An atlas of optical continuum and line emission from low-mass X-ray binaries

We have obtained broad-band optical spectra of a sample of 15 low-mass X-ray binaries (LMXBs) in the range 4000-9000 Å and fitted them with combinations of standard accretion disc models plus secondary star templates. For the sample of LMXBs in which we determined the power-law indices of the accretion disc flux distribution, we can explain the steepness of the slopes in terms of an optically thick accretion disc and X-ray heating of the outer parts of the disc. We present for the first time optical spectra of X 1705 - 250, XB 1905 + 00 and XB 1916 -053. Optical line emission was detected in most of the sample. The Balmer lines Hα and Hβ were generally the strongest lines with mean equivalent widths of 3.7 and 1.9 Å, respectively. The Hen II λ4686 and the Bowen C III/N III blend at λ4640 were also observed, with equivalent widths of 3.0 and 4.1 Å, respectively. The Paschen lines were also present in Cen X - 4, GS 2000 + 25, and for the first time detected in Aql X-1. The only systems in which stellar features were detected were those in which the companions were evolved e.g. GX 1+4, Cyg X - 2 and the source in which the mass accretion rate is low i.e. Cen X - 4.