Atmospheres and Spectra of Strongly Magnetized Neutron Stars -- III. Partially Ionized Hydrogen Models

We construct partially ionized hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with surface fields B=10^12-5 \times 10^14 G and effective temperatures T_eff \sim a few \times 10^5-10^6 K. These models are based on the latest equation of state and opacity results for magnetized, partially ionized hydrogen plasmas that take into account various magnetic and dense medium effects. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars. For the models with B=10^12-10^13 G, the spectral features due to neutral atoms lie at extreme UV and very soft X-ray energy bands and therefore are difficult to observe. However, the continuum flux is also different from the fully ionized case, especially at lower energies. For the superstrong field models (B\ga 10^14 G), we show that the vacuum polarization effect not only suppresses the proton cyclotron line as shown previously, but also suppresses spectral features due to bound species; therefore spectral lines or features in thermal radiation are more difficult to observe when the neutron star magnetic field is \ga 10^14 G.Comment: 12 pages, 10 figures; ApJ, accepted (v599: Dec 20, 2003

Probing Axions with Radiation from Magnetic Stars

Recent experiments suggest that polarized photons may couple significantly to pseudoscalar particles such as axions. We study the possible observational signatures of axion-photon coupling for radiation from magnetic stars, with particular focus on neutron stars. We present general methods for calculating the axion-photon conversion probability during propagation through a varying magnetized vacuum as well as across an inhomogeneous atmosphere. Partial axion-photon conversion may take place in the vacuum region outside the neutron star. Strong axion-photon mixing occurs due to a resonance in the atmosphere, and depending on the axion coupling strength and other parameters, significant axion-photon conversion can take place at the resonance. Such conversions may produce observable effects on the radiation spectra and polarization signals from the star. We also apply our results to axion-photon propagation in the Sun and in magnetic white dwarfs. We find that there is no appreciable conversion of solar axions to photons during the propagation.Comment: 12 pages, 11 figures. Minor changes. PRD accepte

Transfer of Polarized Radiation in Strongly Magnetized Plasmas and Thermal Emission from Magnetars: Effect of Vacuum Polarization

We present a theoretical study of radiative transfer in strongly magnetized electron-ion plasmas, focusing on the effect of vacuum polarization due to quantum electrodynamics. This study is directly relevant to thermal radiation from the surfaces of highly magnetized neutron stars, which have been detected in recent years. Strong-field vacuum polarization modifies the photon propagation modes in the plasma, and induces a ``vacuum resonance'' at which a polarized X-ray photon propagating outward in the neutron star atmosphere can convert from a low-opacity mode to a high-opacity mode and vice versa. The effectiveness of this mode conversion depends on the photon energy and the atmosphere density gradient. For a wide range of field strengths, $7\times 10^{13}< B < 10^{16}$ G, the vacuum resonance lies between the photospheres of the two photon modes, and the emergent radiation spectrum from the neutron star is significantly modified by the vacuum resonance. (For lower field strengths, only the polarization spectrum is affected.) Under certain conditions, which depend on the field strength, photon energy and propagation direction, the vacuum resonance is accompanied by the phenomenon of mode collapse (at which the two photon modes become degenerate) and the breakdown of Faraday depolarization. Thus, the widely used description of radiative transfer based on photon modes is not adequate to treat the vacuum polarization effect rigorously. We study the evolution of polarized X-rays across the vacuum resonance and derive the transfer equation for the photon intensity matrix (Stokes parameters), taking into account the effect of birefringence of the plasma-vacuum medium, free-free absorption, and scatterings by electrons and ions.Comment: 19 pages with 9 figures; minor additions (mainly the at end of sec.5.2); ApJ in press (v588, n2, May 10, 2003 issue

Radiation spectra and polarization in magnetar bursts

We present Monte Carlo simulations of radiative transfer in magnetar atmospheres. We include the effects of vacuum polarization, electron and proton scattering, and free-free absorption. Simulations are performed for the atmosphere model with the magnetic field perpendicular and also tilted with respect to the neutron star surface, and we show that the average spectrum does not strongly depend on the orientation of the magnetic field. We investigate the region of the parameter space where the vacuum absorption-like feature appears in the spectrum and we analyze the shape of the proton cyclotron line. Our results indicate that the existence of the vacuum polarization feature should be a general attribute of soft gamma-ray repeaters burst spectra, provided that the energy release takes place at the sufficiently dense region, and the atmosphere scaleheight is large enough. We discuss the existence of such a feature in recent observational data on these sources.Comment: submitted to Ap

Polarized X-Ray Emission from Magnetized Neutron Stars: Signature of Strong-Field Vacuum Polarization

In the atmospheric plasma of a strongly magnetized neutron star, vacuum polarization can induce a Mikheyev-Smirnov-Wolfenstein like resonance across which a X-ray photon may (depending on its energy) convert from one mode into the other, with significant changes in opacities and polarizations. We show that this vacuum resonance effect gives rise to an unique energy-dependent polarization signature in the surface emission from neutron stars. The detection of polarized X-rays from neutron stars can provide a direct probe of strong-field quantum electrodynamics and constrain the neutron star magnetic field and geometry.Comment: Slightly shortened version published in Phys. Rev. Lett. 91, 071101 (2003, Aug.15

The polarization effects of radiation from magnetized envelopes and extended accretion structures

The results of numerical calculations of linear polarization from magnetized spherical optically thick and optically thin envelopes are presented. We give the methods how to distinguish magnetized optically thin envelopes from optically thick ones using observed spectral distributions of the polarization degree and the positional angle. The results of numerical calculations are used for analysis of polarimetric observations of OB and WR stars, X-ray binaries with black hole candidates (Cyg X-1, SS 433) and supernovae. The developed method allows to estimate magnetic field strength for the objects mentioned above.Comment: 18 pages, 6 figure