Simultaneous Chandra and HST observations of the quiescent neutron star low-mass X-ray binaries in 47 Tucanae

Author:

van den Berg M1ORCID,Rivera Sandoval L E2ORCID,Heinke C O3ORCID,Cohn H N4,Lugger P M4ORCID,Grindlay J E1,Edmonds P D1,Anderson J5,Catuneanu A36

Affiliation:

1. Center for Astrophysics ∣ Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138 , USA

2. Department of Physics and Astronomy, University of Texas Rio Grande Valley , Brownsville, TX 78520 , USA

3. Department of Physics, University of Alberta , CCIS 4-183, Edmonton, AB T6G 2E1 , Canada

4. Department of Astronomy, Indiana University , 727 E. Third Street, Bloomington, IN 47405 , USA

5. Space Telescope Science Institute , 3800 San Martin Drive, Baltimore, MD 21218 , USA

6. Dana Canada Corporation , 656 Kerr Street, Oakville, ON L6K 3E4 , Canada

Abstract

ABSTRACT 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 H α 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 colours, we conclude that the accretion discs 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.

Funder

ESO

NSERC

Publisher

Oxford University Press (OUP)

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