arXiv subjects

Phil Arras

Publications and source records attributed to Phil Arras.

68 records · Page 4Linked to original sources

arXiv2006

Quasi-Periodic Oscillations from Magnetorotational Turbulence

Quasi-periodic oscillations (QPOs) in the X-ray lightcurves of accreting neutron star and black hole binaries have been widely interpreted as being due to standing wave modes in accretion disks. These disks are thought to be highly turbulent due to the magnetorotational instability (MRI). We study wave excitation by MRI turbulence in the shearing box geometry. We demonstrate that axisymmetric sound waves and radial epicyclic motions driven by MRI turbulence give rise to narrow, distinct peaks in the temporal power spectrum. Inertial waves, on the other hand, do not give rise to distinct peaks which rise significantly above the continuum noise spectrum set by MRI turbulence, even when the fluid motions are projected onto the eigenfunctions of the modes. This is a serious problem for QPO models based on inertial waves.

astro-ph↗

arXiv2006

Thermal Structure and Radius Evolution of Irradiated Gas Giant Planets

We consider the thermal structure and radii of strongly irradiated gas giant planets over a range in mass and irradiating flux. The cooling rate of the planet is sensitive to the surface boundary condition, which depends on the detailed manner in which starlight is absorbed and energy redistributed by fluid motion. We parametrize these effects by imposing an isothermal boundary condition $T \equiv T_{\rm deep}$ below the photosphere, and then constrain $T_{\rm deep}$ from the observed masses and radii. We compute the dependence of luminosity and core temperature on mass, $T_{\rm deep}$ and core entropy, finding that simple scalings apply over most of the relevant parameter space. These scalings yield analytic cooling models which exhibit power-law behavior in the observable age range $0.1-10 {\rm Gyr}$, and are confirmed by time-dependent cooling calculations. We compare our model to the radii of observed transiting planets, and derive constraints on $T_{\rm deep}$. Only HD 209458 has a sufficiently accurate radius measurement that $T_{\rm deep}$ is tightly constrained; the lower error bar on the radii for other planets is consistent with no irradiation. More accurate radius and age measurements will allow for a determination of the correlation of $T_{\rm deep}$ with the equilibrium temperature, informing us about both the greenhouse effect and day-night asymmetries.

astro-ph↗

arXiv2005

Radiation from condensed surface of magnetic neutron stars

Recent observations show that the thermal X-ray spectra of many isolated neutron stars are featureless and in some cases (e.g., RX J1856.5-3754) well fit by a blackbody. Such a perfect blackbody spectrum is puzzling since radiative transport through typical neutron star atmospheres causes noticeable deviation from blackbody. Previous studies have shown that in a strong magnetic field, the outermost layer of the neutron star may be in a condensed solid or liquid form because of the greatly enhanced cohesive energy of the condensed matter. The critical temperature of condensation increases with the magnetic field strength, and can be as high as 10^6 K (for Fe surface at B \sim 10^{13} G or H surface at B \sim a few times 10^{14} G). Thus the thermal radiation can directly emerge from the degenerate metallic condensed surface, without going through a gaseous atmosphere. Here we calculate the emission properties (spectrum and polarization) of the condensed Fe and H surfaces of magnetic neutron stars in the regimes where such condensation may be possible. For a smooth condensed surface, the overall emission is reduced from the blackbody by less than a factor of 2. The spectrum exhibits modest deviation from blackbody across a wide energy range, and shows mild absorption features associated with the ion cyclotron frequency and the electron plasma frequency in the condensed matter. The roughness of the solid condensate (in the Fe case) tends to decrease the reflectivity of the surface, and make the emission spectrum even closer to blackbody. We discuss the implications of our results for observations of dim, isolated neutron stars and magnetars.

