Unveiling the X-ray point source population of the Young Massive Cluster Westerlund 1 ^*,**

¹ Department of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK e-mail: s.clark@open.ac.uk
² Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
³ Departamento. de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain
⁴ National Research Council, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton BC. V2A 6K3, Canada
⁵ Department of Physics & Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
⁶ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China

Received: 29 January 2007
Accepted: 3 October 2007

Abstract

Aims.We investigate the nature of the X-ray point source population within the Young Massive Cluster Westerlund 1.

Methods.Chandra observations of 18 ks and 42 ks were used to determine the X-ray properties of emitters within Wd 1, while a comprehensive multiwavelength dataset was employed to constrain their nature.

Results.We find X-ray emission from a multitude of different stellar sources within Wd 1, including both evolved high mass and low mass pre-MS stars. We attribute the X-ray emission from the high mass component to both single stars and colliding wind binaries on the basis of their observed flux and spectral properties, with binaries being systematically harder and more luminous than single stars. We are able to infer a high binary fraction for both WN (10/16) and WC stars (7/8), resulting in a combined Wolf Rayet binary fraction of 70%. These represent the most stringent limits currently placed on the binary fraction of very massive (>45 ) stars. We place the first observational constraints on X-ray emission from stars transitioning between the Main Sequence and Wolf Rayet phases, finding that both hot (B hypergiants) and cool (yellow hypergiants and red supergiants) spectral types appear to be intrinsically X-ray faint. The B[e] star W9 is found to be X-ray bright and shows similarities to both the X-ray binary SS433 and the Luminous Blue Variable η Carinae. Globally, we find the point source population to be systematically fainter than those found in younger massive star forming regions such as NGC 3603 and R136/30 Doradus, consistent with a loss of the most massive stars to SNe and a reduction in emissivity from the low mass pre-Main Sequence stars. No unambiguous evidence for X-ray emission due to accretion onto relativistic objects of any mass is found, although the current data do not exclude the presence of either a High Mass X-ray Binary or an Intermediate Mass Black Hole accreting at a low rate. Finally, we suggest the progenitor mass for the magnetar CXOU J164710.2-455216 is comparable to that of SGR 1806-20 (~55 ), while that for SGR 1900+14 appears significantly lower (~15 ), implying that magnetars may form from stars with a wide range of initial masses.