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
Jun 28, 202113 pages
Published in:
- Astron.Astrophys. 654 (2021) A131,
- Astron.Astrophys. 654 (2021) A131
- Published: Oct 1, 2021
e-Print:
- 2106.14909 [astro-ph.HE]
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Abstract: (EDP Sciences)
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.Key words: stars: neutron / supergiants / X-rays: binaries / stars: individual: IGR J08408-4503 / stars: individual: HD 74194 / stars: individual: LM Vel⋆ Based on observations (ObsID 0861460101) obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.Note:
- 13 pages, 6 figures, 5 tables; accepted for publication in Astronomy & Astrophysics
- stars: neutron
- supergiants
- X-rays: binaries
- stars: individual: IGR J08408-4503
- stars: individual: HD 74194
- stars: individual: LM Vel
References(80)
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