arXiv subjects

Lars Bildsten

Publications and source records attributed to Lars Bildsten.

At least 145 records · Page 8Linked to original sources

arXiv2007

Turbulent Mixing in the Surface Layers of Accreting Neutron Stars

During accretion a neutron star (NS) is spun up as angular momentum is transported through its surface layers. We study the resulting differentially rotating profile, focusing on the impact this has for type I X-ray bursts. The predominant viscosity is likely provided by the Tayler-Spruit dynamo. The radial and azimuthal magnetic field components have strengths of ~10^5 G and ~10^10 G, respectively. This leads to nearly uniform rotation at the depths of interest for X-ray bursts. A remaining small shear transmits the accreted angular momentum inward to the NS interior. Though this shear gives little viscous heating, it can trigger turbulent mixing. Detailed simulations will be required to fully understand the consequences of mixing, but our models illustrate some general features. Mixing has the greatest impact when the buoyancy at the compositional discontinuity between accreted matter and ashes is overcome. This occurs at high accretion rates, at low spin frequencies, or may depend on the ashes from the previous burst. We then find two new regimes of burning. The first is ignition in a layer containing a mixture of heavier elements from the ashes. If ignition occurs at the base of the mixed layer, recurrence times as short as ~5-30 minutes are possible. This may explain the short recurrence time of some bursts, but incomplete burning is still needed to explain these bursts' energetics. When mixing is sufficiently strong, a second regime is found where accreted helium mixes deep enough to burn stably, quenching X-ray bursts. We speculate that the observed change in X-ray burst properties near one-tenth the Eddington accretion rate is from this mechanism. The carbon-rich material produced by stable helium burning would be important for triggering and fueling superbursts. (abridged)

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arXiv2006

Hydrodynamic Thermonuclear Runaways in Superbursts

We calculate the thermal and dynamical evolution of the surface layers of an accreting neutron star during the rise of a superburst. For the first few hours following unstable 12C ignition, the nuclear energy release is transported by convection. However, as the base temperature rises, the heating time becomes shorter than the eddy turnover time and convection becomes inefficient. This results in a hydrodynamic nuclear runaway, in which the heating time becomes shorter than the local dynamical time. Such hydrodynamic burning can drive shock waves into the surrounding layers and may be the trigger for the normal X-ray burst found to immediately precede the onset of the superburst in both cases where the Rossi X-Ray Timing Explorer was observing.

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arXiv2006

The Advanced Compton Telescope Mission

The Advanced Compton Telescope (ACT), the next major step in gamma-ray astronomy, will probe the fires where chemical elements are formed by enabling high-resolution spectroscopy of nuclear emission from supernova explosions. During the past two years, our collaboration has been undertaking a NASA mission concept study for ACT. This study was designed to (1) transform the key scientific objectives into specific instrument requirements, (2) to identify the most promising technologies to meet those requirements, and (3) to design a viable mission concept for this instrument. We present the results of this study, including scientific goals and expected performance, mission design, and technology recommendations.

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arXiv2006

Pulsational Instabilities in Accreting White Dwarfs

(Abridged) The Cataclysmic Variable (CV) population harbors a diverse range of donor stars and accreting white dwarfs (WDs). A range of WD masses is expected, from low mass Helium core WDs, to massive WDs which have previously accreted at rates high enough for Hydrogen to burn steadily. Furthermore, a wide range of Helium enrichment is expected in the accreted material depending on the degree to which the donor star is evolved. We investigate the impact of this diversity on the range of effective temperatures ($T_{\rm eff}$) for which g-modes are unstable. The critical $T_{\rm eff}$ below which modes are unstable ("blue edge") depends on both surface gravity, $g$, and He abundance, $Y$. The Hydrogen/first Helium ionization instability strip is more sensitive to $g$ than $Y$. We find that (for solar composition envelopes), relative to a fiducial WD mass $0.6 M_\odot$, the blue edge for a $0.4 M_\odot$ He core WD shifts downward by $\approx 1000 {\rm K}$, while that for a massive $\approx 1.2 M_\odot$ WD shifts upward by $\approx 2000 {\rm K}$. The second Helium ionization instability strip exhibits strong dependences on both $g$ and $Y$. Surprisingly, increasing $Y$ by only 10% relative to solar creates an instability strip near $15,000 {\rm K}$. Hence CV's below the period gap with evolved donor stars of $Y\ga 0.4$ may have an "intermediate" instability strip well outside of the isolated DA and DB variables. This "intermediate" instability strip also occurs for low mass He WD with solar composition envelopes. The lack of pulsations in CV's with $T_{\rm eff}$ in the pure Hydrogen ZZ Ceti instability strip is also easily explained.

