Wynn C. G. Ho

UBVRI Photometry of the Type IC SN 1994I in M51

We present optical photometry for the type Ic SN 1994I in M51 (NGC 5194) from UT March 31 to Aug 6, 1994, starting eight days before B-band maximum and ending three months later. We estimate the extinction to SN 1994I was considerable and quite uncertain, A V = 1:4 0:5 mag, making a detailed comparison to other supernovae diicult. Using a distance modulus = 29:6 0:3 mag to M51, we calculate absolute magnitudes and a quasi-bolometric light curve for the supernova. It appears that SN 1994I was less luminous than normal type Ia SNe. There is strong evidence that the ejecta of SN 1994I were much less massive than those of other SNe.

Atmospheres and spectra of strongly magnetized neutron stars — II. The effect of vacuum polarization

We study the effect of vacuum polarization on the atmosphere structure and radiation spectra of neutron stars with surface magnetic fields B = 10 1 4 -10 1 5 G, as appropriate for magne-tars. Vacuum polarization modifies the dielectric property of the medium and gives rise to a resonance feature in the opacity; this feature is narrow and occurs at a photon energy that depends on the plasma density. Vacuum polarization can also induce resonant conversion of photon modes via a mechanism analogous to the Mikheyev-Smirnov-Wolfenstein (MSW) mechanism for neutrino oscillation. We construct atmosphere models in radiative equilibrium with an effective temperature of a few × 10 6 K by solving the full radiative transfer equations for both polarization modes in a fully ionized hydrogen plasma. We discuss the subtleties in treating the vacuum polarization effects in the atmosphere models and present approximate solutions to the radiative transfer problem which bracket the true answer. We show from both analytic considerations and numerical calculations that vacuum polarization produces a broad depression in the X-ray flux at high energies (a few keV ≤ E ≤ a few tens of keV) as compared to models without vacuum polarization; this arises from the density dependence of the vacuum resonance feature and the large density gradient present in the atmosphere. Thus the vacuum polarization effect softens the high-energy tail of the thermal spectrum, although the atmospheric emission is still harder than the blackbody spectrum because of the non-grey opacities. We also show that the depression of continuum flux strongly suppresses the equivalent width of the ion cyclotron line and therefore makes the line more difficult to observe.

Atmospheres and spectra of strongly magnetized neutron stars

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.

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

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.

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 × 10 13 < B < a few × 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. Subject headings: magnetic fields – radiative transfer – stars: neutron – stars: atmospheres – X-rays: stars

A search for rapid photometric variability in symbiotic binaries

We report on our survey for rapid (time scale of minutes) photometric variability in symbiotic binaries. These binaries are becoming an increasingly important place to study accretion onto white dwarfs since they are candidate Type Ia supernovae progenitors. Unlike in most cataclysmic variables, the white dwarfs in symbiotics typically accrete from a wind, at rates greater than or equal to 10 −9 M⊙ yr −1 . In order to elucidate the differences between symbiotics and other white dwarf accretors, as well as search for magnetism in symbiotic white dwarfs, we have studied 35 symbiotic binaries via differential optical photometry. Included in our sample are all but one of the symbiotics from the lists of Kenyon (1986) and Downes & Keyes (1988) with published V magnitude less than 14 and declination greater than 20 ◦ . Our study is the most comprehensive to date of rapid variability in symbiotic binaries. We have found one magnetic accretor, Z And, previously reported by Sokoloski & Bildsten (1999). In four systems (EG And, BX Mon, CM Aql, and BF Cyg), some evidence for flickering at a low level (roughly 10 mmag) is seen for the first time. These detections are, however, marginal. For 25 systems, we place tight upper limits on both aperiodic variability (flickering) and periodic variability, highlighting a major difference between symbiotics and cataclysmic variables. The remaining five of the objects included in our sample (the 2 recurrent novae RS Oph and T CrB, plus CH Cyg, o Ceti, and MWC 560) had previous detections of optical flickering. We discuss our extensive observations of these previously-known flickering systems in a separate paper. Five new variable stars were discovered serendipitously in the fields of the survey objects, and the observations of these stars are also presented elsewhere. We discuss the impact of our results on the “standard” picture of wind-fed accretion, and speculate on the possibility that light from quasi-steady nuclear burning on the surface of the white dwarf hides the fluctuating emission from accretion.

Resonant Conversion of Photon Modes Due to Vacuum Polarization in a Magnetized Plasma: Implications for X-Ray Emission from Magnetars

It is known that vacuum polarization can modify the photon propagation modes in the atmospheric plasma of a strongly magnetized neutron star. A resonance occurs when the effect of vacuum polarization on the photon modes balances that of the plasma. We show that a photon (with energy E a few keV) propagating outward in the atmosphere can convert from one polarization mode into another as it traverses the resonant density, ρres Yη-2(B/1014G)2(E/1keV)2xa0gxa0cm-3, where Ye is the electron fraction and η ~ 1 is a slowly varying function of the magnetic field B. The physics of this mode conversion is analogous to the Mikheyev-Smirnov-Wolfenstein mechanism for neutrino oscillation. Because the two photon modes have vastly different opacities in the atmosphere, this vacuum-induced mode conversion can significantly affect radiative transport and surface emission from strongly magnetized neutron stars.

r-Mode Oscillations and Spin-down of Young Rotating Magnetic Neutron Stars

Recent work has shown that a young, rapidly rotating neutron star loses angular momentum to gravitational waves generated by unstable r-mode oscillations. We study the spin evolution of a young, magnetic neutron star including the effects of both gravitational radiation and magnetic braking (modeled as magnetic dipole radiation). Our phenomenological description of nonlinear r-modes is similar to, but distinct from, that given by Owen and colleagues in 1998, in that our treatment is consistent with the principle of adiabatic invariance in the limit when direct driving and damping of the mode are absent. We show that while magnetic braking tends to increase the r-mode amplitude by spinning down the neutron star, it nevertheless reduces the efficiency of gravitational wave emission from the star. For B 1014(νs/300 Hz)2 G, where νs is the spin frequency, the spin-down rate and the gravitational waveforms are significantly modified by the effect of magnetic braking. We also estimate the growth rate of the r-mode due to electromagnetic (fast magnetosonic) wave emission and due to Alfven wave emission in the neutron star magnetosphere. The Alfven wave driving of the r-mode becomes more important than the gravitational radiation driving when B 1013(νs/150 Hz)3 G; the electromagnetic wave driving of the r-mode is much weaker. Finally, we study the properties of local Rossby-Alfven waves inside the neutron star and show that the fractional change of the r-mode frequency due to the magnetic field is of order 0.5(B/1016 G)2(νs/100 Hz)-2.

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 = 1012-5 ? 1014 G and effective temperatures Teff ~ a few ? 105-106 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 = 1012-1013 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 1014 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 1014 G.

Electromagnetic polarization in partially ionized plasmas with strong magnetic fields and neutron star atmosphere models

The polarizability tensor of a strongly magnetized plasma and the polarization vectors and opacities of normal electromagnetic waves are studied for conditions typical of neutron star atmospheres, taking account of partial ionization effects. Vacuum polarization is also included, and a new set of fitting formulae are used that are accurate for wide range of field strengths. The full account of the coupling of the quantum mechanical structure of the atoms to their center-of-mass motion across the magnetic field is shown to be crucial for the correct evaluation of the polarization properties and opacities of the plasma. The self-consistent treatment of the polarizability and absorption coefficients proves to be necessary if the ionization degree of the plasma is low, which can occur in the atmospheres of cool or ultramagnetized neutron stars. Atmosphere models and spectra are presented to illustrate the importance of such self-consistent treatment.