arXiv subjects

Lars Bildsten

Publications and source records attributed to Lars Bildsten.

At least 163 records · Page 9Linked to original sources

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.

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arXiv2004

Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies

Chandra observations of distant elliptical galaxies have revealed large numbers of Low Mass X-ray Binaries (LMXBs) accreting at rates in excess of 10^{-8} solar masses per year. The majority of these LMXBs reside in globular clusters (GCs) and it has been suggested that many of the field LMXBs also originated in GCs. We show here that ultracompact binaries with orbital periods of 8-10 minutes and He or C/O donors of 0.06-0.08 solar masses naturally provide the observed accretion rates from gravitational radiation losses alone. Such systems are predicted to be formed in the dense GC environment, a hypothesis supported by the 11.4 minute binary 4U 1820-30, the brightest persistent LMXB in a Galactic GC. These binaries have short enough lifetimes (less then 3 Myr) while bright that we calculate their luminosity function under a steady-state approximation. This yields a luminosity function slope in agreement with that observed for luminosities in the range of 6E37 ergs/sec to 5E38 ergs/sec, encouraging us to use the observed numbers of LMXBs per GC mass to calculate the accumulated number of ultracompact binaries. For a constant birthrate over 8 Gyrs, the number of ultracompact binaries which have evolved through this bright phase is nearly 4000 in a 10 million solar mass GC, consistent with dynamical interaction calculations. Perhaps most importantly, if all ultracompacts become millisecond radio pulsars, then the observed normalization agrees with the inferred number of millisecond radio pulsars in 47 Tuc and Galactic GCs in general.

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arXiv2004

Spreading of Accreted Material on White Dwarfs

When a white dwarf (WD) is weakly magnetized and its accretion disk is thin, accreted material first reaches the WD's surface at its equator. This matter slows its orbit as it comes into co-rotation with the WD, dissipating kinetic energy into thermal energy and creating a hot band of freshly accreted material around the equator. Radiating in the extreme ultraviolet and soft X-rays, this material moves toward the pole as new material piles behind it, eventually becoming part of the WD once it has a temperature and rotational velocity comparable with the surface. We present a set of solutions which describe the properties of this ``spreading layer'' in the steady state limit based on the conservation equations derived by Inogamov & Sunyaev (1999) for accreting neutron stars. Our analysis and subsequent solutions show that the case of WDs is qualitatively different. We investigate example solutions of the spreading layer for a WD of mass $M=0.6M_\odot$ and radius $R=9\times10^{8}{\rm cm}$. These solutions show that the spreading layer typically extends to an angle of $θ_{\rm SL}\approx0.01-0.1$ (with respect to the equator), depending on accretion rate and the magnitude of the viscosity. At low accretion rates, $\dot{M}\lesssim10^{18}{\rm g s}^{-1}$, the amount of spreading is negligible and most of 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 may spread to latitudes high enough to be directly visible above the accretion disk.

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arXiv2003

Unstable Nonradial Oscillations on Helium Burning Neutron Stars

Material accreted onto a neutron star can stably burn in steady state only when the accretion rate is high (typically super-Eddington) or if a large flux from the neutron star crust permeates the outer atmosphere. For such situations we have analyzed the stability of nonradial oscillations, finding one unstable mode for pure helium accretion. This is a shallow surface wave which resides in the helium atmosphere above the heavier ashes of the ocean. It is excited by the increase in the nuclear reaction rate during the oscillations, and it grows on the timescale of a second. For a slowly rotating star, this mode has a frequency of approximately 20-30 Hz (for l=1), and we calculate the full spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The short period X-ray binary 4U 1820--30 is accreting helium rich material and is the system most likely to show this unstable mode,especially when it is not exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally stable atmosphere shows that nonradial perturbations have a different stability criterion than the spherically symmetric thermal perturbations that generate type I X-ray bursts.

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arXiv2003

Measuring White Dwarf Accretion Rates via their Effective Temperatures

Our previous theoretical study of the impact of an accreting envelope on the thermal state of an underlying white dwarf (WD) has yielded equilibrium core temperatures, classical nova ignition masses and thermal luminosities for WDs accreting at time averaged rates of = 10^-11 - 10^-8 Msun/yr. These 's are appropriate to WDs in cataclysmic variables (CVs) of P_orb <~ 7 hr, many of which accrete sporadically as Dwarf Novae. Approximately thirty nonmagnetic Dwarf Novae have been observed in quiescence, when the accretion rate is low enough for spectral detection of the WD photosphere, and a measurement of T_eff. We use our theoretical work to translate the measured T_eff's into local time-averaged accretion rates, confirming the factor of ten drop in predicted for CV's as they transit the period gap. For DN below the period gap, we show that if is that given by gravitational radiation losses alone, then the WD masses are > 0.8 Msun. An alternative conclusion is that the masses are closer to 0.6 Msun and is 3-4 times larger than that expected from gravitational radiation losses. In either case, it is very plausible that a subset of CVs with P_orb < 2 hours will have T_eff's low enough for them to become non-radial pulsators, as discovered by van Zyl and collaborators in GW Lib.

