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

TL;DR

This paper reports the discovery of a variable near-infrared counterpart to the neutron-star low-mass X-ray binary 4U 1705-440, showing correlation with X-ray flux and likely driven by X-ray heating of the accretion disk and secondary star.

Contribution

First identification of a near-infrared counterpart to 4U 1705-440 with analysis of its variability and correlation with X-ray states.

Findings

nIR brightness correlates with X-ray flux and spectral state

Possible change in nIR/X-ray flux relation between states

nIR emission likely dominated by X-ray heating mechanisms

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 K_s-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 for a change in the slope of the nIR/X-ray flux relation between 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.