Suzaku Observes Weak Flares from IGR J17391-3021 Representing a Common Low-activity State in this Supergiant Fast X-ray Transient

Abstract

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 × 10³³ 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) × 10²² 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.