XMM-Newton RGS spectroscopy of LMC X-3

Abstract

We present soft X-ray spectroscopy of the black hole binary LMC X-3 from the XMM-Newton Reflection Grating Spectrometer. The observations span the full range of spectral states seen in LMC X-3. The spectra are completely dominated by continuum emission, and the neutral absorbing column measured from the OI edge (N_H= 3.8^+0.8_-0.7× 10²⁰ cm^-2) is consistent with the Galactic interstellar column density towards LMC X-3. We find no evidence for variability of the neutral absorbing column. We also constrain the ionized column density using the upper limits to the equivalent width of the OII-OVIII K-shell resonance lines: we find that the equivalent hydrogen column density of gas in which O is partially ionized is < 8× 10²⁰ cm^-2. From this upper limit we can rule out a line-driven stellar wind as the power source for the X-ray emission of LMC X-3 except when it is faint. At wavelengths longward of the peak emission the spectral shape is well described by a multitemperature disc-blackbody spectrum; the power-law component, which dominates at shorter wavelengths, does not continue longward of the disc-blackbody peak. This implies that the multitemperature disc-blackbody component supplies the seed photons that are Compton upscattered in the hot corona, consistent with the standard paradigm for black hole X-ray binary spectral states.