The x-ray source population of the small magellanic cloud

Abstract

In this work we study the X-ray source population of the Small Magellanic Cloud (SMC). The SMC is our second nearest star-forming galaxy, which allows us to identify the optical counterparts of the X-ray binary population (XRB) and therefore directly study their link with star-formation (SF). Using Chandra, XMM-Newton and optical photometric catalogs we study the young XRB population of this galaxy, down to luminosity levels of quiescent X-ray source populations. The Chandra observations in particular cover the central, densest and most active region of the SMC, while the XMM-Newton survey is designed to probe regions of different stellar populations (10? 500Myr). Based on the comparison of these observations with comprehensive stellar catalogs of the SMC, we propose for the first time 25 candidate High Mass X-ray Binaries, of which 16 are candidate Be X-ray binaries (Be-XRBs; 9 new candidates in the Chandra survey and 7 in the XMM-Newton fields). Moreover, we confirm previous classification of Be-XRBs for 19 sources (18 in Chandra, and 1 in the XMM-Newton survey). We also examine the “overabudance” of Be-XRBs in the SMC fields with respect to the Milky Way. In luminosities down to ? 1034 erg s?1 , we find that Be-XRBs are ? 2 times more common in the SMC when compared to the Milky Way even after taking into account the difference in the formation rates of OB stars. This residual excess can be attributed to the lower metallicity of the SMC. Finally, we find that the mixing of Be-XRBs with other than their natal stellar population is not an issue in our comparisons of Be-XRBs and stellar populations in the SMC. Instead we find indication for variation of the XRB populations of the SMC on kiloparsec scales, related to local variations of the formation rate of OB stars and slight variation of their age, which results in different relative numbers of Be stars and therefore XRBs. Comparison of the Be-XRB populations with their local star-formation history shows that they are observed in regions with SF rate bursts ? 30?70 Myr ago. The age range coincides with the age of maximum Be-star formation, based on independent studies of Galactic and SMC Be stars. We also find a strong correlation between the number of Be-XRBs and the strength of the SF at the age of maximum Be-star formation (i.e. ? 40 Myr ago), while regions with strong but more recent SF (e.g. the Wing) are deficient in Be-XRBs. Regions that host HMXBs, which however have not been proved yet to be Be-XRBs, show a similar SF history (with a significant burst at ? 40 Myr ago). We thus conclude that the majority of the optically unidentified SMC HMXBs have a Be nature. The very strong correlation of the young X-ray sources with the stellar population in scales of a few arcminutes (? 0.5kpc) provides strong evidence for relatively small supernova kicks during the formation of the compact object. Finally, we have used the 2dF (Two-degree Field) spectrograph on the 3.9m Anglo-Australian Telescope (AAT) to confirm the classification of candidate Be-XRBs as emission line stars and identify their spectral type and luminosity class when possible. Our sample is selected from the Chandra survey and existing XMM-Newton observations of the SMC. These observations show that the distribution of spectral types of Be-XRBs in the SMC agrees with that of the LMC, while it is in contrast to the population of Be- XRBs in the Milky Way. In addition we confirmed the different distributions of spectral type of Be stars in XRB systems and individual Be stars. With these spectra we also discover the second supergiant X-ray binary in the SMC. Overall this study demonstrates the importance of the Be-XRBs as a dominant component of young XRB populations, and shows evidence for strong evolution of the X-ray source populations of ages < 100Myr and different metallicities.