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

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-metre Magellan telescope and the WIYN High-Resolution Infrared Camera instrument on the 3.5-metre 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 yr between 2003 and 2015, SAX J1806.5−2215 has been active for more than 5 yr 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 United Kingdom Infrared Telescope (UKIRT) Galactic plane observations taken during the same outburst, colour–colour 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 an 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 suggests that there was an increase in flux by at least one magnitude of the detected star during our observations, and hence suggests the detection of the likely counterpart of SAX J1806.5−2215.