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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.

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arXiv2005

The Thermal State of the Accreting White Dwarf in AM Canum Venaticorum Binaries

We calculate the heating and cooling of the accreting white dwarf (WD) in the ultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD can contribute significantly to their optical and ultraviolet emission. We estimate the WD's effective temperature, Teff, using the optical continuum for a number of observed binaries, and show that it agrees well with our theoretical calculations. Driven by gravitational radiation losses, the time averaged accretion rate, , decreases monotonically with increasing Porb, covering six orders of magnitude. If the short period (Porb<10 min) systems accrete at a rate consistent with gravitational radiation via direct impact, we predict their unpulsed optical/UV light to be that of the Teff>50,000$ K accreting WD. At longer Porb we calculate the Teff and absolute visual magnitude, M_V, that the accreting WD will have during low accretion states, and find that the WD naturally crosses the pulsational instability strip. Discovery and study of pulsations could allow for the measurement of the accumulated helium mass on the accreting WD, as well as its rotation rate. Accretion heats the WD core, but for Porb>40 minutes, the WD's Teff is set by its cooling as plummets. For the two long period AM CVn binaries with measured parallaxes, GP Com and CE 315, we show that the optical broadband colors and intensity are that expected from a pure helium atmosphere WD. This confirms that the WD brightness sets the minimum light in wide AM CVn binaries, allowing for meaningful constraints on their population density from deep optical searches, both in the field and in Globular Clusters.

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arXiv2005

Exposing the Nuclear Burning Ashes of Radius Expansion Type I X-ray Bursts

We solve for the evolution of the vertical extent of the convective region of a neutron star atmosphere during a Type I X-ray burst. The convective region is well-mixed with ashes of nuclear burning and its extent determines the rise time of the burst light curve. Using a full nuclear reaction network, we show that the maximum vertical extent of the convective region during photospheric radius expansion (RE) bursts can be sufficiently great that: (1) some ashes of burning are ejected by the radiation driven wind during the RE phase and, (2) some ashes of burning are exposed at the neutron star surface following the RE phase. We find that ashes with mass number A ~ 30 - 60 are mixed in with the ejected material. We calculate the expected column density of ejected and surface ashes in hydrogen-like states and determine the equivalent widths of the resulting photoionization edges from both the wind and neutron star surface. We find that these can exceed 100 eV and are potentially detectable. A detection would probe the nuclear burning processes and might enable a measurement of the neutron star gravitational redshift. In addition, we find that in bursts with pure helium burning layers, protons from (alpha, p) reactions cause a rapid onset of the 12C(p, gamma)13N(alpha, p)16O reaction sequence. The sequence bypasses the relatively slow 12C(alpha, gamma)16O reaction and leads to a sudden surge in energy production that is directly observable as a rapid (~ ms) increase in flux during burst rise.

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arXiv2005

The Energy Dependence of Neutron Star Surface Modes and X-ray Burst Oscillations

We calculate the photon energy dependence of the pulsed amplitude of neutron star (NS) surface modes. Simple approximations demonstrate that it depends most strongly on the bursting NS surface temperature. This result compares well with full integrations that include Doppler shifts from rotation and general relativistic corrections to photon propagation. We show that the energy dependence of type I X-ray burst oscillations agrees with that of a surface mode, lending further support to the hypothesis that they originate from surface waves. The energy dependence of the pulsed emission is rather insensitive to the NS inclination, mass and radius, or type of mode, thus hindering constraints on these parameters. We also show that, for this energy-amplitude relation, the majority of the signal (relative to the noise) comes in the 2-25 keV band, so that the current burst oscillation searches with the Rossi X-Ray Timing Explorer are close to optimal. The critical test of the mode hypothesis for X-ray burst oscillations would be a measurement of the energy dependence of burst oscillations from an accreting millisecond pulsar.

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arXiv2005

The discovery of X-ray binaries in the Sculptor Dwarf Spheroidal Galaxy

We report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with $L_X$ of at least $6\times10^{33}$ ergs/sec with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5-8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries.

