The XMM-Newton survey of the Small Magellanic Cloud: discovery of the 11.866 s Be/X-ray binary pulsar XMMU J004814.0-732204(SXP11.87)

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 South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa
⁴ University of Maryland, Baltimore County, Mail Code 662, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁶ University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW1797, Australia
⁷ Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, 15784 Zografos, Athens, Greece
⁸ Foundation for Research and Technology Hellas, IESL, Greece
⁹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
¹⁰ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 21 September 2010
Accepted: 6 November 2010

Abstract

Aims. One of the goals of the XMM-Newton survey of the Small Magellanic Cloud is to study the Be/X-ray binary population. During one of our first survey observations, a bright new transient  −  XMMU J004814.0-732204 −  was discovered.

Methods. We present the analysis of the EPIC X-ray data, together with optical observations, to investigate the spectral and temporal characteristics of XMMU J004814.0-732204.

Results. We found coherent X-ray pulsations in the EPIC data with a period of (11.86642 ± 0.00017) s. The X-ray spectrum can be modelled by an absorbed power law with an indication for a soft excess. Depending on the modelling of the soft X-ray spectrum, the photon index ranges between 0.53 and 0.66. We identify the optical counterpart as a B = 14.9 mag star that was monitored during the MACHO and OGLE-III projects. The optical light curves show regular outbursts by  ~0.5 mag in B and R and up to 0.9 mag in I, which repeat on a time scale of about 1000 days. The OGLE-III optical colours of the star are consistent with an early B spectral type. An optical spectrum obtained at the 1.9 m telescope of the South African Astronomical Observatory in December 2009 shows Hα emission with an equivalent width of 3.5  ±  0.6 Å.

Conclusions. The X-ray spectrum and the detection of pulsations suggest that XMMU J004814.0-732204is a new high-mass X-ray binary pulsar in the SMC. The long term variability and the Hα emission line in the spectrum of the optical counterpart identify it as a Be/X-ray binary system.