Thermal emission from hot bare strange stars and their observational appearance

Vladimir Usov

Weizmann Institute


Strange stars made almost entirely of deconfined quarks have long been proposed as a possible alternative to neutron stars. Strange quark matter with the density of ~ 5x1014 g cm-3 might exist up to the surface of strange stars. Such bare strange stars (BSSs) differ qualitatively from neutron stars which have the density at the surface (more exactly at the photosphere) of about 0.1-1 g cm-3. This opens observational possibilities to distinguish BSSs from neutrons stars, if indeed the formers exist. We present the results of calculations of the thermal emission of photons and electron-positron pairs from the surface of a hot BSS. Since strange quark matter at the surface of a BSS is bound via strong interaction rather than gravity, such a star is not subject to the Eddington limit in contrast to a neutron star, and its thermal luminosity in photons and pairs may be up to ~ 1052 ergs/s or even more. Using the thermal emission from the surface of a BSS as a boundary condition, we consider numerically the structure of pair winds and the emerging emission from BBSs for total luminosities of L = 1034-1042 ergs/s. We find that for L > 2x1035 ergs/s, photons dominate the emerging emission. As L increases from ~ 1034 to 1042 ergs/s, the mean energy of emergent photons decreases from ~ 400 keV to ~ 40 keV, as the spectrum changes in shape from that of a wide annihilation line to nearly a blackbody spectrum with a high energy (> 100 keV) tail. These results are pertinent to the deduction of the outside appearance of hot BSSs, which might help discern them from neutron stars. Some criteria are suggested for a compact object to be considered as a BSS candidate. Soft gamma-ray repeaters are among such candidates. The bursting activity of a soft gamma-ray repeater may be explained by fast heating of the surface of a BSS up to the temperature of ~ (1-3) x 109 K and its subsequent thermal emission.

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