Hot Outflowing Gas from the X‐Ray Binary Hercules X‐1

Abstract

We present a unified picture of outflowing gas from the X-ray binary system Hercules X-1/HZ Herculis. We suggest that the outflowing gas (a wind) causes UV emission seen in mid-eclipse, narrow UV absorption lines, and broad UV P Cygni lines. Observations with the FOS and STIS spectrographs on the Hubble Space Telescope (HST) show UV emission lines in the middle of X-ray eclipse, when the X-ray-heated atmosphere of the normal star and accretion disk should be entirely hidden from view. Narrow absorption lines (FWHM ≈ 50 km s-1) blueshifted by 500 km s-1 during observations in 1998 and by 400 km s-1 during observations in 1999 were seen from = 0 to 0.3. The line velocity was constant to within 20 km s-1. The P Cygni profiles from Hercules X-1 have optical depths τ 1 with a maximum expansion velocity of ≈600 km s-1, and are seen in the resonance lines N V λλ1238.8, 1242.8, Si IV λλ 1393.7, 1402.8, and C IV λλ1548.2, 1550.8. We discuss whether this wind originates in the accretion disk or on the companion star, and how the relevant ions can survive X-ray ionization by the neutron star.