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

Polarization of Thermal X-rays from Isolated Neutron Stars

Since the opacity of a magnetized plasma depends on polarization of radiation, the radiation emergent from atmospheres of neutron stars with strong magnetic fields is expected to be strongly polarized. The degree of linear polarization, typically ~10-30%, depends on photon energy, effective temperature and magnetic field. The spectrum of polarization is more sensitive to the magnetic field than the spectrum of intensity. Both the degree of polarization and the position angle vary with the neutron star rotation period so that the shape of polarization pulse profiles depends on the orientation of the rotational and magnetic axes. Moreover, as the polarization is substantially modified by the general relativistic effects, observations of polarization of X-ray radiation from isolated neutron stars provide a new method for evaluating the mass-to-radius ratio of these objects, which is particularly important for elucidating the properties of the superdense matter in the neutron star interiors.Comment: 7 figures, to be published in Ap

Space-Time Description of Scalar Particle Creation by a Homogeneous Isotropic Gravitational Field

We give the generalization of the method of the space-time description of particle creation by a gravitational field for a scalar field with nonconformal coupling to the curvature. The space-time correlation function is obtained for a created pair of the quasi-particles, corresponding to a diagonal form of the instantaneous Hamiltonian. The case of an adiabatic change of the metric of homogeneous isotropic space is analyzed. We show that the created pairs of quasi-particles in de Sitter space should be interpreted as pairs of virtual particles.Comment: 7 pages, 3 figure

GEMINGA'S SOFT X-RAY EMISSION AND THE STRUCTURE OF ITS SURFACE

We present a model to explain the decrease in the amplitude of the pulse profile with increasing energy observed in Geminga's soft X-ray surface thermal emission. We assume the presence of plates surrounded by a surface with very distinct physical properties: these two regions emit spectra of very distinct shapes which present a crossover, the warm plates emitting a softer spectrum than the colder surrounding surface. The strongly pulsed emission from the plates dominates at low energy while the surroundings emission dominates at high energy, producing naturally a strong decrease in the pulsed fraction. In our illustrative example the plates are assumed to be magnetized while the rest of the surface is field free. This plate structure may be seen as a schematic representation of a continuous but very nonuniform distribution of the surface magnetic field or as a quasi realistic structure induced by past tectonic activity on Geminga.Comment: 10 pages, AASTeX latex, + 3 figures (compressed 7 uuencoded). Submitted to Ap. J. Let

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

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

Equation of state and opacities for hydrogen atmospheres of magnetars

The equation of state and radiative opacities of partially ionized, strongly magnetized hydrogen plasmas, presented in a previous paper [ApJ 585, 955 (2003), astro-ph/0212062] for the magnetic field strengths 8.e11 G < B < 3.e13 G, are extended to the field strengths 3.e13 G < B < 1.e15 G, relevant for magnetars. The first- and second-order thermodynamic functions and radiative opacities are calculated and tabulated for 5.e5 < T < 4.e7 K in a wide range of densities. We show that bound-free transitions give an important contribution to the opacities in the considered range of B in the outer neutron-star atmosphere layers. Unlike the case of weaker fields, bound-bound transitions are unimportant.Comment: 7 pages, 6 figures, LaTeX using emulateapj.cls (included). Accepted by Ap

High-energy threshold reaction rates on 0.8 GeV proton-irradiated thick Pb-target

This works presents results of activation-aided determination of threshold reaction rates in 92 209Bi, natPb, 197Au, 181Ta, 169Tm, natIn, 93Nb, 64Zn, 65Cu, 63Cu, 59Co, 19F, and 12C samples and in 121 27Al samples. All the samples were aligned with the proton beam axis inside and outside the demountable 92-cm thick Pb target of 15-cm diameter assembled of 23 4-cm thick discs. The samples were placed on 12 target disks to reproduce the long axis distribution of protons and neutrons. In June 2006, the target was exposed for 18 hours to a 800-MeV proton beam extracted from the ITEP U-10 accelerator. The proton fluence and the proton beam shape were determined using the 27Al(p,x)7Be monitor reaction. The reaction rates were determined by the direct gamma-spectrometry techniques. In total, 1196 gamma-spectra have been measured, and about 1500 reaction rates determined. The measured reaction rates were simulated by the MCNPX code using the following databases: ENDF/B6 for neutrons below 20 MeV, MENDL2 for 20-100 MeV neutrons, and MENDL2P for proton cross sections up to 200 MeV. An acceptable agreement of simulations with experimental data has been found.Comment: 4 pages, 5 figures, 3 tables, only pdf file, to be published in Proc. Int. Conf. on Nucl. Data for Sci. and Technology (ND2007), Nice, France, April 22-27, 200