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arXiv2005

The Type Ia Supernova Rate

We explore the idea that the Type Ia supernovae (SNe Ia) rate consists of two components: a prompt piece that is proportional to the star formation rate (SFR) and an extended piece that is proportional to the total stellar mass. We fit the parameters of this model to the local observations of Mannucci and collaborators and then study its impact on three important problems. On cosmic scales, the model reproduces the observed SNe Ia rate density below z=1, and predicts that it will track the measured SFR density at higher redshift, reaching a value of 1-3.5 X 10^-4 per yr per Mpc^3 at z=2. In galaxy clusters, a large prompt contribution helps explain the iron content of the intracluster medium. Within the Galaxy, the model reproduces the observed stellar [O/Fe] abundance ratios if we allow a short (approximately 0.7 Gyr) delay in the prompt component. Ongoing medium-redshift SN surveys will yield more accurate parameters for our model

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arXiv2005

Surface Modes on Bursting Neutron Stars and X-ray Burst Oscillations

Accreting neutron stars (NSs) often show coherent modulations during type I X-ray bursts, called burst oscillations. We consider whether a nonradial mode can serve as an explanation for burst oscillations from those NSs which are not magnetic. We find that a surface wave in the shallow burning layer transitions into a crustal interface wave as the envelope cools, a new and previously uninvestigated phenomenon. The surface modulations decrease dramatically as the mode switches, explaining why burst oscillations often disappear before burst cooling ceases. When we include rotational modifications, we find mode frequencies and drifts consistent with those observed. The large NS spin ($\approx270-620 {\rm Hz}$) needed to make this match implies that accreting NSs are spinning at frequencies $\approx4 {\rm Hz}$ above the burst oscillation. Since the asymptotic frequency is set by the crustal interface wave, the observed late time frequency drifts are a probe of the composition and temperature of NS crusts. We compare our model with the observed drifts and persistent luminosities of X-ray burst sources, and find that NSs with a higher average accretion rate show smaller drifts, as we predict. Furthermore, the drift sizes are consistent with crusts composed of iron-like nuclei, as expected for the ashes of the He-rich bursts that are exhibited by these objects.

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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.

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arXiv2005

Classical Novae as a Probe of the Cataclysmic Variable Population

Classical Novae (CNe) are the brightest manifestation of mass transfer onto a white dwarf in a cataclysmic variable (CV). As such, they are probes of the mass transfer rate, Mdot, and WD mass, Mwd, in these interacting binaries. Our calculations of the dependence of the CN ignition mass, Mign, on Mdot and Mwd yields the recurrence times of these explosions. We show that the observed CNe orbital period distribution is consistent with the interrupted magnetic braking evolutionary scenario, where at orbital periods Porb > 3 hr mass transfer is driven by angular momentum loss via a wind from the companion star and at Porb < 3 hr by gravitational radiation. About 50% of CNe occur in binaries accreting at Mdot ~= 10^{-9} Msun/yr with Porb = 3-4 hr, with the remaining 50% split evenly between Porb longer (higher Mdot) and shorter (lower Mdot) than this. This resolution of the relative contribution to the CN rate from different CVs tells us that 3(9)x10^5 CVs with WD mass 1.0(0.6)Msun are needed to produce one CN per year. Using the K-band specific CN rate measured in external galaxies, we find a CV birthrate of 2(4)x10^{-4}/yr per 10^{10}Lsun,K, very similar to the luminosity specific Type Ia supernova rate in elliptical galaxies. Likewise, we predict that there should be 60-180 CVs for every 10^6Lsun,K in an old stellar population, similar to the number of X-ray identified CVs in the globular cluster 47 Tuc, showing no overabundance relative to the field. Using a two-component steady state model of CV evolution we show that the fraction of CVs which are magnetic (22%) implies a birthrate of 8% relative to non-magnetic CVs, similar to the fraction of strongly magnetic field WDs. (abridged)

