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

A Manca, F. Coti Zelati, J Li, D.F Torres, J Ballet, A Marino, A Sanna, N Rea, T Di Salvo, A Riggio, et al.

<figure><img src="image_1.png" /></figure>

## To cite this version:

A Manca, F. Coti Zelati, J Li, D.F Torres, J Ballet, et al., Identification and characterisation of the gamma-ray counterpart of the transitional pulsar candidate CXOU J110926.4-650224. Astronomy & Astrophysics - A&A, 2025, 695, pp.A187. 10.1051/0004-6361/202453010. hal-04989440

HAL Id: hal-04989440

https://hal.science/hal-04989440v1

Submitted on 22 May 2025

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

A. Manca<fnref n="1" />,*, F. Coti Zelati<fnref n="2" />,<fnref n="3" />, J. Li<fnref n="4" />,<fnref n="5" />, D. F. Torres<fnref n="2" />,<fnref n="3" />,<fnref n="6" />, J. Ballet<fnref n="7" />, A. Marino<fnref n="2" />,<fnref n="3" />, A. Sanna<fnref n="1" />, N. Rea<fnref n="2" />,<fnref n="3" />, T. Di Salvo<fnref n="8" />,<fnref n="9" />,<fnref n="10" />, A. Riggio<fnref n="1" />,<fnref n="10" />, L. Burderi<fnref n="1" />,<fnref n="11" />, and R. Iaria<fnref n="8" />

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⁻<fnref n="2" /> s⁻<fnref n="1" /> in the 0.1–300 GeV range.

**Key words.** accretion, accretion disks – methods: data analysis – stars: neutron – X-rays: binaries – X-rays: individuals: CXOU J110926.4–650224

## 1. Introduction

Millisecond pulsars (MSPs) are a subcategory of pulsars characterised by very short rotation periods (≤10 ms) and weak magnetic fields (~10⁸⁻<fnref n="9" /> G). The existence of MSPs provides evidence for the so-called recycling scenario of pulsars (Alpar et al. 1982): these objects are thought to be the outcome of a phase of accretion within a low-mass X-ray binary (LMXB) system, which causes the neutron star (NS) to spin up to millisecond periods (the accretion-powered state). When the accretion phase ends, after ~10⁸–10⁹ years, the NS becomes detectable as a radio pulsar powered by the rotation of its magnetic field (the rotation-powered state).

Transitional millisecond pulsars (tMSPs) are sources that switch between the accretion- and rotation-powered states and represent the link between accreting X-ray pulsars and evolved radio MSPs. They thus provide strong evidence for the recycling scenario. Only three tMSPs have been identified to date, and only

one has been observed in a full accretion outburst state (for a review, see Papitto & de Martino 2022). The tMSPs exhibit three distinct states depending on the mass transfer rate from the companion – a key parameter that regulates the equilibrium between the gravitational attraction on the matter and the outward pressure exerted by the pulsar wind. The first is an accretion outburst state with an X-ray luminosity of ~10³<fnref n="6" />⁻³⁷ erg s⁻<fnref n="1" /> (only observed in IGR J18245–2452; Papitto et al. 2013; Ferrigno et al. 2014); the second is a radio pulsar state ($L_X \le 10^{33}$ erg s⁻<fnref n="1" />); the third is an intermediate state characterised by the presence of an accretion disc and an X-ray luminosity of 10³<fnref n="3" />⁻³⁴ erg s⁻<fnref n="1" />, that is, between that measured in the accretion state and the radio pulsar state. This state is referred to as the ‘sub-luminous disc state’ or ‘active X-ray state’ and was observed in both PSR J1023+0038 and MSS J1227–4859 (see e.g. Linares 2014). Three distinct intensity modes can be identified in this intermediate state, as observed in the X-ray light curves: the high mode, characterised by a power-law spectrum with a photon index of Γ ~ 1.7 and a luminosity of $L_X \approx 10^{33}$ erg s⁻<fnref n="1" />; the low mode, characterised by a slightly softer power-law spectrum with Γ ~ 1.8 and a

