Journal Article

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

L. Sidoli, P. Esposito and L. Ducci

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 409, issue 2, pages 611-618
Published in print December 2010 | ISSN: 0035-8711
Published online November 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17320.x
The longest observation of a low-intensity state from a supergiant fast X-ray transient: Suzaku observes IGR J08408−4503

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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 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.

Keywords: X-rays: individual: IGR J08408−4503

Journal Article.  5495 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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