Showing papers by "Wynn C. G. Ho published in 2007"

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 consider- able 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 NH ≈ 1 × 10 20 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 × 10 5 K, gravitational redshift zg ∼ 0.22, atmospheric hydrogen column yH ≈ 1gc m −2 , and magnetic field B ≈ (3-4) × 10 12 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 pro- ducing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.

Modelling mid-Z element atmospheres for strongly magnetized neutron stars

Abstract: We construct models for strongly magnetized neutron star atmospheres composed of midZ elements (carbon, oxygen and neon) with magnetic fields B = 10 12 ‐10 13 G and effective temperatures T eff = (1 ‐ 5) × 10 6 K; this is done by first addressing the physics relevant to strongly magnetized plasmas and calculating the equation of state and polarization-dependent opacities. We then obtain the atmosphere structure and spectrum by solving the radiative transfer equations in hydrostatic and radiative equilibrium. In contrast to hydrogen opacities at the relevant temperatures, mid-Z element opacities are dominated by numerous bound‐bound and bound‐free transitions. Consequently, temperature profiles are closer to grey profiles, and photosphere densities are lower than in the hydrogen case. Mid-Z element atmosphere spectra are significantly softer than hydrogen atmosphere spectra and show numerous absorption lines and edges. The atmosphere spectra depend strongly on surface composition and magnetic field but weakly on surface gravity. Absorption lines are primarily broadened by motional Stark effects and the (unknown) surface magnetic field distribution. When magnetic field variation is not severe, substructure in broad absorption features can be resolved by (phase-resolved) CCD spectroscopy from Chandra and XMM‐Newton. Given the multiple absorption features seen in several isolated neutron stars (INSs), it is possible to determine the surface composition, magnetic field, temperature and gravitational redshift with existing X-ray data; we present qualitative comparisons between our model spectra and the neutron stars 1E1207.4−5209 and RX J1605.3+3249. Future high-resolution X-ray missions such as Constellation-X will measure the gravitational redshift with high accuracy by resolving narrow absorption features; when combined with radius measurements, it will be possible to uniquely determine the mass and radius of INSs.

Constraining the geometry of the neutron star RX J1856.5−3754

Abstract: RX J1856.5-3754 is one of the brightest, nearby isolated neutron stars (NSs), and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere match well the entire spectrum, from X-rays to optical (with best-fitting NS radius R ≈ 14 km, gravitational redshift Żg ∼ 0.2, and magnetic field B ≈ 4 x 10 12 G). A remaining puzzle is the non-detection of rotational modulation of the X-ray emission, despite extensive searches. The situation changed recently with XMM-Newton observations that uncovered 7-s pulsations at the ≈1 per cent level. By comparing the predictions of our model (which includes simple dipolar-like surface distributions of magnetic field and temperature) with the observed brightness variations, we are able to constrain the geometry of RX J1856.5-3754, with one angle <6° and the other angle ≈20°-5°, though the solutions are not definitive, given the observational and model uncertainties. These angles indicate a close alignment between the rotation and the magnetic axes or between the rotation axis and the observer. We discuss our results in the context of RX J1856.5-3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS.

Constraining The Geometry Of The Neutron Star RX J1856.5-3754

Abstract: RX J1856.5-3754 is one of the brightest, nearby isolated neutron stars (NSs), and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere match well the entire spectrum, from X-rays to optical (with best-fitting NS radius R ≈ 14 km, gravitational redshift Żg ∼ 0.2, and magnetic field B ≈ 4 x 10 12 G). A remaining puzzle is the non-detection of rotational modulation of the X-ray emission, despite extensive searches. The situation changed recently with XMM-Newton observations that uncovered 7-s pulsations at the ≈1 per cent level. By comparing the predictions of our model (which includes simple dipolar-like surface distributions of magnetic field and temperature) with the observed brightness variations, we are able to constrain the geometry of RX J1856.5-3754, with one angle <6° and the other angle ≈20°-5°, though the solutions are not definitive, given the observational and model uncertainties. These angles indicate a close alignment between the rotation and the magnetic axes or between the rotation axis and the observer. We discuss our results in the context of RX J1856.5-3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS.

Thin magnetic hydrogen atmospheres and the neutron star RX J1856.5–3754

Abstract: RX J1856.5–3754 is one of the brightest nearby isolated neutron stars, 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-fit parameters are an interstellar column density NH≈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.

Atmospheres and Spectra of Strongly Magnetized Neutron Stars

Abstract: We construct atmosphere models for strongly magnetized neutron stars with surface fields B � 10 12 10 15 G and effective temperatures Teff � 10 6 10 7 K. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars, including radio pulsars, soft gamma-ray repeaters, and anomalous Xray pulsars. In our models, the atmosphere is composed of pure hydrogen or helium and is assumed to be fully ionized. The radiative opacities include free-free absorption and scattering by both electrons and ions computed for the two photon polarization modes in the magnetized electron-ion plasma. Since the radiation emerges from deep layers in the atmosphere with � > 10 2 g/cm 3 , plasma effects can significantly modify the photon opacities by changing the properties of the polarization modes. In the case where the magnetic field and the surface normal are parallel, we solve the full, angle-dependent, coupled radiative transfer equations for both polarization modes. We also construct atmosphere models for general field orientations based on the diffusion approximation of the transport equations and compare the results with models based on full radiative transport. In general, the emergent thermal radiation exhibits significant deviation from blackbody, with harder spectra at high energies. The spectra also show a broad feature (�E/EBi � 1) around the ion cyclotron resonance EBi = 0.63(Z/A)(B/10 14 G) keV, where Z and A are the atomic charge and atomic mass of the ion, respectively; this feature is particularly pronounced when EBi > 3kTeff. Detection of the resonance feature would provide a direct measurement of the surface magnetic fields on magnetars.

Constraining the Geometry of the Neutron Star RX J1856.5-3754

Abstract: RX J1856.5-3754 is one of the brightest, nearby isolated neutron stars, and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere matches well the entire spectrum, from X-rays to optical (with best-fitting neutron star radius R=14 km, gravitational redshift z_g~0.2, and magnetic field B~4x10^12 G). A remaining puzzle is the non-detection of rotational modulation of the X-ray emission, despite extensive searches. The situation changed recently with XMM-Newton observations that uncovered 7 s pulsations at the 1% level. By comparing the predictions of our model (which includes simple dipolar-like surface distributions of magnetic field and temperature) with the observed brightness variations, we are able to constrain the geometry of RX J1856.5-3754, with one angle < 6 deg and the other angle = 20-45 deg, though the solutions are not definitive given the observational and model uncertainties. These angles indicate a close alignment between the rotation and magnetic axes or between the rotation axis and the observer. We discuss our results in the context of RX J1856.5-3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS.

Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars

Abstract: Recent observations by XMM-Newton detected rotational pulsations in the total brightness and spectrum of several neutron stars. To properly interpret the data, accurate modeling of neutron star emission is necessary. Detailed analysis of the shape and strength of the rotational variations allows a measurement of the surface composition and magnetic field, as well as constrains the nuclear equation of state. We discuss our models of the spectra and light curves of two of the most observed neutron stars, RX J1856.5-3754 and 1E 1207.4-5209, and discuss some implications of our results and the direction of future work.