ChandraGrating Spectroscopy of the X‐Ray Binary 4U 1700−37 in a Flaring State

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Abstract
Chandra X-Ray Observatory grating spectra of the supergiant X-ray binary 4U 1700-37 reveal emission lines from hydrogen- and helium-like S, Si, Mg, and Ne in the 4-13 Å range. The spectrum also shows fluorescent lines from S and Si and a prominent Fe Kα line at 1.94 Å. The lines contribute to the previously unaccounted "soft excess" in the flux in this range at orbital ≈ 0.7. The X-ray source was observed during intermittent flaring, and the strengths of the lines vary with the source state. The widths of the lines (FWHM ≈ 1000-2000 km s-1) can result from either Compton scattering or Doppler shifts. Power spectra of the hard X-rays show red noise, and the soft X-rays and lines show in addition quasi-periodic oscillations and a power spectral break. Helium-like triplets of Si and Mg suggest that the gas is not in a pure photoionization equilibrium. We discuss whether resonant scattering could affect the line ratios or whether a portion of the wind may be heated to temperatures T ~ 106 K.
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