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Post Info TOPIC: Cygnus X-2
Blobrana


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Cygnus X-2
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Title: On the mass of the neutron star in Cyg X-2
Authors: J. Casares, J.I. Gonzalez Hernandez, G. Israelian, R. Rebolo

We present new high resolution spectroscopy of the low mass X-ray binary Cyg X-2 which enables us to refine the orbital solution and rotational broadening of the donor star. In contrast with Elebert et al (2009) we find a good agreement with results reported in Casares et al. (1998). We measure P=9.84450 ±0.00019 day, K_2=86.5 ±1.2 km s^{-1} and V \sin i=33.7 ±0.9 km s^{-1}. These values imply q=M_{2}/M_{1}=0.34 ± 0.02 and M_{1}=1.71 ± 0.21 solar masses (for i=62.5 ± 4°). Therefore, the neutron star in Cyg X-2 can be more massive than canonical. We also find no evidence for irradiation effects in our radial velocity curve which could explain the discrepancy between Elebert et al's and our K_2 values.

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Blobrana


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Title: Swift monitoring of Cygnus X-2: investigating the NUV-X-ray connection
Authors: E. S. Rykoff (UCSB), E. M. Cackett, J. M. Miller (U. Michigan)

The neutron star X-ray binary (NSXRB) Cyg X-2 was observed by the Swift satellite 51 times over a 4 month period in 2008 with the XRT, UVOT, and BAT instruments. During this campaign, we observed Cyg X-2 in all three branches of the Z track (horizontal, normal, and flaring branch). We find that the NUV emission is uncorrelated with the soft X-ray flux detected with the XRT, and is anticorrelated with the BAT X-ray flux and the hard X-ray colour. The observed anticorrelation is inconsistent with simple models of reprocessing as the source of the NUV emission. We interpret the anticorrelation as a consequence of the high inclination angle of Cyg X-2, where NUV emission is preferentially scattered by a corona that expands as the disk is radiatively heated. In this model the NUV emission is not a good proxy for solar masses in the system. We also discuss the implications of using Swift/XRT to perform spectral modelling of the continuum emission of NSXRBs.

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