Ultraviolet spectroscopy of V1341 Cygni (= Cygnus X-2).

Abstract

Cyg X-2 is a low-mass X-ray binary of orbital period P = 9^d.8. Several IUE spectra in the 1200-3000 Å range are examined. Variability up to a factor of 2 is apparent. A discussion of the emission-line spectrum yields estimates of the temperature (T ≈ 2×10⁴K), density (n ≈ 2×10¹⁰ cm^-3), and radius (r ≈ 5×10¹² cm) of the line-emission region. Two processes may contribute to the UV emission. One is reprocessing of X-rays in the accretion disk. The other process is bremsstrahlung of a hot plasma producing the X-rays. A comparison with Sco X-1, whose similarity with Cyg X-2 has been noted, suggests that the two processes are likely to coexist.