Magnetic hydrogen atmosphere models and the neutron star RX J1856.5-3754

Abstract

RX J1856.5-3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5-3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density N H ≈ 1 × 1020 cm-2 and an emitting area with R∞ ≈ 17 km (assuming a distance to RX J1856.5-3754 of 140 pc), temperature T∞ ≈ 4.3 × 105 K, gravitational redshift zg ∼ 0.22, atmospheric hydrogen column yH ≈ 1 g cm-2, and magnetic field B ≈ (3-4) × 1012 G; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5-3754. © 2007 RAS.