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arXiv2005

Arbitrarily Degenerate Helium White Dwarfs as Donors in AM CVn Binaries

We apply the Deloye & Bildsten (2003) isentropic models for donors in ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact, interacting binaries. The mass-radius relations of these systems' donors in the mass range of interest ($M_2<0.1 \msun$) are not single-valued, but parameterized by the donor's specific entropy. This produces a range in the relationships between system observables, such as orbital period, $\Porb$, and mass transfer rate, $\Mdot$. For a reasonable range in donor specific entropy, $\Mdot$ can range over several orders of magnitude at fixed $\Porb$. We determine the unique relation between $\Mdot$ and $M_2$ in the AM CVn systems with known donor to accretor mass ratios, $q=M_2/M_1$. We use structural arguments, as well as each system's photometric behavior, to place limits on $\Mdot$ and $M_2$ in each. Most systems allow a factor of about 3 variation in $\Mdot$, although V803 Cen, if the current estimates of its $q$ are accurate, is an exception and must have $M_2 \approx 0.02 \msun$ and $\Mdot \approx 10^{-10} \msun$ yr$^{-1}$. Our donor models also constrain each donor's core temperature, $T_c$, range and correlate $T_c$ with $M_2$. We examine how variations in donor specific entropy across the white dwarf family \citep{nele01a} of AM CVn systems affects this population's current galactic distribution. Allowing for donors that are not fully degenerate produces a shift in systems towards longer $\Porb$ and higher $\Mdot$ increasing the parameter space in which these systems can be found. This shift increases the fraction of systems whose $\Porb$ is long enough that their gravity wave (GW) signal is obscured by the background of detached double white dwarf binaries that dominate the GW spectrum below a frequency $\approx 2$ mHz.

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arXiv2004

The Spreading Layer and Dwarf Nova Oscillations

We describe recent theoretical work on the final stage of accretion when material passes from an accretion disk onto a white dwarf surface. Our calculations focus on understanding the latitudinal spreading and differentially rotating profile of this material, so we call it the ``spreading layer'' (SL) model. The SL typically extends to an angle of 0.01-0.1 radians, with respect to the equator. At low accretion rates ($\dot{M}\la10^{18} {\rm g s}^{-1}$) the amount of spreading is small, so that the dissipated energy is radiated back into the accretion disk. When the accretion rate is high, such as in dwarf novae, symbiotic binaries, and supersoft sources, the material spreads to higher latitudes to be more easily observed. The SL may contain shallow surface modes, and we propose that such waves could produce dwarf nova oscillations (DNOs). This hypothesis naturally explains many key properties of DNOs, including their frequency range, sinusoidal nature, sensitivity to accretion rate, and why they are only seen during outburst.

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arXiv2004

A Spreading Layer Origin for Dwarf Nova Oscillations

Dwarf nova outbursts often show coherent ($Q\sim10^4-10^6$) sinusoidal oscillations with the largest pulsed fraction in the extreme ultraviolet. Called dwarf nova oscillations (DNOs), they have periods of $P\approx3-40 {\rm s}$ and scale with luminosity as $P\propto L^{-β}$ with $β\approx0.1-0.2$. We propose that DNOs may be produced by nonradial oscillations in a thin hydrostatic layer of freshly accreted material, the ``spreading layer'' (SL), at the white dwarf (WD) equator. This would naturally explain a number of key properties of DNOs, including their frequency range, sinusoidal nature, sensitivity to accretion rate, and why they are only seen during outburst. In support of this hypothesis we construct a simple model that treats the SL as a cavity containing shallow surface waves, each with the same radial structure, but split into three different modes denoted by their azimuthal wavenumber, $m$. The $m=0$ latitudinally propagating mode best matches the periods and scalings associated with most DNOs, and DNOs with periods shorter than the WD Keplerian period are explained by the $m=-1$ prograde mode. We also predict a third set of oscillations, produced by the $m=1$ retrograde mode, and show its expected dependence on accretion rate.

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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.

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arXiv2004

Neutron Star Crustal Interface Waves

The eigenfrequencies of nonradial oscillations are a powerful probe of a star's interior structure. This is especially true when there exist discontinuities such as at the neutron star (NS) ocean/crust boundary, as first noted by McDermott, Van Horn & Hansen. The interface mode associated with this boundary has subsequently been neglected in studies of stellar nonradial oscillations. We revisit this mode, investigating its properties both analytically and numerically for a simple NS envelope model. We find that it acts like a shallow surface ocean wave, but with a large radial displacement at the ocean/crust boundary due to flexing of the crust with shear modulus $μ\ll P$, the pressure. This displacement lowers the mode's frequency by a factor of $\sim(μ/P)^{1/2}\sim0.1$ in comparison to a shallow surface wave frequency on a hard surface. The interface mode may be excited on accreting or bursting NSs and future work on nonradial oscillations should consider this mode. Our work also implies an additional mode on massive and/or cold white dwarfs with crystalline cores, which may have a frequency between the f-mode and g-modes, an otherwise empty part of the frequency domain.

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