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 K ≲ T S ≪ mc 2 / k ≃ 6 × 10 9 K is enough to produce X-rays with high luminosity L X ≃ (10 32 -10 34 )[Δ M /(10 -22 M ☉ )] 2 ergs s -1 . For a total atmospheric mass Δ M ~ 10 -20 to 10 -19 M ☉ , this luminosity may be as high as the Eddington limit. The mean energy of X-ray photons that are radiated from such a low-mass atmosphere of a strange star is ~10 2 [ T S /(10 8 K)] 0.45 , ≃30-300 times greater than the mean energy of X-ray photons that are radiated from the surface of both a neutron star and a strange star with a massive normal-matter envelope, Δ M ~ 10 -5 M ☉ , 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.