<footnote n="1">¹ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu km 0.7, I-09042 Monserrato, Italy</footnote>

<footnote n="2">² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain</footnote>

<footnote n="3">³ Institut d'Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, Barcelona, Spain</footnote>

<footnote n="4">⁴ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, PR China</footnote>

<footnote n="5">⁵ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, PR China</footnote>

<footnote n="6">⁶ Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain</footnote>

<footnote n="7">⁷ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France</footnote>

<footnote n="8">⁸ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica – Emilio Segrè, Via Archirafi 36, 90123 Palermo, Italy</footnote>

<footnote n="9">⁹ INFN, Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy</footnote>

<footnote n="10">¹⁰ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy</footnote>

<footnote n="11">¹¹ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 - Palermo, Italy</footnote>

<footnote>* Corresponding author; arianna.manca@inaf.it</footnote>
luminosity of $L_X \approx 10^{32}$ erg s$^{-1}$; and the flaring mode, characterised by sudden increases in count rates on timescales of less than one minute to hours, which can reach luminosities of $\approx 10^{34}$ erg s$^{-1}$ (Tendulkar et al. 2014). The switches between modes can occur in about 10 s.

Transitional MSPs have been observed across several bands, ranging from gamma-ray to radio frequencies. The gamma-ray emission offers valuable insights into the processes occurring in tMSPs and is one of their distinctive features. Emission at such high energies implies active non-thermal processes are at play. In the sub-luminous state of confirmed tMSPs, the gamma-ray emission is several times more intense than that observed in the rotation-powered state, indicating an enhancement of these non-thermal processes. For instance, PSR J1023+0038 exhibits gamma-ray emission that is around five times brighter in its sub-luminous state compared to its rotation-powered state (Stappers et al. 2014). Notably, similar levels of gamma-ray emission have not been detected in other LMXBs (neither in quiescence nor during accretion phases), making tMSPs the only LMXBs confirmed to emit in the GeV range (though some LMXB systems have potential gamma-ray counterparts that require further evidence for a definitive association; e.g. de Oña Wilhelmi et al. 2016). The gamma-ray emission of tMSPs in the sub-luminous state is characterised by a power-law spectrum with a photon index of $\Gamma \sim 2$ and a cutoff energy between 4 and 10 GeV, as observed in both PSR J1023+0038 and MSS J1227–4859 (Torres et al. 2017; Torres & Li 2022).

The increased gamma-ray emission of tMSPs in their sub-luminous state raises questions about its origin – whether it results from the combined gamma-ray radiation of a central rotation-powered pulsar and an additional non-thermal mechanism, or if it arises entirely from a novel non-thermal process. Two primary scenarios have been proposed to explain the origin of this gamma-ray emission. In the propeller scenario, the ejected matter emits synchrotron radiation, which is subsequently up-scattered to gamma-ray energies through inverse Compton scattering (ICS) in the boundary region where the accretion disc is truncated by the NS magnetosphere (Papitto et al. 2014; Papitto & Torres 2015). In the mini-pulsar wind nebula model, the rotation-powered pulsar emits a pulsar wind that interacts with the inner regions of the accretion disc, creating a shock. This shock emits radiation in the X-ray, UV, and optical bands. The synchrotron emission produced at the shock is then up-scattered via the ICS of UV photons from the disc by the pulsar wind, producing detectable GeV emission. This model accounts for the periodic pulsations observed in the X-ray, UV, and optical emissions, as the shock emission is modulated at the pulsar spin period (Papitto et al. 2019; Veledina et al. 2019; Miraval Zanon et al. 2022; Illiano et al. 2023).

