The longest observation of a low intensity state from a Supergiant Fast X-ray Transient: Suzaku observes IGRJ08408-4503

TL;DR

This paper presents the longest deep X-ray observation of a Supergiant Fast X-ray Transient in a low intensity state, revealing persistent accretion and constraining hard X-ray emission during quiescence.

Contribution

It provides the first detailed spectral and temporal analysis of a SFXT in a prolonged low luminosity state outside outburst, with stringent limits on hard X-ray emission.

Findings

Source was in a low intensity state with luminosity ~4E32 erg/s.

Two long flares observed, peaking at ~3 times the initial flux.

Hard X-ray emission constrained to <6E33 erg/s during low state.

Abstract

We report here on the longest deep X-ray observation of a SFXT outside outburst, with an average luminosity level of 1E33 erg/s (assuming 3 kpc distance). This observation was performed with Suzaku in December 2009 and was targeted on IGRJ08408-4503, 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 ks and 64.7 ks, respectively, spanning about three days. The source was caught in a low intensity state characterized by an initially average X-ray luminosity level of 4E32 erg/s (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 6E33 erg/s (15-40 keV; 3 kpc distance) to the hard X-ray emission, which is the most stringent constrain to the hard X-ray emission during a low intensity state in a SFXT, to date. The timescale observed for the two low intensity long flares is indicative of an orbital separation of the order of 1E13 cm in IGRJ08408-4503.