray pulsar SWIFTJ1626.6-5156 during a major X-ray outburst in order to unveil its nature andinvestigate its flaring activity. Epoch- and pulse-folding techniques were usedto derive the spin period. Time-average and pulse-phase spectroscopy wereemployed to study the spectral variability in the flare and out-of-flare statesand energy variations with pulse phase. Power spectra were obtained toinvestigate the periodic and aperiodic variability. Two large flares, with aduration of ~450 seconds were observed on 24 and 25 December 2005. During theflares, the X-ray intensity increased by a factor of 3.5, while thepeak-to-peak pulsed amplitude increased from 45% to 70%. A third, smaller flareof duration ~180 s was observed on 27 December 2005. The flares seen in SWIFTJ1626.6-5156 constitute the shortest events of this kind ever reported in ahigh-mass X-ray binary. In addition to the flaring activity, strong X-raypulsations with Pspin=15.3714+-0.0003 s characterise the X-ray emission inSWIFT J1626.6-5156. After the major outburst, the light curve exhibits stronglong-term variations modulated with a 45-day period. We relate this modulationto the orbital period of the system or to a harmonic. Power density spectrashow, in addition to the harmonic components of the pulsation, strongband-limited noise with an integrated 0.01-100 Hz fractional rms of around 40%that increased to 64% during the flares. A weak QPO (fractional rms 4.7%) withcharacteristic frequency of 1 Hz was detected in the non-flare emission. Thetiming (short X-ray pulsations, long orbital period) and spectral (power-lawwith cut off energy and neutral iron line) properties of SWIFT J1626.6-5156 arecharacteristic of Be/X-ray binaries.

Bright flares from the X-ray pulsar SWIFT J1626.6-5156