Candidate tMSPs are efficiently identified by searching for counterparts of unidentified gamma-ray sources that exhibit time variability in the X-rays, analogous to the high and low modes characteristic of tMSPs, and their spectral shape is consistent with a power-law model with a photon index of $\Gamma \sim 1.7$. Additionally, tMSPs display peculiar double-peaked emission lines in the optical band, which arise from the presence of an accretion disc (see e.g. Bogdanov & Halpern 2015; Bogdanov 2016, for references on 1RXS J154439.4–112820).

The candidate tMSP CXOU J110926.4–650224 was first detected by Chandra in 2008 and identified as the soft X-ray counterpart of the hard X-ray source IGR J11098–6457 (Tomsick et al. 2009). The source showed a behaviour typical of tMSPs in the sub-luminous state, with a bimodal variability in

the soft X-ray emission and the presence of a hard X-ray counterpart (Coti Zelati et al. 2019). An optical counterpart, of magnitude $G \sim 20.1$, was also identified at an estimated distance of $\approx 4$ kpc. Subsequent coordinated X-ray and radio observations detected a radio counterpart with an average flux density of $\sim 33$ µJy at 1.28 GHz, which exhibited significant variability and occasional flaring events shortly after X-ray flares. However, no clear correlated or anti-correlated variability was observed between the X-ray and radio emissions (Coti Zelati et al. 2021). More recently, high-time-resolution X-ray, optical, and near-infrared observations revealed that the variability patterns of the X-ray and optical emissions are strongly correlated and that the near-infrared emission was strongly variable. Spectroscopic analyses suggest that the companion star's spectral type is between K0 and K5 (Coti Zelati et al. 2024).

A gamma-ray source with a position that is fully compatible with that of CXOU J110926.4–650224 was initially detected by the Fermi Large Area Telescope (LAT) based on 8 years of data collection<fnref n="1" />. However, it was not detected in the first 4FGL catalogue (Abdollahi et al. 2020) or in the second data release (4FGL-DR2; Ballet et al. 2020). A gamma-ray source, 4FGL J1110.3–6501, was detected again in the 4FGL-DR3 and DR4 data releases (see Abdollahi et al. 2022 and Ballet et al. 2023 for more details on the latest release of the Fermi catalogue). However, the position in the 4FGL-DR3 and DR4, derived from 12 years of data collection, is not compatible with that of CXOU J110926.4–650224 at the 95% confidence level (c.l.).

We analysed about 15 years of Fermi-LAT data in order to establish a connection between the X-ray and gamma-ray sources. This paper is organised as follows: In Sect. 2 we describe the Fermi-LAT data analysis methods used to investigate the gamma-ray emission from 4FGL J1110.3–6501. Section 3 presents the results of our analysis, including the source localisation and spectral characterisation. In Sect. 4 we discuss the implications of our findings within the context of tMSPs. Finally, Sect. 5 summarises our conclusions and outlines prospects for future research.

## 2. Data analysis

In our analysis, we used data collected by the Fermi-LAT between August 5, 2008 (MJD 54683) and June 29, 2023 (MJD 60124), approximately 14 years and 11 months in total. Our analysis was restricted to events located within a circular region of interest (ROI) with a radius of 15°, centred on the nominal position of the Fermi source 4FGL J1110.3–6501 (RA = $11^h10^m16^s$, Dec = $-65^\circ01'06''$; see Abdollahi et al. 2020). We considered events with energies ranging from 0.1 to 300 GeV.

The data extraction and analysis were performed using the FERMITOOLS software package (v2.2.0; Fermi Science Support Development Team 2019) and the FERMIPY python suite (v. 1.3.1; Wood et al. 2017). We selected the source events separating between front and back events of the LAT tracker and having zenith angle $< 90^\circ$ so as to reduce contamination from the Earth limb. Corresponding good time intervals were created with the gtmktime tool.

