tokens: 2010 ; Bahramian et al. 2021 ). In addition, very hard spectra in a bright hard state have been observed in a couple of NS LMXBs (Parikh et al. 2017 ) and BH LMXBs including V 404 Cyg (e.g., Kimura et al. 2016 ), V4641 Sgr (e.g., Maitra Bailyn 2006 ), and Swift J1858.6−0814 (Hare et al. 2020 ). All these sources have a strong short-term flux variation by a more than order of magnitude (e.g., Kitamoto et al. 1989 ; Revnivtsev et al. 2002 ; Wijnands et al. 2017 ), also in agreement with the present case. In addition, partial covering absorption is sometimes required in these sources, especially in high-flux phases (Kimura et al. 2016 ), consistent with Swift J0840.7−3516 in a bright phase. We note that the photon index of Swift J0840.7−3516 after the decay, ≲2, is reminiscent of a BH rather than an NS LMXB, which usually shows a larger value, Γ ∼ 3 (Wijnands et al. 2015 ), although the NS LMXB EXO 1745−248 showed a similar hard spectrum in quiescence (Rivera Sandoval et al. 2018 ). On the other hand, an NS LMXB would rather be favored, in that a very small radius of the thermal component, 1–2( D /8 kpc) km, consistent with emission from an NS surface, was obtained from the NuSTAR+Swift/XRT+NICER spectrum in a faint phase. This holds even if the observed power-law component was entirely produced by Comptonized disk photons. Assuming conservation of the number of photons and an isotropic scattered emission (Kubota Makishima 2004 ), and using the intrinsic disk photons estimated from the best-fit diskbb+cutoffpl model, we obtain the inner disk radius as ∼3( D /8 kpc) km (where i = 60° is assumed). Similar thermal emission likely emitted by the NS surface can be seen in NS LMXBs at low X-ray luminosities below ∼10 35 erg s −1 but not in a more luminous hard state, where the power-law component dominates the X-ray flux (e.g., Sakurai et al. 2014 ; Shidatsu et al. 2017 ). The thermal component in Swift J0840.7−3516 was observed at a consistent luminosity; the second rapid decay from 10 5 s since the discovery, in which this component was found, started at ∼10 35 ( D /8 kpc) 2 erg s −1 . Even if an LMXB scenario works for all the properties raised above, the observed absorption feature is difficult to explain. Although ionized Si and S lines from disk winds are sometimes seen at similar energies (e.g., Ueda et al. 2009 ), they are much weaker than the present case and always with strong iron K α lines at ∼7 keV. Assuming the observed feature to be the redshifted Fe K edge, we obtained z = 1.1. If this is the gravitational redshift caused by a Schwarzschild stellar-mass BH, the feature should be produced at ∼1.3 R S , where R S is the Schwarzschild radius. It is unclear that an absorber with such a small size can exist in the vicinity of a BH. Another possibility is that the observed feature is a different blueshifted line produced by an outflow at a relativistic speed, like baryonic jets observed in SS 433. However, it is unclear whether such
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