A search for near-infrared counterparts of two faint neutron star X-ray transients : XMMU J174716.1-281048 and SAX J1806.5-2215

TL;DR

This study used near-infrared imaging to search for counterparts of two faint neutron star X-ray binaries, identifying a probable counterpart for SAX J1806.5-2215 but not for XMMU J174716.1-281048, providing insights into their nature.

Contribution

First near-infrared observations of these faint X-ray binaries, identifying a likely counterpart for SAX J1806.5-2215 and suggesting the counterparts are probably not high-mass stars.

Findings

Detected a likely NIR counterpart for SAX J1806.5-2215 during outburst.

Did not find a true NIR counterpart for XMMU J174716.1-281048.

Observed flux increase indicating variability of the SAX J1806.5-2215 counterpart.

Abstract

We present our near-infrared (NIR) imaging observations of two neutron star low mass X-ray binaries XMMU J174716.1-281048 and SAX J1806.5-2215 obtained using the PANIC instrument on the 6.5-meter Magellan telescope and the WHIRC instrument on the 3.5-meter WIYN telescope respectively. Both sources are members of the class of faint to very-faint X-ray binaries and undergo very long X-ray outburst, hence classified as `quasi persistent X-ray binaries'. While XMMU J174716.1-281048 was active for almost 12 years between 2003 and 2015, SAX J1806.5-2215 has been active for more than 5 years now since 2011. From our observations, we identify two NIR stars consistent with the Chandra X-ray error circle of XMMU J174716.1-281048. The comparison of our observations with the UKIRT Galactic plane observations taken during the same outburst, color-color diagram analysis and spectral energy distribution suggest that both stars are probably a part of the field population and are likely high mass stars. Hence possibly neither of the two stars is a true NIR counterpart. For the faint X-ray binary SAX J1806.5-2215 during its current outburst, we detected a NIR star in our K band WIYN observations consistent with its Chandra error circle. The identified NIR star was not detected during the UKIRT observations taken during its quiescent state. The comparison of two observations suggest that there was an increase in flux by at least one magnitude of the detected star during our observations, hence suggests the detection of the likely counterpart of SAX J1806.5-2215.