SUZAKUOBSERVES WEAK FLARES FROM IGR J17391–3021 REPRESENTING A COMMON LOW-ACTIVITY STATE IN THIS SUPERGIANT FAST X-RAY TRANSIENT

We present an analysis of a 37 ks observation of the supergiant fast X-ray transient IGR J17391−3021 (= XTE J1739−302) gathered with Suzaku . The source evolved from quiescence to a low-activity level culminating in three weak flares lasting ∼3 ks each in which the peak luminosity is only a factor of five times that of the pre-flare luminosity. The minimum observed luminosity was 1.3 × 10 33 erg s −1 ( d /2.7 kpc) 2 in the 0.5–10 keV range. The weak flares are accompanied by significant changes in the spectral parameters including a column density ( N H =(4.1 +0.4 −0.5 ) × 10 22 cm −2 ) that is ∼2–9 times the absorption measured during quiescence. Accretion of obscuring clumps of stellar wind material can explain both the small flares and the increase in N H . Placing this observation in the context of the recent Swift monitoring campaign, we find that weak-flaring episodes, or at least epochs of enhanced activity just above the quiescent level but well below the moderately bright or high-luminosity outbursts, represent more than 60% ± 5% of all observations in the 0.5–10 keV energy range making this the most common state in the emission behavior of IGR J17391−3021.