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Low-mass normal-matter atmospheres of strange stars and their radiation

The quark surface of a strange star has a very low emissivity for X-ray photons. I find that a small amount of normal matter at the quark surface with temperature in the range $10^7\la T_{_S}} \ll mc^2/k\simeq 6\times 10^9$ K is enough to produce X-rays with high luminosity, $L_X\simeq 10^{32}- 10^{34}(ΔM/10^{-22}M_\odot)^2 erg s^{-1}$. For the total atmosphere mass $ΔM\sim (10^{-20}-10^{-19})M_\odot$, this luminosity may be as high as the Eddington limit. The mean energy of X-ray photons which are radiated from such a low-mass atmosphere of a strange star is $\sim 10^2(T_S/10^8 K)^{0.45} \simeq 30-300$ times larger than the mean energy of X-ray photons which are radiated from the surface of both a neutron star and a strange star with a massive normal-matter envelope, $ΔM\sim 10^{-5}M_\odot$, for a fixed temperature at the stellar core. This raises the possibility that some black hole candidates with hard X-ray spectra are, in fact, such strange stars with a low-mass atmosphere. The X-ray emission from single strange stars is estimated.