Infrared observations of low-mass X-ray binaries – II. Discovery of a variable infrared counterpart to GX13 + 1

Using the IRCAM on UKIRT, it has been discovered that the bright (K ~ 11 mag), infrared source (NCL 101) at the precise radio position of the Galactic Bulge X-ray source, GX13 + 1, is variable, making it the first of the bright central sources to be so identified. The variability (~ 1 mag) is on a time-scale of days or longer, the source remaining constant to within 0.05 mag (in K) during a single 5-hr monitoring run. If this variation is an orbital modulation, then the long period implies that the distinction between Z and Atoll sources is more complex than simply the orbital period. A deep INT Prime Focus image of the field reveals no visible counterpart to this object, with a limit of R > 22, indicating heavy obscuration that is consistent with the observed X-ray absorption ($${A}_{V}\sim17$$).