We have examined the soft X-ray plus optical/UV spectrum of the nearby isolated neutron star RX J1856 − 3754, comparing with detailed models of a thermally emitting surface. Like previous investigators, we find the spectrum is best fit by a two-temperature blackbody model. In addition, our simulations constrain the allowed viewing geometry from the observed pulse fraction upper limits. These simulations show that RX J1856 − 3754 is very likely to be a normal young pulsar, with the non-thermal radio beam missing Earth's line of sight. The SED limits on the model parameter space put a strong constraint on the star's M/R. At the measured parallax distance, the allowed range for M NS = 1.5M ⊙ is R NS = 13.7 ± 0.6 km. Under this interpretation, the EOS is relatively stiff near nuclear density and the 'Quark Star' EOS posited in some previous studies is strongly excluded. The data also constrain the surface T distribution over the polar cap.

We consider the e †ect of rapid rotation on the light curves of neutron stars with hot polar caps. For P B 3 ms spin periods, the pulse fractions can be as much as an order of magnitude larger than with simple, slowly rotating (Schwarzschild) estimates. Doppler boosting, in particular, leads to characteristic distortion and "" soft lags ÏÏ in the pulse proÐles, which are easily measurable in light curves with moderate energy resolution. With D105 photons it should also be possible to isolate the more subtle distortions of light travel time variations and frame dragging. Detailed analysis of high-quality millisecond pulsar data from upcoming X-ray missions must include these e †ects.

Abstract. We present a new grid of LTE model atmospheres for weakly magnetic (B < ∼ 10 10 G) neutron stars, using opacity and equation of state data from the OPAL project and employing a fully frequency-and angle-dependent radiation transfer. We discuss the differences from earlier models, including a comparison with a detailed NLTE calculation. We suggest heating of the outer layers of the neutron star atmosphere as an explanation for the featureless X-ray spectra of RX J1856.5-3754 and RX J0720.4-3125 recently observed with Chandra and XMM.

We report on Chandra imaging observations of the Galactic Unidentified γ-ray source GEV J1809-2327, comparing the X-ray images with new VLA 1.46 GHz and 4.86 GHz maps. The X-ray images reveal a point source connected to a non-thermal X-ray/radio nebula, supporting a pulsar/wind model for the γ-ray emitter. We also detect numerous X-ray sources from the young stellar association in the adjacent HII region S32.

No abstract