Upper limit on the mass of RX J1856.5--3754 as a possible quark star
  Recent deep Chandra LETG+HRC-S observations suggest the possibility that RX J1856.5--3754 is a compact star whose radiation radius is 3.8--8.2 km. In this paper, we systematically calculate the mass-radius relations of quark stars within the bag model. Assuming that RX J1856.5--3754 is a pure quark star, we derive an upper limit on its mass for various sets of the bag-model parameters. 
Kazunori Kohri, Kei Iida, and Katsuhiko Sato