... box

Based on observations made with telescopes at the Centro Astronómico Hispano Alemán (1.23 m + 3.50 m), at the Observatorio del Roque de los Muchachos (NOT + WHT), at the United States Naval Observatory (1.00 m) and at the Russian Academy of Sciences's Special Astrophysical Observatory (6.05 m). The NOT is operated on the island of San Miguel de la Palma jointly by Denmark, Finland, Iceland, Norway and Sweden, in Spain's Observatorio del Roque de los Muchahos of the Instituto de Astrofísica de Canarias. The Centro Astronómico Hispano Alemán is operated in Calar Alto by the Max-Planck Institut für Astronomie of Heidelberg, jointly with Spain's Comisión Nacional de Astronomía.

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... SExtractor

http://terapix.iap.fr/soft/sextractor/

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... limits

We have assumed $K^\prime\simeq K_{\rm S}$.

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... ratio

The optical to X-ray flux ratio ($f_{\rm0X}$) is defined as the R band flux (or upper limit), in units of $\mu$Jy, divided by the 1.6-10 keV X-ray flux, in units of 10^-13 cg (de Pasquale et al. 2003).

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