Swift J0513.4-6547 = LXP 27.2 : a new Be/X-ray binary system in the Large Magellanic Cloud

TL;DR

This paper reports the discovery and detailed analysis of a new Be/X-ray binary system in the Large Magellanic Cloud, including its timing, optical properties, and magnetic field, revealing new insights into such systems.

Contribution

It presents the first detailed timing, optical, and magnetic field measurements of a new Be/X-ray binary in the LMC, highlighting discrepancies in mass estimates and magnetic field strengths.

Findings

Orbital period of 27.4 days determined from optical modulation.

Neutron star magnetic field estimated at ~10^13 G, higher than typical.

Dynamical mass estimate for the Be star does not match expected for B1V stars.

Abstract

An exceptionally bright new X-ray source in the Large Magellanic Cloud was discovered by the Swift/BAT telescope on MJD 54923 (2 April 2009), and shown to have a pulse period of 27s using follow-up observations by RXTE/PCA. We report here on detailed timing observations taken over the following weeks using Fermi/GBM which reveal an excellent orbital solution and indicate that the source flux peaked at $\sim10^{38}$ erg s$^{-1}$. In addition, we report on follow-up optical observations (spectroscopic and photometric) which permit a classification of the mass donor star as B1Ve, and furthermore reveal a strong optical modulation at a period consistent with the binary period found from the Fermi/GBM data - 27.4 d. The dynamical mass estimate for the Be star is not in agreement with that expected for a B1V star - a mismatch similar to this has been previously reported for the few other Magellanic Cloud stars that also have dynamical mass estimates. In addition, the neutron star magnetic field determination from the X-ray data ($\sim10^{13}$ G) adds further evidence to the possibility that many such stars in High Mass X-ray Binary systems may have magnetic fields greater than previously expected ($\sim10^{12}$ G).