Coti Zelati, FrancescoFrancescoCoti ZelatiPAPITTO, ALESSANDROALESSANDROPAPITTODE MARTINO, DomitillaDomitillaDE MARTINOBuckley, David A. H.David A. H.BuckleyOdendaal, AlidaAlidaOdendaalLi, JianJianLiRussell, Thomas D.Thomas D.RussellTorres, Diego F.Diego F.TorresMazzola, Simona M.Simona M.MazzolaBozzo, EnricoEnricoBozzoGromadzki, MariuszMariuszGromadzkiCAMPANA, SergioSergioCAMPANARea, NandaNandaReaFerrigno, CarloCarloFerrignoMigliari, SimoneSimoneMigliari2020-12-282020-12-2820190004-6361http://hdl.handle.net/20.500.12386/29209We 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 ∼20.1 (3300-10500 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 Γ = 1.63 ± 0.01 (at 1σ c.l.), and the X-ray luminosity was (2.16 ± 0.04) × 10<SUP>34</SUP> erg s<SUP>-1</SUP> for a distance of 4 kpc. Based on observations with different instruments, the X-ray luminosity has remained relatively steady over the past ∼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 ± 0.2) × 10<SUP>34</SUP> erg s<SUP>-1</SUP> (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 3σ flux upper limit of 18 μJy beam<SUP>-1</SUP> (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.STAMPAenArticle10.1051/0004-6361/2018348352-s2.0-85062194120000459240000002http://arxiv.org/abs/1903.04526v1https://www.aanda.org/articles/aa/abs/2019/02/aa34835-18/aa34835-18.html2019A&A...622A.211CFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering