Identification and characterisation of the gamma-ray counterpart of the transitional pulsar candidate CXOU J110926.4-650224

Abstract

Transitional millisecond pulsars (tMSPs) represent a crucial link between the rotation-powered and accretion-powered states of binary pulsars. During their active X-ray state, tMSPs are the only low-mass X-ray binary systems detected up to GeV energies by the Fermi Large Area Telescope (LAT). CXOU J110926.4-650224 is a newly discovered tMSP candidate in an active X-ray state and is potentially spatially compatible with a faint gamma-ray source listed in the latest Fermi-LAT point-source catalogue as 4FGL J1110.3-6501. Confirming the association between CXOU J110926.4-650224 and the Fermi source is a key step towards validating its classification as a tMSP. We analysed Fermi-LAT data collected from August 2008 to June 2023 to determine a more accurate localisation of the gamma-ray source, characterise its spectral properties, and investigate potential time variability. By thoroughly reconstructing the gamma-ray background around the source using a weighted likelihood model, we obtain a new localisation that aligns with the position of the X-ray source at the 95% confidence level, with a test statistic value of ∼42. This establishes a spatial association between the gamma-ray source and CXOU J110926.4-650224. The gamma-ray emission is adequately described by a power-law model with a photon index of Γ = 2.5 ± 0.1 and a corresponding flux of (3.7 ± 0.9)×10-12 erg cm-2-s-1 in the 0.1-300 GeV range.