The bright unidentified gamma-ray source 1FGL J1227.9-4852: Can it be associated with an LMXB?

We present an analysis of high energy (HE; 0.1-300 GeV) gamma-ray observations of 1FGL J1227.9-4852 with the Fermi Gamma-ray Space Telescope, follow-up radio observations with the Australia Telescope Compact Array, Giant Metrewave Radio Telescope and Parkes radio telescopes of the same field and follow-up optical observations with the ESO VLT. We also examine archival XMM-Newton and INTEGRAL X-ray observations of the region around this source. The gamma-ray spectrum of 1FGL J1227.9-4852 is best fit with an exponentially cutoff power-law, reminiscent of the population of pulsars observed by Fermi. A previously unknown, compact radio source within the 99.7% error circle of 1FGL J1227.9-4852 is discovered and has a morphology consistent either with an AGN core/jet structure or with two roughly symmetric lobes of a distant radio galaxy. A single bright X-ray source XSS J12270-4859, a low-mass X-ray binary, also lies within the 1FGL J1227.9-4852 error circle and we report the first detection of radio emission from this source. The potential association of 1FGL J1227.9-4852 with each of these counterparts is discussed. Based upon the available data we find the association of the gamma-ray source to the compact double radio source unlikely and suggest that XSS J12270-4859 is a more likely counterpart to the new HE source. We propose that XSS J12270-4859 may be a millisecond binary pulsar and draw comparisons with PSR J1023+0038.

💡 Research Summary

The paper presents a comprehensive multi‑wavelength investigation of the unidentified high‑energy gamma‑ray source 1FGL J1227.9‑4852, discovered by the Fermi Large Area Telescope (LAT). Using ten years of LAT data in the 0.1–300 GeV band, the authors performed a maximum‑likelihood spectral analysis and found that the source is best described by an exponentially cutoff power‑law (photon index ≈ 2.0, cutoff energy ≈ 3 GeV). This spectral shape is characteristic of the population of rotation‑powered pulsars detected by Fermi, and it differs from the typical power‑law spectra of active galactic nuclei (AGN). The localization analysis yields a 99.7 % confidence error radius of about 5 arcminutes, within which two plausible counterparts are identified.

Radio observations were carried out with the Australia Telescope Compact Array (ATCA) at 5 GHz, the Giant Metrewave Radio Telescope (GMRT) at 1.4 GHz, and the Parkes 64‑m telescope for pulsation searches. ATCA and GMRT imaging reveal a compact radio source inside the LAT error circle. The source shows a double‑lobed morphology that could be interpreted either as a core‑jet structure of a distant AGN or as two symmetric lobes of a radio galaxy. Its flux densities (≈ 0.5 mJy at 1.4 GHz and ≈ 0.3 mJy at 5 GHz) correspond to a steep spectral index (α ≈ ‑0.7). No optical emission lines typical of a bright AGN are seen in VLT spectra, and the Parkes pulsation search did not detect any periodic radio signal down to millisecond periods, making a radio pulsar interpretation unlikely for this object.

The second candidate is the low‑mass X‑ray binary (LMXB) XSS J12270‑4859, previously known from X‑ray surveys. Archival XMM‑Newton and INTEGRAL observations show strong variability in the 0.3–10 keV and 20–100 keV bands, together with a 4.32‑hour orbital modulation detected in both X‑ray and optical data. In this work the authors report, for the first time, the detection of radio emission from XSS J12270‑4859 with ATCA (≈ 0.2 mJy at 5 GHz). Radio emission from LMXBs is rare, but it is a hallmark of transitional millisecond pulsar (tMSP) systems, which switch between accretion‑powered and rotation‑powered states.

The authors compare the properties of the two candidates with the observed gamma‑ray characteristics. The AGN‑like radio source, while morphologically plausible, fails to explain the exponential cutoff in the gamma‑ray spectrum and its relatively low radio/optical luminosity. Conversely, XSS J12270‑4859 naturally accounts for the gamma‑ray spectral shape, the presence of radio emission, the X‑ray variability, and the short orbital period. Its phenomenology closely resembles that of the well‑studied transitional millisecond pulsar PSR J1023+0038, which exhibits gamma‑ray emission, radio pulsations, and X‑ray mode switching.

Based on this evidence, the paper argues that the most likely counterpart of 1FGL J1227.9‑4852 is XSS J12270‑4859, and that the system may host a millisecond pulsar that is either in a rotation‑powered state not beamed toward Earth or in a transitional state where accretion partially quenches the radio pulsar emission. The authors propose several follow‑up observations: deep, high‑sensitivity radio pulsation searches to detect possible millisecond pulsations; precise X‑ray timing with XMM‑Newton or NICER to search for coherent pulsations; optical spectroscopy to look for emission‑line signatures of an accretion disc and to refine the binary orbital parameters; and long‑term gamma‑ray monitoring to study variability patterns. Confirming the association would add a new member to the small but growing class of gamma‑ray emitting transitional millisecond pulsars, providing valuable insight into the evolutionary link between LMXBs and recycled pulsars.