Wynn C. Ho - Univ of Southampton

Wynn C. Ho
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Wynn C. Ho
Univ of Southampton
United Kingdom

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Pub Categories

High Energy Astrophysical Phenomena (26)
Solar and Stellar Astrophysics (20)
Astrophysics (17)
General Relativity and Quantum Cosmology (7)
High Energy Physics - Phenomenology (5)
Nuclear Theory (3)
Astrophysics of Galaxies (1)

Publications Authored By Wynn C. Ho

The radio and gamma-ray pulsar PSR J2032+4127 was recently found to be in a decades-long orbit with the Be star MT91 213, with the pulsar moving rapidly towards periastron. This binary shares many similar characteristics with the previously unique binary system PSR B1259-63/LS 2883. Here, we describe radio, X-ray, and optical monitoring of PSR J2032+4127/MT91 213. Read More

The X-ray source 1E 161348-5055 in the supernova remnant RCW 103 recently exhibited X-ray activity typical of magnetars, i.e. neutron stars with magnetic fields > 10^14-10^15 G. Read More

The light curve of many supernovae (SNe) and gamma-ray bursts (GRBs) can be explained by a sustained injection of extra energy from its possible central engine, a rapidly rotating strongly magnetic neutron star (i.e. magnetar). Read More

Pulsars are known for their superb timing precision, although glitches can interrupt the regular timing behavior when the stars are young. These glitches are thought to be caused by interactions between normal and superfluid matter in the crust of the star. However, glitching pulsars such as Vela have been shown to require a superfluid reservoir that greatly exceeds that available in the crust. Read More

In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on timescales $<10^4$ years, while continuing accretion may produce variations on a range of timescales. While some quiescent neutron stars (e. Read More

In the standard scenario for spin evolution of isolated neutron stars, a young pulsar slows down with a surface magnetic field B that does not change. Thus the pulsar follows a constant B trajectory in the phase space of spin period and spin period time derivative. Such an evolution predicts a braking index n = 3 while the field is constant and n > 3 when the field decays. Read More

The observed rapid cooling of the Cassiopeia A neutron star can be interpreted as being caused by neutron and proton transitions from normal to superfluid and superconducting states in the stellar core. Here we present two new Chandra ACIS-S Graded observations of this neutron star and measurements of the neutron star mass M and radius R found from consistent fitting of both the X-ray spectra and cooling behavior. This comparison is only possible for individual nuclear equations of state. Read More

Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed as an interesting source of gravitational waves. In this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. First of all we focus on the r-mode instability and show that a 'minimal' neutron star model (which does not incorporate exotica in the core, dynamically important magnetic fields or superfluid degrees of freedom), is not consistent with observations. Read More

A number of radio pulsars exhibit intriguing mode-switching behavior. Recent observations of PSR B0943+10 revealed correlated radio and X-ray mode switches, providing a new avenue for understanding this class of objects. The large X-ray pulse fraction observed during the radio quiet phase (Q mode) was previously interpreted as a result of changing obscuration of X-rays by dense magnetosphere plasma. Read More

The excellent sensitivity of X-ray telescopes, such as Chandra and XMM-Newton, is ideal for the study of cooling neutron stars, which can emit at these energies. In order to exploit the wealth of information contained in the high quality data, a thorough knowledge of the radiative properties of neutron star atmospheres is necessary. A key factor affecting photon emission is magnetic fields, and neutron stars are known to have strong surface magnetic fields. Read More

Affiliations: 1University of Southampton, 2University of Southampton, 3University of Southampton, 4University of Southampton

Many pulsars are formed with a binary companion from which they can accrete matter. Torque exerted by accreting matter can cause the pulsar spin to increase or decrease, and over long times, an equilibrium spin rate is achieved. Application of accretion theory to these systems provides a probe of the pulsar magnetic field. Read More

We present recent work on using astronomical observations of neutron stars to reveal unique insights into nuclear matter that cannot be obtained from laboratories on Earth. First, we discuss our measurement of the rapid cooling of the youngest neutron star in the Galaxy; this provides the first direct evidence for superfluidity and superconductivity in the supra-nuclear core of neutron stars. We show that observations of thermonuclear X-ray bursts on neutron stars can be used to constrain properties of neutron superfluidity and neutrino emission. Read More

The mass and radius of the neutron star (NS) in low-mass X-ray binaries can be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass and radius constrains the properties of dense matter in NS cores. A critical ingredient for spectral fits is the composition of the NS atmosphere: hydrogen atmospheres are assumed in most prior work, but helium atmospheres are possible if the donor star is a helium white dwarf. Here we perform spectral fits to XMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13. Read More

Central compact objects (CCOs) are neutron stars that are found near the center of supernova remnants, and their association with supernova remnants indicates these neutron stars are young (<~ 10^4 yr). Here we review the observational properties of CCOs and discuss implications, especially their inferred magnetic fields. X-ray timing and spectral measurements suggest CCOs have relatively weak surface magnetic fields (~ 10^10 - 10^11 G). Read More