astro-ph↗

arXiv2005

White Dwarf Heating and Subsequent Cooling in Dwarf Nova Outbursts

We follow the time dependent thermal evolution of a white dwarf (WD) undergoing sudden accretion in a dwarf nova outburst, using both simulations and analytic estimates. The post-outburst lightcurve clearly separates into early times when the WD flux is high, and late times when the flux is near the quiescent level. The break between these two regimes, occurring at a time of order the outburst duration, corresponds to a thermal diffusion wave reaching the base of the freshly accreted layer. Our principal result is that long after the outburst, the fractional flux perturbation about the quiescent flux decays as a power law with time (and {\it not} as an exponential). We use this result to construct a simple fitting formula that yields estimates for both the quiescent flux and the accreted column, i.e. the total accreted mass divided by WD surface area. The WD mass is not well constrained by the late time lightcurve alone, but it can be inferred if the accreted mass is known from observations. We compare our work with the well-studied outburst of WZ Sge, finding that the cooling is well described by our model, giving an effective temperature $T_{\rm eff}=14,500 {\rm K}$ and accreted column $Δy\approx10^6 {\rm g cm^{-2}}$, in agreement with the modeling of Godon et al. To reconcile this accreted column with the accreted mass inferred from the bolometric accretion luminosity, a large WD mass $\gtrsim1.1M_\odot$ is needed. Our power law result is a valuable tool for making quick estimates of the outburst properties. We show that fitting the late time lightcurve with this formula yields a predicted column within 20% of that estimated from our full numerical calculations.

astro-ph↗

arXiv2004

Seismology of Accreting White Dwarfs

Pulsation modes have recently been observed in a handful of white dwarf (WD) primaries of cataclysmic variables, allowing an interesting new probe into the structure of accreting WD's. We briefly discuss the seismology of these objects, how stellar properties may be inferred from the observed mode frequencies, and mode driving mechanisms. For one pulsator, GW Lib, we have shown that a WD mass $M=1.05 M_\odot$ and accreted envelope mass $M_{\rm env}=0.4\times 10^{-4} M_\odot$ give the best match to the observed pulsation periods. A first exploration of mode driving favors $T_{\rm eff} = 14 000 K$ and a massive WD, but more work is necessary.

astro-ph↗

arXiv2004

Seismology of the Accreting White Dwarf in GW Lib

We present a first analysis of the g-mode oscillation spectrum for the white dwarf (WD) primary of GW Lib, a faint cataclysmic variable (CV). Stable periodicities have been observed from this WD for a number of years, but their interpretation as stellar pulsations has been hampered by a lack of theoretical models appropriate to an accreting WD. Using the results of Townsley and Bildsten, we construct accreting models for the observed effective temperature and approximate mass of the WD in GW Lib. We compute g-mode frequencies for a range of accreted layer masses, Macc, and long term accretion rates, . If we assume that the observed oscillations are from l=1 g-modes, then the observed periods are matched when M ~= 1.02 Msun, Macc ~= 0.31 x 10^-4 Msun and ~= 7.3 x 10^-11 Msun/yr. Much more sensitive observations are needed to discover more modes, after which we will be able to more accurately measure these parameters and constrain or measure the WD's rotation rate.

astro-ph↗

arXiv2004

Magnetic Field Evolution in Neutron Star Crusts due to the Hall Effect and Ohmic Decay

We present calculations of magnetic field evolution by the Hall effect and Ohmic decay in neutron star crusts. We first discuss when and for which neutron stars the Hall effect is important relative to Ohmic decay. We compute the evolution of an initial field distribution by Ohmic decay, and give approximate analytic formulas for both the surface and interior fields as a function of time. We then discuss the subsequent evolution by the Hall effect, giving several examples which illustrate how the field configuration evolves. Hall wave eigenfunctions are computed, including the effect of the large density change across the crust. We estimate the response of the crust to the magnetic stresses induced by Hall waves, and give a detailed discussion of the boundary conditions at the solid-liquid interface. Finally, we discuss the implications for the Hall cascade proposed by Goldreich & Reisenegger.