We computed the gamma-ray flux and spectral parameters of 4FGL J1110.3–6501 via a joint weighted binned maximum likelihood fitting technique (Abdollahi et al. 2020), adopting the latest instrument response function, P8R3_SOURCE_V3. Starting

<footnote n="1">¹ https://fermi.gsfc.nasa.gov/ssc/data/access/lat/fl8y/</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Sensitivity to detectable gamma-ray emission by the Fermi-LAT at the position of 4FGL J1110.3–6501 as a function of time. The inset shows a zoomed-in view for the most recent time span. Flux values have been estimated assuming a TS threshold of 25 and performing a scan over photon indices ranging from 1 to 5 to mimic various spectral scenarios. The calculation accounts for the local background and exposure at the source position.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Count spectrum and residuals resulting from the likelihood fit of the data within the considered RoI. The fit was performed using a total of 184 sources (see the text for details). The black line represents the total model, where the predominant component is the Galactic diffuse emission (in azure), followed by the isotropic background gamma-ray emission component (in orange). Some of the brightest sources in our RoI are also highlighted.</figcaption></figure>

from our filtered data, we created a square counts map of size 21.2°, with an image scale of 0.1° pix⁻¹, using the Aitoff projection method (Calabretta & Greisen 2002); this allowed us to visualise the Galactic diffuse emission component. We estimated the LAT flux sensitivity at the position of 4FGL J1110.3–6501 to be ≈(2–4) × 10⁻¹² erg cm⁻<fnref n="2" /> s⁻¹ (see Fig. 1).

One key aspect of the analysis involves accurately modelling the background gamma-ray emission in a sky region that is densely populated with sources, as is the case in our study. We used the make4FGLxml script<fnref n="2" /> to generate a spatial-spectral model that includes the model for the Galactic interstellar diffuse gamma-ray emission (gll_iem_v07.fits), the spectral template for the extragalactic isotropic background

emission (iso_P8R3_SOURCE_V3_v1.txt), and all the sources listed in the latest Fermi-LAT data release (4FGL-DR4) located within the RoI. Sources listed in the 4FGL-DR4 and lying within a square region of width 25° centred on the target source position were also added in the modelling. The emission from these sources is described using the best-fitting models derived in the 4FGL-DR4, such as a power law, a LogParabola (usually adopted for blazars), and a power law with a high-energy exponential cutoff (typical for pulsars). The model includes 184 sources (174 point-like sources and 10 extended sources). To enhance accuracy of our starting parameters for the fit, we executed the optimize() tool prior to initiating the fitting process.

We performed a joint weighted likelihood fit employing the gtlike tool with the NEWMINUIT optimisation algorithm. We derived the weighted maps from our filtered files and used them in the likelihood fit<fnref n="3" />. In the fitting process, we allowed the normalisations of all sources within 2° of the target position to vary, along with the parameters of the Galactic and isotropic background components, as well as the parameters of our target (which is modelled using a power law).

To determine the significance of the source, we used the test statistic (TS), defined as (Mattox et al. 1996)

$$TS = 2(\log L_1 - \log L_0), \quad (1)$$

where log L₁ represents the likelihood including our source in the model, and log L₀ represents the likelihood without our source. We generated a TS map using the gttsmap tool (Fig. 3a), excluding our source to highlight it as an excess. Next, we determined the gamma-ray source position with the localize() tool, which employs a TS map to determine the source position through a likelihood fit. Finally, we ran gtlike again at the newly determined position to obtain the final spectral results.

### 3. Results

Figure 2 presents the count spectrum and residuals resulting from the joint likelihood fit of the background model and the target source emission, as a function of energy. At higher energies, starting from ≈10 GeV, we observe residuals that were not accurately accounted for by the fitting process. Nevertheless, these residuals exhibit relatively small scatter and occur in an energy range where significant emission from 4FGL J1110.3–6501 is not expected. The dominant component in the spectrum is the Galactic diffuse emission (shown by the azure line), followed by the isotropic background emission component (depicted by the orange line). We have highlighted the contributions from four of the brightest sources in our RoI in the Fermi 4FGL-DR4 catalogue: 4FGL J1018.9–5856, 4FGL J1028.5–5819, 4FGL J1045.1–5940, and 4FGL J1048.2–5832, all located along the Galactic plane. The remaining sources in our model are represented by grey lines.