Pulsars are rotating neutron stars that are seen to slow down, and the spin-down rate is thought to be due to magnetic dipole radiation. This leads to a prediction for the braking index n, which is a combination of spin period and its first and second time derivatives. However, all observed values of n are below the predicted value of 3. Read More

It has recently been proposed that radio emission from magnetars can be evaluated using a "fundamental plane" in parameter space between pulsar voltage gap and ratio of X-ray luminosity Lx to rotational energy loss rate Edot. In particular, radio emission from magnetars will occur if Lx/Edot<1 and the voltage gap is large, and there is no radio emission if Lx/Edot>1. Here we clarify several issues regarding this fundamental plane, including demonstrating that the fundamental plane is not uniquely defined. Read More

Conversion of photons into axions under the presence of a strong magnetic field can dim the radiation from magnetized astrophysical objects. Here we perform a detailed calculation aimed at quantifying the signatures of photon-axion conversion in the spectra, light curves, and polarization of neutron stars (NSs). We take into account the energy and angle-dependence of the conversion probability and the surface thermal emission from NSs. Read More

Rapidly rotating Neutron Stars in Low Mass X-ray Binaries (LMXBs) may be an interesting source of Gravitational Waves (GWs). In particular, several modes of stellar oscillation may be driven unstable by GW emission, and this can lead to a detectable signal. Here we illustrate how current X-ray and ultra-violet (UV) observations can constrain the physics of the r-mode instability. Read More

We examine to what extent the inferred surface temperature of magnetars in quiescence can constrain the presence of a superfluid in the neutron star core and the role of magnetic field decay in the core. By performing detailed simulations of neutron star cooling, we show that extremely strong heating from field decay in the core cannot produce the high observed surface temperatures nor delay the onset of neutron superfluidity in the core. We verify the results of Kaminker et al. Read More

Neutron stars accreting matter from low-mass binary companions are observed to undergo bursts of X-rays due to the thermonuclear explosion of material on the neutron star surface. We use recent results on superfluid and superconducting properties to show that the core temperature in these neutron stars may not be uniquely determined for a range of observed accretion rates. The degeneracy in inferred core temperatures could contribute to explaining the difference between neutron stars which have very short recurrence times between multiple bursts and those which have long recurrence times between bursts: short bursting sources have higher temperatures and normal neutrons in the stellar core, while long bursting sources have lower temperatures and superfluid neutrons. Read More

We consider the astrophysical constraints on the gravitational-wave driven r-mode instability in accreting neutron stars in low-mass X-ray binaries. We use recent results on superfluid and superconducting properties to infer the core temperature in these neutron stars and show the diversity of the observed population. Simple theoretical models indicate that many of these systems reside inside the r-mode instability region. Read More

We present Hubble Space Telescope optical and ultraviolet photometry for five nearby, thermally emitting neutron stars. With these measurements, all seven such objects have confirmed optical and ultraviolet counterparts. Combining our data with archival space-based photometry, we present spectral energy distributions for all sources and measure the "optical excess": the factor by which the measured photometry exceeds that extrapolated from X-ray spectra. Read More

The central compact objects are a newly-emerging class of young neutron stars near the centre of supernova remnants. From X-ray timing and spectral measurements, their magnetic fields are determined to be ~ 10^10-10^11 G, which is significantly lower than that found on most pulsars. Using the latest electrical and thermal conductivity calculations, we solve the induction equation to determine the evolution of a buried crustal or core magnetic field. Read More

Affiliations: 1University of Southampton, 2University of Southampton, 3University of Southampton

We compare the rotation rate of neutron stars in low-mass X-ray binaries (LMXBs) with the orbital period of the binaries. We find that, while short orbital period LMXBs span a range of neutron star rotation rates, all the long period LMXBs have fast rotators. We also find that the rotation rates are highest for the systems with the highest mean mass accretion rates, as can be expected if the accretion rate correlates with the orbital period. Read More

According to recent results of Ho & Heinke (2009) and Heinke & Ho (2010), the Cassiopeia A supernova remnant contains a young neutron star which has carbon atmosphere and shows noticeable decline of the effective surface temperature. We report a new (November 2010) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet-state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. Read More

We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke (Nature, 462, 71 (2009)) that the thermal radiation of this star is well-described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling stage from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles -- stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. Read More

Affiliations: 1University of Alberta, 2University of Southampton

The cooling rate of young neutron stars gives direct insight into their internal makeup. Although the temperatures of several young neutron stars have been measured, until now a young neutron star has never been observed to decrease in temperature over time. We fit 9 years of archival Chandra ACIS spectra of the likely neutron star in the ~330 years old Cassiopeia A supernova remnant with our non-magnetic carbon atmosphere model. Read More