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arXiv2003

Theoretical Modeling of the Thermal State of Accreting White Dwarfs Undergoing Classical Novae

White dwarfs experience a thermal renaissance when they receive mass from a stellar companion in a binary. For accretion rates < 10^-8 Msun/yr, the freshly accumulated hydrogen/helium envelope ignites in a thermally unstable manner that results in a classical novae (CN) outburst and ejection of material. We have undertaken a theoretical study of the impact of the accumulating envelope on the thermal state of the underlying white dwarf (WD). This has allowed us to find the equilibrium WD core temperatures (T_c), the classical nova ignition masses (M_ign) and the thermal luminosities for WDs accreting at rates of 10^-11 - 10^-8 Msun/yr. These accretion rates are most appropriate to WDs in cataclysmic variables (CVs) of P_orb <~ 7 hr, many of which accrete sporadically as dwarf novae. We have included ^3He in the accreted material at levels appropriate for CVs and find that it significantly modifies the CN ignition mass. We compare our results with several others from the CN literature and find that the inclusion of ^3He leads to lower M_ign for >~ 10^-10 Msun/yr, and that for below this the particular author's assumption concerning T_c, which we calculate consistently, is a determining factor. Initial comparisons of our CN ignition masses with measured ejected masses find reasonable agreement and point to ejection of material comparable to that accreted.

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arXiv2003

White Dwarf Donors in Ultracompact Binaries: The Stellar Structure of Finite Entropy Objects

We discuss the mass-radius (M-R) relations for low-mass (M<0.1 Msun) white dwarfs (WDs) of arbitrary degeneracy and evolved (He, C, O) composition. We do so with both a simple analytical model and models calculated by integration of hydrostatic balance using a modern equation of state valid for fully ionized plasmas. The M-R plane is divided into three regions where either Coulomb physics, degenerate electrons or a classical gas dominate the WD structure. For a given M and central temperature, T_c, the M-R relation has two branches differentiated by the model's entropy content. We present the M-R relations for a sequence of constant entropy WDs of arbitrary degeneracy parameterized by M and T_c for pure He, C, and O. We discuss the applications of these models to the recently discovered accreting millisecond pulsars. We show the relationship between the orbital inclination for these binaries and the donor's composition and T_c. In particular we find from orbital inclination constraints that the probability XTE J1807-294 can accommodate a He donor is approximately 15% while for XTE J0929-304, it is approximately 35%. We argue that if the donors in ultracompact systems evolve adiabatically, there should be 60-160 more systems at orbital periods of 40 min than at orbital periods of 10 min, depending on the donor's composition.

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arXiv2003

New Views of Thermonuclear Bursts

Since the advent of powerful new X-ray observatories, NASA's Rossi X-ray Timing Explorer (RXTE), the Italian - Dutch BeppoSAX mission, XMM-Newton and Chandra, a number of entirely new phenomena associated with thermonuclear burning on neutron stars have been discovered. These include: (i) the discovery of millisecond (300 - 600 Hz) oscillations during bursts, so called ``burst oscillations'', (ii) a new regime of nuclear burning on neutron stars which manifests itself through the generation of hours long flares about once a decade, now referred to as ``superbursts'',(iii) discoveries of bursts from low accretion rate neutron stars, and (iv) new evidence for discrete spectral features from bursting neutron stars. In this article we review our current understanding of thermonuclear bursts on neutron stars, with a focus on these new phenomena.

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arXiv2002

Arresting Accretion Torques with Gravitational Radiation

Recent theoretical work has made it plausible for neutron stars (NSs) to lose angular momentum via gravitational radiation on long timescales (around Myrs) while actively accreting. The gravitational waves (GWs) can either be emitted via the excitation of r-modes or from a deformed crust. GW emission can thus intervene to slow-down or halt the otherwise relentless spin-up from accretion. Prior to this theoretical work (and the measurements of NS rotation rates in LMXBs) the community was rather confident that an accreting NS would be spun-up to rotation rates near breakup, motivating searches for sub-millisecond objects. After only briefly describing the physics of the GW processes, I argue that the limiting spin frequency might be appreciably lower than the breakup frequency. Millisecond radio pulsar observers would likely discover the impact of GW emission as a dropoff in the number of pulsars beyond 600 Hz, and I show here that the millisecond pulsar inventory in 47 Tuc might already exhibit such a cutoff. These theoretical ideas will be tested by GW searches with ground-based interferometers, such as the advanced LIGO instrument proposed for operation by 2008.

