Simultaneous Chandra and HST observations of the quiescent neutron-star low-mass X-ray binaries in 47 Tucanae
arxiv(2024)
摘要
We present simultaneous Chandra X-ray Observatory and Hubble Space Telescope
observations of three certain (X5, X7, W37) and two likely (X4, W17) quiescent
neutron-star low-mass X-ray binaries (qLMXBs) in the globular cluster 47 Tuc.
We study these systems in the X-ray, optical and near-ultraviolet (NUV) using
the simultaneous data and additional non-contemporaneous HST data. We have
discovered a blue and variable NUV counterpart to W17. We have not securely
identified the eclipsing qLMXB W37 in the optical or NUV. Deeper
high-resolution imaging is needed to further investigate the faint NUV excess
near the centre of the W37 error circle. We suggest that a previously
identified optical astrometric match to X7 is likely the true counterpart. The
Halpha emission and the location of the counterpart in the colour-magnitude
diagram, indicate that the secondary is probably a non-degenerate, H-rich star.
This is consistent with previous results from fitting X7's X-ray spectrum. In
X4, the simultaneous X-ray and optical behaviour supports the earlier
suggestion that the X-ray variability is driven by changes in accretion rate.
The X-ray eclipses in X5 coincide with minima in the optical/NUV light curves.
Comparison of the 47 Tuc qLMXBs with the cataclysmic variables (CVs) in the
cluster confirms that overall the qLMXBs have larger X-ray-to-optical flux
ratios. Based on their optical/NUV colors, we conclude that the accretion disks
in the qLMXBs are less prominent than in CVs. This makes the ratio of X-ray
flux to excess blue optical flux a powerful discriminator between CVs and
qLMXBs.
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