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, 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 toward validating its classification as a tMSP. In this study, we analyse Fermi-LAT data collected from August 2008 to June 2023 to achieve 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 $\sim 42$. 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 $\Gamma = 2.5 \pm 0.1$ and a corresponding flux of $(3.7\pm0.9) \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1-300 GeV range.

Identification and Characterisation of the Gamma-Ray Counterpart of CXOU J110926.4--650224

The study of transitional millisecond pulsars (tMSPs) offers critical insights into the binary evolution and lifecycle of pulsars, situated at the intersection of accretion- and rotation-powered stages. The paper addresses the identification and characterisation of the gamma-ray counterpart to the transitional pulsar candidate CXOU J110926.4--650224 using 15 years of observations from the Fermi Large Area Telescope (LAT).

Context and Importance

tMSPs, which oscillate between accreting X-ray states and radio pulsar states, form the cornerstone evidence for the recycling theory of pulsar evolution. This study focuses on a specific tMSP candidate, CXOU J110926.4--650224, and its potential association with a gamma-ray source detected by the Fermi LAT. Previously detected tMSPs have been confirmed by their gamma-ray emissions, with noted enhancements during sub-luminous states suggesting involved non-thermal processes.

Methodology

The paper utilises data collected by Fermi LAT from August 2008 to June 2023 to perform an in-depth analysis of the gamma-ray source associated with CXOU J110926.4--650224. A weighted likelihood model was developed to reconstruct the gamma-ray background and achieve precise localisation and characterisation. The Test Statistic (TS) value derived was approximately 42, establishing a spatial association at 95% confidence.

The gamma-ray source displayed emission that aligned well with a power-law spectral model with a photon index $\Gamma = 2.5 \pm 0.1$ and a flux of $(3.7 \pm 0.9) \times 10^{-12}$ ergs cm$^{-2}$ s$^{-1}$ over the 0.1--300 GeV range. This spectrum is consistent with other known tMSPs, although no significant spectral curvature, which is an attribute of such systems, was detected due to the source's relative faintness.

Implications and Future Work

This research positions CXOU J110926.4--650224 as a promising candidate for an additional tMSP, potentially augmenting the limited roster of known systems. The study, though unable to capture conclusive state-switching indicative of confirmed tMSPs, establishes a crucial groundwork for further research into the source's nature. Future observations, possibly facilitated by continued Fermi LAT monitoring and upcoming missions like the Einstein Probe, might ascertain the source's behavior conclusively.

Conclusion

CXOU J110926.4--650224 represents a significant milestone in expanding our interpretive knowledge of tMSPs' gamma-ray characteristics. Continued exploration of gamma-ray emissions from tMSPs will deepen our understanding of the dynamic processes underpinning state transitions between accretion-powered and rotation-powered phases. This data-and-analysis-driven approach to accurately associating terrestrial wavelengths with gamma-ray sources is critical for cataloguing celestial phenomena and reinforces the relevance of extended observational datasets in high-energy astrophysical studies.