Program: GN-2012A-Q-72

We propose target-of-opportunity near-infrared (NIR) imaging observations of sub-luminous transient neutron star X-ray binaries during X-ray outburst. These sources belong to a new class of X-ray binaries with accretion luminosities 2-3 orders of magnitude fainter than the well studied bright X-ray binaries. The cause of their sub-luminous nature is not known and the current binary evolution models fail to explain these systems. We propose NIRI Ks, Brackett gamma, and H imaging to search for their NIR counterparts, and search for evidence of hydrogen. If brighter than Ks=17, we will trigger GNIRS spectroscopic followup, focused on detecting hydrogen and/or helium lines in the spectra. Non-detection of hydrogen lines, and detection of helium, would indicate that the donor stars are white dwarfs, helping us understand these unusual systems' evolution.