Discovery of the neutron star spin and a possible orbital period from the Be/X-ray binary IGR J05414-6858 in the LMC^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: rsturm@mpe.mpg.de
² School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
³ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 22 February 2012
Accepted: 29 April 2012

Abstract

Context. The number of known Be/X-ray binaries in the Large Magellanic Cloud is small compared to the observed population of the Galaxy or the Small Magellanic Cloud. The discovery of a system in outburst provides the rare opportunity to measure its X-ray properties in detail.

Aims. IGR J05414-6858 was discovered in 2010 with INTEGRAL and was found in another outburst with the Swift satellite in 2011. To characterise the system, we analysed the data from a follow-up XMM-Newton target of opportunity observation of the 2011 outburst and investigated the stellar counterpart with photometry and spectroscopy.

Methods. We modelled the X-ray spectra from the EPIC instruments on XMM-Newton and compared them with Swift archival data. We searched for periodicities and variability in the X-ray and optical light curves. The optical counterpart was classified using spectroscopy obtained with ESO’s Faint Object Spectrograph at NTT.

Results. The X-ray spectra as seen in 2011 are relatively hard with a photon index of  ~0.3−0.4 and show only low absorption. They deviate significantly from earlier spectra of a probable type-II outburst in 2010. The neutron star spin period of P_spin = 4.4208 s was discovered with EPIC-pn. The I-band light curve revealed a transition from a low to a high state around MJD 54 500. The optical counterpart is classified as B0-1 IIIe and shows Hα emission and a variable near-infrared excess that vanishes during the 2010 outburst. In the optical high state, we found a periodicity at 19.9 days, probably caused by binarity and indicating the orbital period.