Soft X-ray emission lines in the X-ray binary Swift J1858.6-0814 observed with XMM-Newton Reflection Grating Spectrometer: disc atmosphere or wind?

We find soft X-ray emission lines from the X-ray binary Swift J1858.6-0814 in data from XMM-Newton Reflection Grating Spectrometer (RGS): NVII, OVII, and OVIII, as well as notable residuals short of a detection at Ne IX and other higher ionization transitions. These could be associated with the disc atmosphere, as in accretion disc corona sources, or with a wind, as has been detected in Swift J1858.6-0814 in emission lines at optical wavelengths. Indeed, the NVII line is redshifted, consistent with being the emitting component of a P-Cygni profile. We find that the emitting plasma has an ionization parameter log (xi) = 1.35 +/- 0.2 and a density n > 1.5 x 10(11) cm(-3). From this, we infer that the emitting plasma must be within 10(13) cm of the ionizing source, similar to 5 x 10(7) r(g) for a 1.4 M-circle dot neutron star, and from the line width that it is at least 10(4) r(g) away [2 x 10(9)(M/1.4M(circle dot)) cm]. We compare this with known classes of emission-line regions in other X-ray binaries and active galactic nuclei.