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-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.

Near-infrared counterparts of three transient very faint neutron star X-ray binaries

“…We measure magnitudes of the NIR counterpart of H = 17.94 ± 0.06 and Ks = 17.22 ± 0.02. The measured Ks magnitude is consistent with 2011 K-band observations of the source (Kaur et al 2017). Fig.…”

“…6 shows the long term Swift/XRT light curve. Though it initially seems as if the X-ray count rate had decayed significantly between the observations of Kaur et al (2017) and those of this work, the inset shows that the two NIR observations occurred at similar X-ray fluxes, consistent with the near identical NIR magnitudes observed at the two epochs. SAX1806 is not detected in UKIDSS, which observed the field in July 2006 when SAX1806 was in quiescence (Campana 2009), placing a 5σ upper limit on the quiescent magnitude of K > 18.05, H > 19.00.…”

“…Monitoring with Swift/BAT indicates that the source is still exhibiting low-level activity and hence has likely remained in outburst for 6 years. 1 During the X-ray outburst that began in 2011, a NIR counterpart was discovered with K = 17.25 ± 0.03 (Kaur et al 2017).…”

“…Through optical observations, M15 X-3 has been shown to contain a main sequence star in a ∼ 4 hr orbit with a NS (Heinke et al 2009;Arnason et al 2015). In addition, XTE J1719-291, Swift J1357.2-0933, SAX J1806.5-2215 and CXOGC J174540.0-290031 have all been suggested to contain mainsequence companions from optical/NIR observations (Muno et al 2005b;Greiner et al 2008;Corral-Santana et al 2013;Kaur et al 2017).…”

“…Long term Swift/XRT light curve of SAX1806, the green arrow represents the time of the observation presented byKaur et al (2017), the red arrow indicates the time of the Gemini/NIRI observation. A zoomed in portion of the light curve is shown inset.…”