Figure 3a presents the TS map of the RoI. Bright residuals are evident along the Galactic plane. Our source is detected with a TS value of approximately 42 (calculated as defined in Eq. (1)). The best-fitting photon index is Γ = 2.5 ± 0.1, and the flux is (3.7 ± 0.9) × 10⁻¹² erg cm⁻<fnref n="2" /> s⁻¹ in the 0.1–300 GeV energy range.

Figure 3b compares the localisation provided in the Fermi 4FGL-DR4 catalogue (red ellipses) with the updated localisation derived from our analysis (blue ellipses; see Table 1 for details). These are superimposed on an archival Chandra X-ray image of the field. Although CXOU J110926.4–650224 falls outside the

<footnote n="2">² https://github.com/physicsranger/make4FGLxml</footnote>

<footnote n="3">³ https://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/weighted_like.pdf</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Left: TS map of the RoI. Regions with significant, un-modelled contributions are shown in brighter colours. The dashed line indicates the position of the Galactic plane. Our source, located in the centre of the RoI, is shown as a yellow spot, as depicted in the inset. Right: comparison of our source localisation with the previous *Fermi* 4FGL-DR4 catalogue position, overlaid on the *Chandra* X-ray image. The catalogue position is shown with red ellipses and the position from our study in blue. The solid ellipses indicate the 68% c.l. and the dashed ellipses the 95% c.l. The *Chandra* source CXOU J110926.4–650224, depicted as the dark spot, lies within the 95% c.l. contour, consistent with our localisation of the *Fermi* source.</figcaption></figure>

<table><caption>**Table 1.** Coordinates and parameters of the confidence regions for 4FGL J1110.3–6501 as derived from the analysis presented in this work.</caption><thead><tr><th rowspan="2">RA</th><th rowspan="2">Dec</th><th rowspan="2">Position angle <sup>(1)</sup></th><th colspan="2">68% c.l.</th><th colspan="2">95% c.l.</th></tr><tr><th>Semi-major axis</th><th>Semi-minor axis</th><th>Semi-major axis</th><th>Semi-minor axis</th></tr></thead><tbody><tr><td>11<sup>h</sup>10<sup>m</sup>12.04<sup>s</sup></td><td>-65°02′07.4″</td><td>327.2°</td><td>0.054°</td><td>0.045°</td><td>0.087°</td><td>0.073°</td></tr></tbody></table>

**Notes.** <sup>(1)</sup>The position angle is measured from north to east

previously catalogued *Fermi* positional uncertainty, it is compatible with the newly determined position at the 95% c.l. To compare our results with the position from the 4FGL-DR4 catalogue, we calculated the ratio of the areas of the error ellipses at the 95% c.l., using the product of the two semi-axes for each ellipse. The confidence region from our analysis is smaller by a factor of ≈0.7.

Considering the large predominance of the Galactic diffuse component and the use of a weighted fit, we double-checked the goodness-of-fit using a PS map, as described by Bruel (2021). The PS map calculation takes into account the variation of the *Fermi*-LAT point spread function with energy, offering a more reliable estimation of the significance with respect to the more commonly used TS map. Contrary to the TS analysis, the PS is sensitive to both positive and negative deviations from the model. Moreover, the PS map analysis is better suited for weighted likelihood fitting procedures, since it can incorporate the weights that are necessary to model the Galactic emission accurately. This PS analysis approach has been validated with previous *Fermi*-LAT data releases, showcasing its effectiveness.

In the PS map analysis, 4FGL J1110.3–6501 is included in the model.

