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We report on the results of an XMM-Newton observation of the Supergiant FastX-ray Transient (SFXT) IGR J08408-4503 performed in June 2020. The source iscomposed by a compact object (likely a neutron star) orbiting around anO8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level ofemission, punctuated by a single, faint flare. Analysis of spectra measuredduring the flare and during quiescence is performed. The quiescent state showsa continuum spectrum well deconvolved to three spectral models: two componentsare from a collisionally-ionized plasma (with temperatures kT1=0.24 keV andkT2=0.76 keV), together with a power law model (photon index of 2.55),dominating above 2 keV. The X-ray flux emitted at this lowest level is3.2$\times10^{-13}$ erg/cm2/s (0.5-10 keV, corrected for the interstellarabsorption), implying an X-ray luminosity of 1.85$\times10^{32}$ erg/s (at 2.2kpc). The two temperature collisionally-ionized plasma is intrinsic to thestellar wind of the donor star, while the power law can be interpreted asemission due to residual, low level accretion onto the compact object. TheX-ray luminosity contributed by the power law component only, in the loweststate, is (4.8$\pm{1.4})\times10^{31}$ erg/s, the lowest quiescent luminositydetected from the compact object in an SFXT. Thanks to this very faint X-raystate caught by XMM-Newton, X-ray emission from the wind of the donor star LMVel could be well-established and studied in detail for the first time, as wellas a very low level of accretion onto the compact object. The residualaccretion rate onto the compact object in IGR J08408-4503 can be interpreted asthe Bohm diffusion of (possibly magnetized) plasma entering the neutron starmagnetosphere at low Bondi capture rates from the supergiant donor wind at thequasi-spherical radiation-driven settling accretion stage.

astronomy and astrophysics

Detecting the intrinsic X-ray emission from the O-type donor star and  the residual accretion in a Supergiant Fast X-ray Transient during its  faintest state