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Published July 2015 | Submitted + Published
Journal Article Open

The VLT-FLAMES Tarantula survey. XX. The nature of the X-ray bright emission-line star VFTS 399

Abstract

Context. The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain. Aims. One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity − in this study we aim to determine its physical nature and the cause of this behaviour. Methods. To accomplish this we performed a time-resolved analysis of optical, infrared and X-ray observations. Results. We found VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select He i lines − taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star. Conclusions. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor, sharing many observational characteristics with classical Be X-ray binaries. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass ≳25 M_⊙ for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating "single" O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population.

Additional Information

© 2015 ESO. Received 18 June 2014. Accepted 16 February 2015. Published online 17 July 2015. Based on observations collected at the European Southern Observatory under program ID 182.D-0222. We thank the referee and Danny Lennon and Ignacio Negueruela for their insightful comments which have greatly improved the paper. S.d.M. acknowledges support by NASA through and Einstein Fellowship grant, PF3-140105. STScI is operated by AURA, Inc. under NASA contract NASA 5-26555. L.K.T. and P.S.B. were supported by Chandra X-ray Observatory general observer grants GO4-15131X and GO5-6080X and by the Penn State ACIS Instrument Team Contract SV4-74018, issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. The Guaranteed Time Observations included here were selected by the ACIS Instrument Principal Investigator, Gordon P. Garmire, of the Huntingdon Institute for X-ray Astronomy, LLC, which is under contract to the Smithsonian Astrophysical Observatory; Contract SV2-82024.

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Published - aa24427-14.pdf

Submitted - 1503.00930v1.pdf

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August 20, 2023
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