Figure 4a displays the PS map of the RoI, where significant detections are indicated by brighter colours. The PS map facilitates the identification of both positive and negative deviations of the model from the observed data. We observe that the brightest sources are located close to the Galactic plane, which is marked with a dashed line. The abundance of bright areas underscores the crowded nature of the region near the Galactic plane. The inset provides a zoomed-in view of the PS map around the position of 4FGL J1110.3–6501.

From the weighted PS map, we derived the histogram of the PS values (Fig. 4b), which indicates the presence of some un-modelled residuals. To quantify the asymmetry of the residual distribution, we computed the skewness of the histogram and obtained a skewness value of 0.43. This suggests a nearly symmetrical distribution around the mean, with a slight rightward skew. The slight positive shift in the mean indicates a minor systematic tendency for the model to under-predict the data counts, though this bias remains minimal. Overall, the PS map analysis
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Left panel: PS map of the RoI. The gradient represents positive and negative residuals in the RoI, rescaled in terms of statistical significance according to the definition in Bruel (2021). Negative values imply an overestimation of the background. The Galactic plane is highlighted with a dashed line. In the inset, the white cross shows the position of our source, included in the model and therefore blended with the background. Right panel: histogram of the PS values in units of standard deviations, derived from the joint weighted likelihood fit that includes the source under examination in the spectral model. Negative values correspond to areas where the model overestimates the background, and positive values areas where the model underestimates it.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** SED with three equally spaced logarithmic energy bins, extracted in the 1–10 GeV energy range. The blue data points represent detections above a $3\sigma$ c.l., while the red arrow represents an upper limit. The dashed red line in the bottom panel corresponds to TS = 9, meaning a $3\sigma$ detection level. The source is well-modelled with a power law with photon index $\Gamma = 2.5$, here represented by the dashed black line. The two solid black lines represent the $1\sigma$ bounds with respect to the model.</figcaption></figure>

confirms that the model accurately represents the data, with few significant deviations detected.

**4. Discussion**

In this work we analysed 15 years of Fermi-LAT data of the candidate tMSP CXOU J110926.4–650224. Our detailed study of

the background and the addition of 3 more years of data led to a more accurate localisation of the gamma-ray source, demonstrating that its position is compatible with that of the candidate tMSP CXOU J110926.4–650224 at the 95% c.l. This supports the identification of 4FGL J1110.3–6501 as the gamma-ray counterpart of CXOU J110926.4–650224. Earlier localisations provided by the most recent Fermi data releases – namely the third and fourth data releases (DR3 and DR4) – which utilised 12 years of data, failed to establish a compatible localisation within the 95% c.l. As an additional test for the gamma-ray source localisation, we performed the fitting procedure using the Chandra X-ray coordinates as the initial reference point. This yielded results consistent with our previous analysis, which employed the 4FGL-DR4 catalogued position as the starting point.

Our analysis benefits from three additional years of data compared to DR3 and DR4. We replicated the analysis procedures that led to the compilation of the DR4 catalogue by employing a joint weighted likelihood fit and incorporating all sources from the DR4 catalogue within the RoI. The extended dataset enabled a more precise characterisation of the gamma-ray emission from 4FGL J1110.3–6501 and, consequently, a more accurate localisation of the source. We find that the gamma-ray source location aligns with the Chandra X-ray source CXOU J110926.4–650224. We anticipate that accumulating further data in the coming years will enable even more precise localisations.

To compare CXOU J110926.4–650224 to the known tMSPs, after localising the gamma-ray source, we conducted a study of its spectral energy distribution (SED). Figure 5 displays the SED extracted using three equally spaced logarithmic energy bins covering the 1–10 GeV energy range. The lower panel of Fig. 5 shows the TS value for each energy bin. In this analysis, we aimed to balance achieving significant detections with characterising the spectral shape. As a result, we obtained two
detections above the TS = 9 threshold and an upper limit just below the threshold.

