A variable near-infrared counterpart to the neutron-star low-mass X-ray binary 4U 1705-440

Abstract

We report the discovery of a near-infrared (NIR) counterpart to the persistent neutron-star low-mass X-ray binary 4U 1705-440, at a location consistent with its recently determined Chandra X-ray position. The NIR source is highly variable, with Ks-band magnitudes varying between 15.2 and 17.3 and additional J- and H-band observations revealing color variations. A comparison with contemporaneous X-ray monitoring observations shows that the NIR brightness correlates well with X-ray flux and X-ray spectral state. We also find possible indications of a change in the slope of the NIR/X-ray flux relation among different X-ray states. We discuss and test various proposed mechanisms for the NIR emission from neutron-star low-mass X-ray binaries and conclude that the NIR emission in 4U 1705-440 is most likely dominated by X-ray heating of the outer accretion disk and the secondary star.