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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 10 33  erg s −1  (assuming 3 kpc distance). This observation was performed with  Suzaku  in 2009 December and was targeted on IGR J 08408− 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 × 10 32   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 × 10 33   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 10 13  cm in IGR J 08408− 4503.

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