The cooling history of a quark star in the color superconductive phase is
investigated. Here we specifically focus on the 2-flavour color (2SC) phase
where novel process of photon generation via glueball (GLB) decay have been
already investigated (Ouyed & Sannino 2001). The picture we present here can in
principle be generalized to quark stars entering a superconductive phase where
similar photon generation mechanisms are at play. As much as 10^{45}-10^{47}
erg of energy is provided by the GLB decay in the 2SC phase. The generated
photons slowly diffuse out of the quark star keeping it hot and radiating as a
black-body (with possibly a Wien spectrum in gamma-rays) for millions of years.
We discuss hot radio-quiet isolated neutron stars in our picture (such as RX
J185635-3754 and RX J0720.4-3125) and argue that their nearly blackbody spectra
(with a few broad features) and their remarkably tiny hydrogen atmosphere are
indications that these might be quark stars in the color superconductive phase
where some sort of photon generation mechanism (reminiscent of the GLB decay)
has taken place. Fits to observed data of cooling compact stars favor models
with superconductive gaps of Delta_2SC = 15-35 MeV and densities
rho_2SC=(2.5-3.0)rho_N (rho_N being the nuclear matter saturation density) for
quark matter in the 2SC phase. If correct, our model combined with more
observations of isolated compact stars could provide vital information to
studies of quark matter and its exotic phases.Comment: 7 journal pages, 4 figures, accepted for publication in MNRAS (more
discussions on photon cooling versus neutrino cooling before and after
pairing of quarks