astro-ph↗

arXiv2003

A Free, Fast, Simple and Efficient TVD MHD Code

We describe a numerical method to solve the magnetohydrodynamic (MHD) equations. The fluid variables are updated along each direction using the flux conservative, 2nd order, total variation diminishing (TVD), upwind scheme of Jin and Xin. The magnetic field is updated separately in two-dimensional advection-constraint steps. The electromotive force (EMF) is computed in the advection step using the TVD scheme, and this same EMF is used immediately in the constraint step in order to preserve \grad.B=0 without the need to store intermediate fluxes. Operator splitting is used to extend the code to three dimensions, and Runge-Kutta is used to get second order accuracy in time. The advantages of this code are high resolution per grid cell, second order accuracy in space and time, enforcement of the \grad.B=0 constraint to machine precision, no memory overhead, speed, and simplicity. A 3-D Fortran implementation less than 400 lines long is made freely available. We also implemented a fully scalable message-passing parallel MPI version. We present tests of the code on MHD waves and shocks.

astro-ph↗

arXiv2003

Saturation of the R-mode Instability

Rossby waves (r-modes) in rapidly rotating neutron stars are unstable because of the emission of gravitational radiation. We study saturation of this instability by nonlinear transfer of energy to stellar "inertial" oscillation modes. We present detailed calculations of stellar inertial modes in the WKB limit, their linear damping by bulk and shear viscosity, and the nonlinear coupling forces among these modes. The saturation amplitude is derived in the extreme limits of strong or weak driving by radiation reaction, as compared to the damping rate of low order inertial modes. We find the saturation energy is {\it extremely small}, at least four orders of magnitude smaller than that found by previous investigators. We discuss the consequences of this result for spin evolution of young neutron stars, and neutron stars being spun up by accretion in Low Mass X-ray Binaries.We also discuss the detection of these gravitational waves by LIGO.

astro-ph↗

arXiv2001

Nonlinear mode coupling in rotating stars and the r-mode instability in neutron stars

We develop the formalism required to study the nonlinear interaction of modes in rotating Newtonian stars in the weakly nonlinear regime. The formalism simplifies and extends previous treatments. At linear order, we elucidate and extend slightly a formalism due to Schutz, show how to decompose a general motion of a rotating star into a sum over modes, and obtain uncoupled equations of motion for the mode amplitudes under the influence of an external force. Nonlinear effects are added perturbatively via three-mode couplings. We describe a new, efficient way to compute the coupling coefficients, to zeroth order in the stellar rotation rate, using spin-weighted spherical harmonics. We apply this formalism to derive some properties of the coupling coefficients relevant to the nonlinear interactions of unstable r-modes in neutron stars, postponing numerical integrations of the coupled equations of motion to a later paper. From an astrophysical viewpoint, the most interesting result of this paper is that many couplings of r-modes to other rotational modes (modes with zero frequencies in the non-rotating limit) are small: either they vanish altogether because of various selection rules, or they vanish to lowest order in the angular velocity. In zero-buoyancy stars, the coupling of three r-modes is forbidden entirely and the coupling of two r-modes to one hybrid rotational mode vanishes to zeroth order in rotation frequency. In incompressible stars, the coupling of any three rotational modes vanishes to zeroth order in rotation frequency.

gr-qc↗

arXiv2000

R-modes in Neutron Stars with Crusts: Turbulent Saturation, Spin-down, and Crust Melting

Rossby waves (r-modes) have been suggested as a means to regulate the spin periods of young or accreting neutron stars, and also to produce observable gravitational wave radiation. R-modes involve primarily transverse, incompressive motions of the star's fluid core. However, neutron stars gain crusts early in their lives: therefore, r-modes also imply shear in the fluid beneath the crust. We examine the criterion for this shear layer to become turbulent, and derive the rate of dissipation in the turbulent regime. Unlike dissipation from a viscous boundary layer, turbulent energy loss is nonlinear in mode energy and can therefore cause the mode to saturate at amplitudes typically much less than unity. This energy loss also reappears as heat below the crust. We study the possibility of crust melting as well as its implications for the spin evolution of low-mass X-ray binaries. Lastly, we identify some universal features of the spin evolution that may have observational consequences.

