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.

9 pages, 8 figures, accepted for the publicatin in MNRAS

References in corpus (3)

• RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs
• An explanation of the Z-track sources
• Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source

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