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

¹ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu km 0.7, I-09042 Monserrato, Italy
² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, Barcelona, Spain
⁴ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, PR China
⁵ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, PR China
⁶ Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain
⁷ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁸ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica – Emilio Segrè, Via Archirafi 36, 90123 Palermo, Italy
⁹ INFN, Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy
¹⁰ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
¹¹ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 - Palermo, Italy

^⋆ Corresponding author; arianna.manca@inaf.it

Received: 15 November 2024
Accepted: 23 February 2025

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.