astro-ph↗

arXiv1998

Neutrino-Nucleon Interactions in Magnetized Neutron-Star Matter: The Effects of Parity Violation

We study neutrino-nucleon scattering and absorption in a dense, magnetized nuclear medium. These are the most important sources of neutrino opacity governing the cooling of a proto-neutron star in the first tens of seconds after its formation. Because the weak interaction is parity violating, the absorption and scattering cross-sections depend asymmetrically on the directions of the neutrino momenta with respect to the magnetic field. We develop the moment formalism of neutrino transport in the presence of such asymmetric opacities and derive explicit expressions for the neutrino flux and other angular moments of the Boltzmann transport equation. For a given neutrino species, there is a drift flux of neutrinos along the magnetic field in addition to the usual diffusive flux. This drift flux depends on the deviation of the neutrino distribution function from thermal equilibrium. Hence, despite the fact that the neutrino cross-sections are asymmetric throughout the star, asymmetric neutrino flux can be generated only in the outer region of the proto-neutron star where the neutrino distribution deviates significantly from thermal equilibrium. In addition to the asymmetric absorption opacity arising from nucleon polarization, we find the contribution of the electron (or positron) ground state Landau level. For neutrinos of energy less than a few times the temperature, this is the dominant source of asymmetric opacity. Lastly, we discuss the implication of our result to the origin of pulsar kicks: in order to generate kick velocity of a few hundred km/s from asymmetric neutrino emission using the parity violation effect, the proto-neutron star must have a dipole magnetic field of at least $10^{15}-10^{16}$ G.

astro-ph↗

arXiv1998

Canaries in a Coal Mine: Using Globular Clusters to Place Limits on Massive Black Holes in the Galactic Halo

We explore the possibility that massive black holes comprise a significant fraction of the dark matter of our galaxy by studying the dissolution of galactic globular clusters bombarded by them. In our simulations, we evolve the clusters along a sequence of King models determined by changes of state resulting from collisions with the black holes. The results divide naturally into regimes of `small' and `large' black hole mass. `Small' black holes do not destroy clusters in single collisions; their effect is primarily cumulative, leading to a relation between $\mbh$ and $\fhalo$, the fraction of the halo in black holes of mass $\mbh$, which is $\fhalo\mbh < $ constant (up to logarithmic corrections). For $\fhalo=1$, we find $\mbh \simless 10^{3} \msun$ by requiring survival of the same clusters studied by Moore (1993), who neglected cluster evolution, mass loss, and stochasticity of energy inputs in his estimates, but reached a similar conclusion. `Large' black holes may not penetrate a cluster without disrupting it; their effect is mainly catastrophic (close collisions), but also partly cumulative (distant collisions). In the large $\mbh$ limit, $\fhalo$ (but not $\mbh$) can be constrained by computing the probability that a cluster survives a combination of close, destructive encounters and distant, nondestructive encounters. We find that it is unlikely that $\fhalo \simgreat 0.3$ by requiring 50 per cent survival probability for Moore's clusters over $10^{10}$ years.

astro-ph↗

arXiv1998

Can Parity Violation in Neutrino Transport Lead to Pulsar Kicks?

In magnetized proto-neutron stars, neutrino cross sections depend asymmetrically on the neutrino momenta due to parity violation. However, these asymmetric opacities do not induce any asymmetric flux in the bulk interior of the star where neutrinos are nearly in thermal equilibrium. Consequently, parity violation in neutrino absorption and scattering can only give rise to asymmetric neutrino flux above the neutrino-matter decoupling layer. The kick velocity is substantially reduced from previous estimates, requiring a dipole field $B \sim 10^{16}$~G to get $v_{kick}$ of order a few hundred km~s$^{-1}$.

astro-ph↗