The longest observation of a low-intensity state from a supergiant fast X-ray transient: Suzaku observes IGR-J08408-4503

Abstract

We report here on the longest deep X-ray observation of a supergiant fast X-ray transient (SFXT) outside outburst, with an average luminosity level of 1033 erg s-1 (assuming 3 kpc distance). This observation was performed with Suzaku in 2009 December and was targeted on IGR-J08408-4503, with a net exposure with the X-ray Imaging Spectrometer (XIS; 0.4-10 keV) and the Hard X-ray Detector (HXD; 15-100 keV) of 67.4 and 64.7 ks, respectively, spanning about 3 d. The source was caught in a low-intensity state characterized by an initially average X-ray luminosity level of 4 × 1032- erg s-1 (0.5-10 keV) during the first 120 ks, followed by two long flares (about 45 ks each) peaking at a flux a factor of about 3 higher than the initial pre-flare emission. Both XIS spectra (initial emission and the two subsequent long flares) can be fitted with a double-component spectrum, with a soft thermal plasma model together with a power law, differently absorbed. The spectral characteristics suggest that the source is accreting matter even at this very low-intensity level. From the HXD observation we place an upper limit of 6 × 1033- erg s-1 (15-40 keV; 3-kpc distance) to the hard X-ray emission, which is the most stringent constraint on the hard X-ray emission during a low-intensity state in a SFXT, to date. The time-scale observed for the two low-intensity long flares is indicative of an orbital separation of the order of 1013 cm in IGR-J08408-4503. © 2010 The Authors. Journal compilation © 2010 RAS.