Progress of Theoretical Physics Supplement No. 151, 2003 181 Possible Quark Star RX J1856.5-3754 and Its Mass

The possibility that RX J1856.5–3754 is a compact star whose radiation radius is 3.8–8.2 km was suggested by recent deep Chandra LETG+HRC-S observations. If we assume that RX J1856.5–3754 is a pure quark star, we obtain an upper limit on its mass for various sets of the bag-model parameters [K. Kohri, K. Iida and K. Sato, Prog. Theor. Phys. 109 (2003), 765]. These days Drake et al. 2) reported that the deep Chandra LETG+HRC-S ob-servations of a soft X-ray source RX J1856.5–3754 provide the X-ray spectrum well represented by a blackbody of temperature T = 61.2 ± 1.0 eV, and the data contain neither evidence for spectral features nor pulsation. They argued that the derive