Highly absorbed X-ray binaries in the Small Magellanic Cloud^⋆

¹ INAF - IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica “G.Occhialini”, via Bassini 15, 20133 Milano, Italy
e-mail: novara@iasf-milano.inaf.it
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
³ School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
⁴ University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany
⁶ Department of Mathematical Sciences, University of South Africa, UNISA, Pretoria 0003, South Africa
⁷ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 31 March 2011
Accepted: 20 June 2011

Abstract

Many of the high mass X-ray binaries (HMXRBs) discovered in recent years in our Galaxy are characterized by a high absorption, most likely intrinsic to the system, that can impede their detection at the softest X-ray energies. Exploiting the good coverage obtained with sensitive XMM-Newton observations, we have undertaken a search for highly absorbed X-ray sources in the Small Magellanic Cloud (SMC), which is known to contain a large number of HMXRBs. After a systematic analysis of 62 XMM-Newton SMC observations, we obtained a sample of 30 sources with evidence of an equivalent hydrogen column density larger than 3 × 10²³ cm^-2. Five of these sources are clearly identified as HMXRBs, four being previously known (including three X-ray pulsars) and one, XMMU J005605.8–720012, being reported here for the first time. For the latter, we present optical spectroscopy confirming the association with a Be star in the SMC. The other sources in our sample have optical counterparts fainter than magnitude  ~16 in the V band, and many have possible NIR counterparts consistent with highly reddened early-type stars in the SMC. While their number is broadly consistent with the expected population of background highly absorbed active galactic nuclei, a few of them could be HMXRBs in which an early-type companion is severely reddened by local material.