X-ray spectral evolution of the extragalactic Z source LMC X-2

Abstract

We present the results obtained by a detailed study of the extragalactic Z source, LMC X-2, using broad band Suzaku data and a large ($ \sim 750$ ksec) data set obtained with the proportional counter array (PCA) onboard RXTE. The PCA data allows for studying the complete spectral evolution along the horizontal, normal and flaring branches of the Z-track. Comparison with previous study show that the details of spectral evolution (like variation of Comptonizing electron temperature), is similar to that of GX 17+2 but unlike that of Cyg X-2 and GX 349+2. This suggests that Z sources are heterogeneous group with perhaps LMC X-2 and GX 17+2 being member of a subclass. However non monotonic evolution of the Compton y-parameter seems to be generic to all sources. The broad band {\it Suzaku} data reveals that the additional soft component of the source modelled as a disk blackbody emission is strongly preferred over one where it is taken to be a blackbody spectrum. This component as well as the temperature of seed photons do not vary when source goes into a flaring mode and the entire variation can be ascribed to the Comptonizing cloud. The bolometric unabsorbed luminosity of the source is well constrained to be $ \sim 2.23 \times 10^{38}$ ergs/sec which if the source is Eddington limited implies a neutron star mass of 1.6 M$_\odot$. We discuss the implications of these results.