We report on a multi-wavelength study of the unclassified X-ray source CXOU J110926.4 - 650224 ( J1109 ) .
We identified the optical counterpart as a blue star with a magnitude of \sim 20.1 ( 3300–10 500 Å ) .
The optical emission was variable on timescales from hundreds to thousands of seconds .
The spectrum showed prominent emission lines with variable profiles at different epochs .
Simultaneous XMM–Newton and NuSTAR observations revealed a bimodal distribution of the X-ray count rates on timescales as short as tens of seconds , as well as sporadic flaring activity .
The average broad-band ( 0.3–79 keV ) spectrum was adequately described by an absorbed power law model with photon index of \Gamma = 1.63 \pm 0.01 ( at 1 \sigma c.l .
) , and the X-ray luminosity was ( 2.16 \pm 0.04 ) \times 10 ^ { 34 } erg s ^ { -1 } for a distance of 4 kpc .
Based on observations with different instruments , the X-ray luminosity has remained relatively steady over the past \sim 15 years .
J1109 is spatially associated with the gamma-ray source FL8Y J1109.8 - 6500 , which was detected with Fermi at an average luminosity of ( 1.5 \pm 0.2 ) \times 10 ^ { 34 } erg s ^ { -1 } ( assuming the distance of J1109 ) over the 0.1–300 GeV energy band between 2008 and 2016 .
The source was undetected during ATCA radio observations that were simultaneous with NuSTAR , down to a 3 \sigma flux upper limit of 18 \mu Jy/beam ( at 7.25 GHz ) .
We show that the phenomenological properties of J1109 point to a binary transitional pulsar candidate currently in a sub-luminous accretion disk state , and that the upper limits derived for the radio emission are consistent with the expected radio luminosity for accreting neutron stars at similar X-ray luminosities .