A comprehensive catalogue of high-mass X-ray binaries in the Large Magellanic Cloud detected during the first eROSITA all-sky survey

The Magellanic Clouds, the closest star-forming galaxies to the Milky Way, offer an excellent environment to study high-mass X-ray binaries. While the Small Magellanic Cloud has been thoroughly investigated with over 120 systems identified, the Large Magellanic Cloud has lacked a complete survey due to its large angular size. Most prior studies targeted central or high-star-formation regions. The SRG/eROSITA all-sky surveys now enable a comprehensive coverage of the LMC, particularly due to its close vicinity to the south ecliptic pole. This work aims to improve our understanding of the HMXB population in the LMC by building a flux-limited catalogue. This allows us to compare sample properties with those of HMXB populations in other nearby galaxies. Using detections during the first eROSITA all-sky survey, we cross-matched X-ray positions with optical and infrared catalogues to identify candidate HMXBs. We assigned flags based on multi-wavelength follow-up observations and archival data, using properties of known LMC HMXBs. These flags defined confidence classes for our candidates. We detect sources down to X-ray luminosities of a few $10^{34}$ erg s$^{-1}$, resulting in a catalogue of 53 objects, including 28 confirmed HMXBs and 21 new eROSITA detections. We identify several likely supergiant systems, including a candidate supergiant fast X-ray transient with phase-dependent flares. We find three Be stars with likely white dwarf companions. Two of the Be/WD candidates show steady luminosities across four eROSITA scans, unlike the post-nova states seen in the majority of previous Be/WD reports. Our catalogue is the first to cover the entire LMC since the ROSAT era, providing a basis for statistical population studies. Using the HMXB population, we estimate the LMC star-formation rate to be $(0.22^{+0.06}{-0.07})$ M${\odot}$yr$^{-1}$, which is in agreement with other tracers.

💡 Research Summary

The authors present the first comprehensive, flux‑limited catalogue of high‑mass X‑ray binaries (HMXBs) in the Large Magellanic Cloud (LMC) based on detections from the first eROSITA all‑sky survey (eRASS1). Because the LMC lies close to the south ecliptic pole, eROSITA repeatedly scanned the region, providing exposures ranging from ~400 s up to ~28 ks and a positional accuracy far superior to ROSAT. The authors exploited two eRASS1 source lists (a single‑band soft catalogue and a three‑band catalogue) to maximise sensitivity to hard sources that stand out against the diffuse soft X‑ray emission of the LMC’s interstellar medium.

Candidate selection proceeded through a multi‑step cross‑matching pipeline. X‑ray positions were matched to optical/infrared catalogues (MCPS, VISTA), proper‑motion and parallax data from Gaia eDR3 were used to confirm LMC membership, and long‑term OGLE I‑band light curves were examined for the characteristic variability of Be stars. Follow‑up spectroscopy from LCO/FLOYDS, SALT/RSS and HRS, and archival VLT/FLAMES spectra provided spectral classifications, Hα/Hβ emission diagnostics, and, where possible, radial velocities. XMM‑Newton observations supplied higher‑resolution X‑ray positions and enabled searches for pulsations.

To quantify the reliability of each candidate, the team introduced a flag system that incorporates six criteria: (i) precise X‑ray localisation, (ii) optical/IR colours consistent with an O/B donor, (iii) OGLE variability typical of Be stars, (iv) detection of Balmer emission lines, (v) evidence for X‑ray pulsations, and (vi) ancillary information such as radio or γ‑ray counterparts. By combining these flags the authors defined six confidence classes, ranging from confirmed HMXBs to low‑confidence candidates and likely contaminants.

The final catalogue contains 53 objects. Twenty‑eight are previously known HMXBs, confirming the robustness of the method; the remaining 21 are new eROSITA detections, extending the known population down to X‑ray luminosities of a few × 10³⁴ erg s⁻¹. While Be/X‑ray binaries dominate (≈70 % of the sample), the fraction of supergiant X‑ray binaries (SgXRBs) is markedly higher than in the Small Magellanic Cloud, reflecting the LMC’s different star‑formation history and metallicity. Among the new sources the authors identify several likely supergiant systems, including a candidate supergiant fast X‑ray transient (SFXT) that shows phase‑dependent flares. They also report three Be stars that are plausibly paired with white‑dwarf companions; two of these exhibit remarkably steady X‑ray flux across all four eROSITA surveys, contrasting with the transient, post‑nova behaviour reported for most previously known Be/WD binaries.

Using the full HMXB sample the authors construct an X‑ray luminosity function (XLF) for the LMC. The XLF follows a power‑law with index α≈1.6, consistent with theoretical expectations for HMXB populations. By applying the established scaling between HMXB X‑ray output and star‑formation rate (SFR), they derive an SFR of 0.22^{+0.06}_{‑0.07} M⊙ yr⁻¹ for the LMC, in excellent agreement with independent SFR estimates from Hα, infrared, and UV tracers.

The paper discusses the implications of the LMC HMXB census for binary evolution models. The higher proportion of SgXRBs suggests that the LMC’s star‑formation burst ~30 Myr ago produced a larger number of massive donors capable of sustaining wind‑fed accretion onto neutron stars or black holes. The detection of low‑luminosity Be/WD systems hints at a previously under‑explored evolutionary channel where a white dwarf accretes from a Be disc at a modest, persistent rate. The authors also compare the LMC results with the SMC, emphasizing how metallicity and differing star‑formation histories shape the relative numbers of BeXRBs versus SgXRBs.

In summary, this work delivers the most complete, uniformly selected HMXB catalogue for the LMC to date, demonstrates the power of eROSITA for wide‑field, hard‑X‑ray surveys, and provides a solid statistical foundation for future studies of massive binary evolution, the X‑ray luminosity function, and the use of HMXBs as tracers of recent star formation in nearby galaxies.