2003MNRAS.340L..13B

Using CGS4 on UKIRT, we have obtained a 1.95-2.45 µm infrared (IR) spectrum of the primary candidate counterpart to the bright Z LMXB GX5-1. IR photometry by Naylor, Charles & Longmore and the astrometry of Jonker et al. had previously identified this star as the most likely counterpart to GX5-1. The spectrum presented here clearly shows Brackett γ and He lines in emission, for the first time confirming the identity of the counterpart. Similar to our previous spectroscopy of the Z source LMXBs Sco X-1 and Sco X-2 (Bandyopadhyay et al. 1999]), the K -band spectrum of GX5-1 shows emission lines only. We briefly discuss the implications of this spectrum for the nature of the Z sources.