Abstract
Abstract V723 Mon was recently reported to be hosting the nearest (d = 460 pc) candidate black hole by Jayasinghe et al. An X-ray detection was claimed based on deep Swift XRT observations with a luminosity of L X ≈ 8 × 1029 erg s−1 or L X/L Edd ≈ 10−9. Here we show that the X-ray photons detected by Swift XRT are due to optical loading from the bright (V = 8.3 mag) star. Using events in the 2–10 keV energy band, where optical loading does not strongly contribute, we find a 3σ upper-limit on the unabsorbed 2–10 keV X-ray flux of F X < 2.8 × 10−14 erg cm−2 s−1, corresponding to a luminosity L X < 7.2 × 1029 erg s−1 at a distance of 460 pc or L X/L Edd ≲ 2 × 10−9 for a M ≈ 3M ⊙ black hole.
Highlights
V723 Mon is a variable red giant star that was found to be in an almost circular binary orbit with a "dark" companion having a period of 59.9 days (Jayasinghe et al 2021; J+21, hereafter)
We show that the X-ray photons detected by Swift XRT are due to optical loading from the bright (V = 8.3 mag) star
Using events in the [2–10] keV energy band, where optical loading does not strongly contribute, we find a 3σ upper-limit on the unabsorbed [2–10] keV X
Summary
V723 Mon is a variable red giant star that was found to be in an almost circular binary orbit with a "dark" companion having a period of 59.9 days (Jayasinghe et al 2021; J+21, hereafter). Masuda & Hirano (2021) noted that periodic (with P/Pbin = 3) radial velocity residuals remain after accounting for the binary motion and attribute them to the tidal deformation of the red giant whose rotation is synchronized with the binary orbit. The source was observed for about 19.6 ks with the Neil Gehrels Swift Observatory (Gehrels et al 2004) and an X-ray detection was claimed, having a position consistent with that of V723 Mon. The source was claimed to be detected with 18.5 ± 4.9 net counts in the 0.3–2.0 keV energy range, suggesting a soft spectrum (J+21). We derive upper limits on the source's X-ray flux in the [2–10] keV energy band, where optical loading has little to no effect
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