The source appears relatively faint in the gamma-ray band, making it challenging to detect any time variability. The count spectrum is well-modelled by a simple power law with a photon index of $\Gamma = 2.5 \pm 0.1$. Notably, known tMSPs (e.g. PSR J1023+0038, XSS J1227-4853, and IGR J1824-2452) exhibit spectral curvature at energies below 1 GeV (Abdollahi et al. 2020; Ballet et al. 2023). In the case of CXOU J110926.4-650224, identifying a similar curvature is difficult due to its faintness. Nonetheless, we tested the presence of curvature in the SED using the *curvature()* command implemented in the *FERMIPY* suite. This algorithm evaluates the improvement in the fit when the source spectral model is changed from a power law (the baseline hypothesis, corresponding to $L_0$ in Eq. (1)) to either a LogParabola or a power law with a high-energy exponential cutoff (the spectral model commonly used to characterise pulsar spectra). In both cases, the TS values were below 2 (~1.1 and ~1.6, respectively), indicating no significant curvature in the SED. Therefore, although the gamma-ray flux and photon index of our source are comparable to those of confirmed tMSPs, we cannot draw any conclusions regarding the curvature of the SED. We anticipate that additional years of data collection and further refinement of background model components will enable us to test for the presence of spectral curvature as observed in other members of this class.

Figure 6 shows the ratio between gamma-ray and X-ray luminosities as a function of gamma-ray luminosity in the sub-luminous disc state. These luminosities represent average values and do not differentiate between emission modes. The luminosities for CXOU J110926.4-650224 are calculated based on an assumed distance of 4 kpc, derived from the best-fitting parallax value obtained from Gaia data (Gaia Collaboration 2016; Gavras et al. 2023). We emphasise that this calculation does not account for the uncertainties associated with the parallax measurements; hence, caution should be exercised (see Coti Zelati et al. 2019 for details). Taken at face value, this estimated distance is greater than those of other known tMSPs, except for IGR J18245-2452. The figure reveals that the two known tMSPs with well-established gamma-ray counterparts are the brightest in the gamma-ray band among the sources examined. In contrast, CXOU J110926.4-650224 exhibits the lowest gamma-ray-to-X-ray luminosity ratio, being brighter in the X-ray band than in the gamma-ray band. Establishing a correlation between these variables is challenging due to the small sample size. With a gamma-ray luminosity of $7.1 \times 10^{33}$ erg s$^{-1}$ in the 0.1-300 GeV range, CXOU J110926.4-650224 is fainter than all known tMSPs; only the candidate 4FGL J0427.8-6704 is less luminous. Notably, the 4FGL J0427.8-6704 is also known to exhibit flaring behaviour (Kennedy et al. 2020). The new estimation of the gamma-ray luminosity based on the aforementioned distance assumption is lower by a factor of 2 compared to the value estimated by Coti Zelati et al. (2019). In that study, the gamma-ray luminosity was derived from the FL8Y catalogue and was therefore considered as a preliminary estimate. Notably, the power-law index remains virtually unchanged.

The identification of a gamma-ray counterpart associated with an X-ray source that is a candidate tMSP marks a significant step in classifying the true nature of the source. However, this finding does not yet nail down CXOU J110926.4-650224 as a tMSP, as final confirmation requires detecting a state transition between a radio pulsar state and an X-ray state (either a sub-luminous or accreting outburst state). If CXOU J110926.4-650224 is confirmed to be a tMSP, it would stand out as either the

