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^⋆

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, 20133 Milano, Italy
e-mail: lara.sidoli@inaf.it
² Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskij pr. 13, 119234 Moscow, Russia
³ Kazan Federal University, Kremlyovskaya 18, 420008 Kazan, Russia
⁴ Institute for Physics and Astronomy, University Potsdam, 14476 Potsdam, Germany
⁵ Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, 27100 Pavia, Italy
⁶ INFN, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy

Received: 25 May 2021
Accepted: 22 June 2021

Abstract

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 kT₁ = 0.24 keV and kT₂ = 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 × 10³² 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)×10³¹ 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.