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Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent lowmass X-ray binaries (qLMXBs). This has motivated us to calculate new hydrogen-atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and selfirradiation by photons from the compact object. We have constructed a self-consistent grid of neutron star models covering a wide range of surface gravities as well as effective temperatures, which we make available to the scientific community.We present multi-epoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fit by our hydrogen-atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 M ⊙ is assumed, our spectral fits require that its radius be in the range R ns = 14.5 +1.8 −1.6 km (90% confidence), larger than expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of M ns = 2.20 +0.03 −0.16 M ⊙ is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.

The spectra of the first galaxies and quasars in the Universe should be strongly absorbed shortward of their rest-frame Lyα wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Lyα line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape due to the Hubble expansion of the surrounding intergalactic HI. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyα line emission around high redshift sources, before the intergalactic medium was reionized. Typically, the Lyα photons emitted by a source at z s ∼ 10 scatter over a characteristic angular radius of ∼ 15 ′′ around the source and compose a line which is broadened and redshifted by ∼ 10 3 km s −1 relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyα halos around high redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales where the Hubble flow is smooth and the gas is neutral, the Lyα brightness distribution can be used to determine the cosmological mass densities of baryons and matter.