The observational appearance of strange stars

Strange stars that are entirely made of strange quark matter (SQM) have been long ago proposed as an alternative to neutron stars (e.g., [1, 2]). The possible existence of strange stars is a direct consequence of the conjecture that SQM composed of roughly equal numbers of up, down, and strange quarks plus a smaller numbers of electrons (to neutralize the electric charge of the quarks) may be the absolute ground state of the strong interaction, i.e., absolutely stable with respect to Fe [1, 3]. The bulk properties (size, moment of inertia, etc.) of models of strange and neutron stars in the observed mass range (1 < M/M⊙ < 2) are rather similar, and it is very difficult to discriminate between strange and neutron stars [4]. SQM with the density of ∼ 5 × 10 g cm might exist up to the surface of strange stars [2, 4]. Such a bare strange star differs qualitatively from a neutron star which has the density at the stellar surface (more exactly at the stellar photosphere) of about 0.1 − 1 g cm. This opens observational possibilities to distinguish bare strange stars from neutron stars.