The Morphology of the Neutral and Ionized Interstellar Medium toward Cyg X-1.

Coadded archival ultraviolet Hubble Space Telescope (HST/E140M) spectra provide further insights in the structure of the ISM toward Cyg X-1. Although the line-of-sight to Cyg X-1 is highly attenuated by dust, the coadded HST spectra, although still at low S/N, together with previous finding indicate the existence of multiple distinct regions in the ISM toward Cyg X-1, ranging in heliocentric radial velocities from -9 km s^-1 to -105 km s^-1. The species, CO, C I, C I*, C I**, O I, O I*, O I**, and S I indicate cool, dense gas with number densities ≥ 100 cm^-3. Profile fitting of stronger neutral and low-ionization features of O I, S II, and Si II, and Fe II supports previous findings showing four distinct components at or near 0, -13, -26, and -43 km s^-1. Additional profile fitting of absorption from the ground C II and 13 cm-1 level of C II* reveals a velocity component near -80 km/s. Additional C II absorption is present near -57 and -105 km s^-1, not seen in C II*, and indicates very low density gas at these velocities. The Si II* absorption at 1265A shows higher density gas similar to that inferred from the C I, C I*, and C I**. Analysis of the highly-ionized Si IV and C IV show common components at -8 and -38 km s^-1 with b-values 9-10 km s^-1. This is consistent with these ions formed in H II regions photoionized by stellar continua of the hot OB stars in the OB3 association, which includes Cyg X-1. Absorption features of the C IV resonance doublet indicate additional broad absorption at or near -65 km s^-1, requiring b~20 km s^-1 to yield a fit to the profiles. The N V doublet at 1238 and 1242A only shows a single sharp absorption at – 8 km/s. The b-value, 9 km s^-1, suggests a temperature, T ≤ 68,000K, too low to be produced by the hot coronal substrate of the ISM. Thus, the N V is likely a result of Auger X-ray photo-ionization from Cyg X-1, and in agreement with previous findings. We compare our HST results with prior IUE studies of the Cygnus region and explain the observed features and the implied ionization in terms of the well-known Loop II detected at 21-cm, expanding supershells around OB associations, and shocks near Cyg X-1.