Capturing the lowest luminosity state of the Supergiant Fast X-ray Transient XTE J1739-302

Abstract

We report here on the results of the analysis of Chandra, XMM-Newton and NuSTAR recent observations of the Supergiant Fast X-ray Transient XTEJ1739-302. The source was caught in a low X-ray luminosity state, from a few $10^{31}$ to $10^{34}$ erg/s (0.5-10 keV). In particular, a very low X-ray luminosity was captured during an XMM-Newton observation performed in October 2022, at a few $10^{31}$ erg/s (0.5-10 keV), never observed before in XTEJ1739-302. The XMM-Newton spectrum could be well fitted either by an absorbed, steep power law model (photon index of 3.5) or by a collisionally-ionized diffuse gas with a temperature of 0.7 keV, very likely produced by shocks in the supergiant donor wind. These observations covered different orbital phases, but all appear compatible with the low luminosity level expected from the orbital INTEGRAL light curve. The absorbing column density is variable in the range $10^{22}-10^{23}$ cm${}^{-2}$. A broad-band X-ray spectrum could be investigated at $10^{34}$ erg/s (0.5-30 keV) for the first time in XTEJ1739-302 with not simultaneous (but at similar orbital phases) Chandra and NuSTAR data, showing a power law spectral shape with a photon index of about 2.2 and an absorbing column density of $\sim$$10^{23}$ cm${}^{-2}$. Remarkably, owing to the XMM-Newton observation, the amplitude of the X-ray variability has increased to five orders of magnitude, making XTEJ1739-302 one of the most extreme SFXTs.

10 pages, 9 Figures, 2 Tables; accepted for publication in Astronomy & Astrophysics on 24 January 2023

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