Detecting the intrinsic X-ray emission from the O-type donor star and the residual accretion in a supergiant fast X-ray transient in its faintest state

• Published in: Astronomy and astrophysics (Berlin) Vol. 654; p. A131
• Main Authors: Sidoli, L., Postnov, K., Oskinova, L., Esposito, P., De Luca, A., Marelli, M., Salvaterra, R.
• Format: Journal Article
• Language: English
• Published: Heidelberg EDP Sciences 01.10.2021
• Subjects: Deposition; Emission analysis; Light curve; Low level; Luminosity; Magnetospheres; Neutron stars; Neutrons; Power law; Stellar flares; Stellar winds
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202141378

We report on the results of an XMM–Newton observation of the supergiant fast X-ray transient (SFXT) IGR J08408-4503 performed in June 2020. The source is composed of a compact object (likely a neutron star) orbiting around an O8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. We analysed spectra measured during the flare and during quiescence. The quiescent state shows a continuum spectrum that is well deconvolved to three spectral models: two components are from a collisionally ionized plasma (with temperatures of k T 1  = 0.24 keV and k T 2  = 0.76 keV), together with a power-law model (photon index, Γ, of ∼2.55), dominating above ∼2 keV. The X-ray flux emitted at this lowest level is 3.2 × 10 −13 erg cm −2 s −1 (0.5–10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85 × 10 32 erg s −1 (at 2.2 kpc). The two-temperature collisionally ionized plasma is intrinsic to the stellar wind of the donor star, while the power-law can be interpreted as emission due to residual, low-level accretion onto the compact object. The X-ray luminosity contributed by the power-law component only, in the lowest state, is (4.8 ± 1.4)×10 31 erg s −1 , which is the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by XMM–Newton , X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, along with a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical, radiation-driven settling accretion stage.