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-rayphotons. I find that a small amount of normal matter at the quark surface withtemperature in the range $10^7\la T_{_S}} \ll mc^2/k\simeq 6\times 10^9$ K isenough to produce X-rays with high luminosity, $L_X\simeq 10^{32}-10^{34}(\Delta M/10^{-22}M_\odot)^2 erg s^{-1}$. For the total atmosphere mass$\Delta M\sim (10^{-20}-10^{-19})M_\odot$, this luminosity may be as high asthe Eddington limit. The mean energy of X-ray photons which are radiated fromsuch 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 areradiated from the surface of both a neutron star and a strange star with amassive normal-matter envelope, $\Delta M\sim 10^{-5}M_\odot$, for a fixedtemperature at the stellar core. This raises the possibility that some blackhole candidates with hard X-ray spectra are, in fact, such strange stars with alow-mass atmosphere. The X-ray emission from single strange stars is estimated.Comment: Accepted for publication in The Astrophysical Journal Letters, AAS LATEX macroc v4.0, 9 page