Capturing the lowest luminosity state of the supergiant fast X-ray transient XTE J1739−302

Abstract

Here, we report the results of our analysis of recent Chandra, XMM-Newton, and NuSTAR observations of the supergiant fast X-ray transient XTE J1739−302. The source was caught in a low X-ray luminosity state, from a few 1031–1034 erg s−1 (0.5–10 keV). A very low X-ray luminosity was captured during an XMM-Newton observation performed in October 2022, at a few 1031 erg s−1 (0.5–10 keV), which had never been observed before in XTE J1739−302. 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 that would very likely have been produced by shocks in the supergiant donor wind. These observations covered different orbital phases, but they all appear compatible with the low luminosity level expected from the orbital INTEGRAL light curve. The absorbing column density is variable in the range between 1022 and 1023 cm−2. The broadband X-ray spectrum was feasibly investigated at 1034 erg s−1 (0.5–30 keV) for the first time in XTE J1739−302 with non-simultaneous (albeit at similar orbital phases) Chandra and NuSTAR data, showing a power-law spectral shape with a photon index of ∼2.2 and an absorbing column density of ∼1023 cm−2. Remarkably, owing to the XMM-Newton observation, the amplitude of the X-ray variability now exceeds five orders of magnitude, making XTE J1739−302 one of the most extreme SFXTs.