The surface of hot neutron stars is covered by a thin atmosphere. If there is accretion after neutron star formation, the atmosphere could be composed of light elements (H or He); if no accretion takes place or if thermonuclear reactions occur after accretion, heavy elements (for example, Fe) are expected. Despite detailed searches, observations have been unable to confirm the atmospheric composition of isolated neutron stars. Read More

We review the polarization properties of X-ray emission from highly magnetized neutron stars, focusing on emission from the stellar surfaces. We discuss how x-ray polarization can be used to constrain neutron star magnetic field and emission geometry, and to probe strong-field quantum electrodynamics and possibly constrain the properties of axions. Read More

Affiliations: 1CfA, 2Ioffe Inst., St. Petersburg, 3CRAL, ENS-Lyon
Category: Astrophysics

We construct partially ionized hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with fixed surface fields between B=10^12 and 2x10^13 G and effective temperatures logT_eff=5.5-6.8, as well as with surface B and T_eff distributions around these values. Read More

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. Read More

Authors: Wynn C. G. Ho1
Affiliations: 1Harvard-Smithsonian CfA
Category: Astrophysics

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. Read More

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. Read More

Affiliations: 1University of Toronto, 2Harvard-Smithsonian CfA
Category: Astrophysics

We construct models for strongly-magnetized neutron star atmospheres composed of mid-Z elements (carbon, oxygen and neon) with magnetic fields B=10^{12}-10^{13} G and effective temperatures Teff=(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. Read More

We study several effects that influence the strength of the proton cyclotron and atomic features in the thermal spectra of magnetic neutron stars. We show that it is possible for vacuum polarization to strongly suppress the spectral lines when the magnetic field B\ga 10^14 G. For weaker fields (B\la 7\times 10^13 G), the surface spectrum is unaffected by vacuum polarization; thus the proton cyclotron absorption line can have a large equivalent width. Read More


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. Read More

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. Read More

Affiliations: 1Cornell, 2Cornell, 3Ioffe Institute, St. Petersburg, 4ENS-Lyon
Category: Astrophysics

We construct hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with surface fields B=10^12-5x10^14 G and effective temperatures T_eff a few x 10^5-10^6 K by solving the full radiative transfer equations for both polarization modes in the magnetized hydrogen plasma. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars. We study the effects of vacuum polarization and bound atoms on the atmosphere structure and spectra. Read More

We have recently shown that in a highly magnetized neutron star atmospheric plasma, vacuum polarization can induce resonant conversion of photon polarization modes via a mechanism analogous to MSW neutrino oscillation. In a recent paper Ozel has dismissed this mode conversion effect as ``mistakes''. Here we explain why our arguments/calculations of this effect are correct. Read More

We study the effect of vacuum polarization on the atmosphere structure and radiation spectra of neutron stars with surface magnetic fields B=10^14-10^15 G, as appropriate for magnetars. 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 MSW mechanism for neutrino oscillation. Read More

We present optical photometry of one Type IIn supernova (1994Y) and nine Type Ia supernovae (1993Y, 1993Z, 1993ae, 1994B, 1994C, 1994M, 1994Q, 1994ae, and 1995D). SN 1993Y and SN 1993Z appear to be normal SN Ia events with similar rates of decline, but we do not have data near maximum brightness. The colors of SN 1994C suggest that it suffers from significant reddening or is intrinsically red. Read More

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 $E\go$ a few keV) propagating outward in the atmosphere can convert from one polarization mode into another as it traverses the resonant density, $\rho_{res}\simeq Y_e^{-1}\eta^{-2}(B/10^{14} G)^2(E/1 keV)^2$ g cm$^{-3}$, where $Y_e$ is the electron fraction, and $\eta\sim 1$ is a slowly varying function of the magnetic field $B$. Read More


We construct atmosphere models for strongly magnetized neutron stars with surface fields $B\sim 10^{12}-10^{15}$ G and effective temperatures $T_{\rm eff}\sim 10^6-10^7$ K. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars, including radio pulsars, soft gamma-ray repeaters, and anomalous X-ray pulsars. In our models, the atmosphere is composed of pure hydrogen or helium and is assumed to be fully ionized. Read More

Affiliations: 1Harvard-Smithsonian Center for Astrophysics, 2Institute for Theoretical Physics, UCSB, 3Cornell University
Category: Astrophysics

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} solar masses per year. Read More

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 both the effects of gravitational radiation and magnetic braking (modeled as magnetic dipole radiation). Our phenomenological description of nonlinear r-modes is similar to, but distinct from, that of Owen et al. Read More

In a coalescing neutron star-neutron star (NS-NS) or neutron star-black hole (NS-BH) binary, oscillation modes of the NS can be resonantly excited by the companion during the final minutes of the inspiral. The resonant energy transfer between the orbit and NS speeds up or slows down the inspiral and induces a phase change in the emitted gravitational waves from the binary. A tidal resonance, (jk,m), occurs when the mode frequency equals m times the orbital frequency. Read More