Study Identifies Companion Star Type in Pulsating Ultraluminous X-ray Binary NGC 1313 X-2
A new preprint reports that the companion star in the pulsating ultraluminous X-ray source (ULX) NGC 1313 X-2 is likely an A-type supergiant, based on archival Gemini-South spectroscopic data from 2009. The study stacked spectra from ten nights of observations, detecting a possible Balmer break below 4000 Ångströms consistent with an A-type supergiant donor star. These findings provide updated constraints on the binary's orbital parameters, accretion disc size, and the kinetic power of its wind or jet relative to accretion power.
A preprint submitted to arXiv presents deep optical spectroscopic monitoring of NGC 1313 X-2, one of the few known pulsating ultraluminous X-ray sources (ULXs), which harbor neutron stars accreting at extreme rates. Using archival long-slit data from the Gemini Multi-Object Spectrograph South (GMOS-S) spanning ten nights in 2009, the authors stacked flux-calibrated spectra and fitted them against stellar templates, identifying a possible Balmer break below 4000 Ångströms. This spectral feature is indicative of an A-type supergiant as the mass-donating companion star. From the inferred stellar radii, the team derived updated constraints on the system's orbital parameters and binary configuration. The study also examined X-ray and optical variability through lag-frequency analysis to probe the accretion disc size scale, and modelled [O III] emission line profiles to constrain the expansion rate of the surrounding gas bubble and the kinetic power of the wind or jet. Additionally, the authors discuss the binary's formation history using multi-wavelength observations, building on prior work in the field.
What's missing
The study relies solely on archival data from 2009, raising questions about whether the system's properties have evolved over the intervening ~17 years. The detection of the Balmer break is described as a 'possible' feature, and the authors do not report a formal statistical significance for the A-type supergiant classification. Independent confirmation through contemporaneous spectroscopy or higher-resolution observations is not yet available. The paper has not yet undergone peer review.
What different sources said
- arXiv astro-phCenter
Deep optical spectroscopic monitoring of the pulsating ULX NGC 1313 X-2 with longslit Gemini observations
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