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

Chandra X-ray Observatory grating spectra of the supergiant X-ray Binary 4U1700-37 reveal emission lines from hydrogen and helium-like S, Si, Mg, and Nein the 4-13 A range. The spectrum also shows fluorescent lines from S, Si, anda prominent Fe K alpha line at 1.94 A. The lines contribute to the previouslyunaccounted "soft excess" in the flux in this range at orbital phi~0.7. TheX-ray source was observed during intermittent flaring, and the strengths of thelines vary with the source state. The widths of the lines (FWHM approximately1000-2000 km/s) 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 linesshow in addition quasiperiodic oscillations (QPOs) and a power-spectral break.Helium-like triplets of Si and Mg suggest that the gas is not in a purephotoionization equilibrium. We discuss whether resonant scattering couldaffect the line ratios or whether a portion of the wind may be heated totemperatures T~10^6 K.Comment: 43 pages, 12 figures (4 in color), submitted to The Astrophysical Journa