<figure><img src="image_8.png" /><figcaption>Fig. 6. Gamma-ray (0.1–300 GeV)-to-X-ray luminosity (0.3–79 keV) ratio in the sub-luminous disc state as a function of the gamma-ray luminosity for known and candidate tMSPs. The green data points represent the two confirmed tMSPs with identified gamma-ray counterparts. The red data points represent candidate tMSPs, including CXOU J110926.4-650224. The gamma-ray luminosity for CXOU J110926.4-650224 was calculated assuming a distance of 4 kpc. X-ray fluxes and distances are taken from Coti Zelati et al. (2019). The gamma-ray fluxes of 4FGL J1023.7+0038 and 4FGL J1227.1-4437 are taken from Torres et al. (2017), while the fluxes for 4FGL J1544.5-1126 and 4FGL J0427.8-6704 are estimated from the latest *Fermi*-LAT DR4 (Ballet et al. 2023). The gamma-ray flux of CXOU J110926.4-650224 (4FGL J1110.3-6501 in the *Fermi*-LAT catalogue) is derived in this work.</figcaption></figure>

most distant or one of the faintest tMSPs known to date, given the currently small sample.

## 5. Conclusions

We have presented the results of the analysis of 15 years of *Fermi*-LAT data of the gamma-ray source 4FGL J1110.3-6501. Our goal was to establish a spatial connection with the candidate tMSP CXOU J110926.4-650224, which is currently in an active sub-luminous X-ray state. By modelling the gamma-ray background of the region around 4FGL J1110.3-6501, we detected the gamma-ray source with a TS of ~40. We determined a new location for the gamma-ray source that is more accurate than the previously catalogued position and aligns with the position derived from *Chandra* data at the 95% c.l. Additionally, we extracted a binned SED that shows two significant detections above the $3\sigma$ c.l. and an upper limit. The source spectrum is modelled by a power law with a photon index of $\Gamma = 2.5 \pm 0.1$.

Our findings underscore the critical importance of conducting detailed analyses on individual unidentified *Fermi*-LAT sources to accurately pinpoint their locations and conclusively identify possible counterparts at lower energies. In particular, linking an unidentified *Fermi*-LAT source with a variable X-ray source is a highly effective method for detecting new, faint tMSPs. Following the launch of the Einstein Probe mission (Yuan et al. 2022), the research prospect in the field of tMSPs and related systems appears promising. This X-ray mission is set to conduct a comprehensive all-sky X-ray survey with unmatched sensitivity in the soft X-ray range. When combined with the expected increase in the number of unidentified gamma-ray sources detected by *Fermi* in the coming years, we anticipate the discovery of more such astrophysically compelling sources.

*Acknowledgements.* We thank Nestor Mirabal for the help and suggestions with the data analysis. We thank Colin Clark, David Smith, Toby Burnett, Philippe
Bruel, Xian Hou, and Matthew Kerr from the *Fermi* collaboration for the feedback on this work. We acknowledge financial contribution from the agreement ASI-INAF n.2017-14-H.0 and INAF mainstream (PI: A. De Rosa, T. Belloni), from the HERMES project financed by the Italian Space Agency (ASI) Agreement n. 2016/13 U.O and from the ASI-INAF Accordo Attuativo HERMES Technologic Pathfinder n. 2018-10-H.1-2020. We also acknowledge support from the European Union Horizon 2020 Research and Innovation Frame- work Programme under grant agreement HERMES-Scientific Pathfinder n. 821896 and from PRIN-INAF 2019 with the project "Probing the geometry of accretion: from theory to observations" (PI: Belloni). FCZ, AMarino and NR are supported by the H2020 ERC Consolidator Grant "MAGNESIA" under grant agreement No. 817661 (PI: Rea) and from grant SGR2021-01269 (PI: Graber/Rea). FCZ is supported by a Ramón y Cajal fellowship (grant agreement RYC2021-030888-1). This work was also partially supported by the program Unidad de Excelencia Maria de Maeztu CEX2020-001058-M. DFT acknowledges support from grants PID2021-124581OB-I00 funded by MCIN/AEI/10.13039/501100011033 and SGR2021-00426. The Fermi-LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA and DOE (United States), CEA/Irfu and IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK, and JAXA (Japan), and the K.A. Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from INAF (Italy) and CNES (France) is also gratefully acknowledged. This work was performed in part under DOE Contract DE-AC02-76SF00515.

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