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arXiv2002

Sinking of Ne-22 in Liquid White Dwarf Interiors

We assess the impact of the trace element Ne-22 on the cooling and seismology of a liquid C/O white dwarf (WD). Due to this element's neutron excess, it sinks towards the interior as the liquid WD cools. The gravitational energy released slows the WD's cooling by 0.5-1.6 Gyr. In addition the Ne-22 abundance gradient changes the periods of the high radial order g-modes at the 1% level.

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arXiv2002

White Dwarf Colors in Low Accretion Rate Binaries

Our recent theoretical work (Townsley and Bildsten 2002) on the thermal state of white dwarfs (WDs) in low mass transfer rate binaries allows us to predict the broadband colors of the binary from those of the WD and companion when the disk is dim. The results based on standard CV evolution are presented h ere. These will aid the discovery of such objects in field surveys and proper-motion selected globular cluster surveys with HST; especially for the largely unexplored post period minim um Cataclysmic Variables (CVs) with the lowest accretion rates and degenerate companions. We have also calculated the fraction of time that the WD resides in the ZZ Ceti instability strip thus clarifying that we expect many accreting WDs to exhibit non-radial oscillations. The study of these will provide new insights into the rotational and thermal structure of an actively accreting WD.

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arXiv2002

Internal Conditions of Accreting White Dwarfs

We explain the physics of compressional heating of the deep interior of an accreting white dwarf (WD) at accretion rates low enough so that the accumulated hydrogen burns unstably and initiates a classical nova (CN). In this limit, the WD core temperature (T_c) reaches an equilibrium value (T_c,eq) after accreting an amount of mass much less than the WD's mass. Once this equilibrium is reached, the compressional heating from within the envelope exits the surface. This equilibrium yields useful relations between the WD surface temperature, accretion rate and mass that can be employed to measure accretion rates from observed WD effective temperatures, thus testing binary evolution models for cataclysmic variables.

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arXiv2002

Hot White Dwarf Donors in Ultracompact X-Ray Binaries

The discovery of two accreting millisecond X-ray pulsars in binaries with 43 minute orbital periods allows for a new probe of the donor's structure. For XTE J1751-305, only a hot white dwarf (WD) can fill the Roche Lobe. A cold He WD is a possible solution for XTE J0929-314, though I will show that evolutionary arguments make a hot WD more likely. In addition to being larger than the T=0 models, these finite entropy, low-mass (<0.03 solar masses) WDs have a minimum mass for a fixed core temperature. If they remain hot as they lose mass and expand, they can ``evaporate'' to leave an isolated millisecond radio pulsar. They also adiabatically expand upon mass loss at a rate faster than the growth of the Roche radius if the angular momentum deposited in the disk is not returned to the donor. If the timescale of the resulting runaway mass transfer is shorter than the viscous timescale in the outer disk, then the mass transfer instability of Ruderman and Shaham for He WDs would be realized. However, my estimates of these timescales still makes the instability unlikely for adiabatic responses. I close by noting the possible impact of finite T WDs on our understanding of AM CVn binaries.

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arXiv2002

Gravitational Settling of ^{22}Ne in Liquid White Dwarf Interiors--Cooling and Seismological Effects

We assess the impact of the trace element ^{22}Ne on the cooling and seismology of a liquid C/O white dwarf (WD). Due to this elements' neutron excess, it sinks towards the interior as the liquid WD cools. The subsequent gravitational energy released slows the cooling of the WD by 0.25--1.6 Gyrs by the time it has completely crystallized, depending on the WD mass and the adopted sedimentation rate. The effects will make massive WDs or those in metal rich clusters (such as NGC 6791) appear younger than their true age. Our diffusion calculations show that the ^{22}Ne mass fraction in the crystallized core actually increases outwards. The stability of this configuration has not yet been determined. In the liquid state, the settled ^{22}Ne enhances the internal buoyancy of the interior and changes the periods of the high radial order g-modes by approximately 1%. Though a small adjustment, this level of change far exceeds the accuracy of the period measurements. A full assessment and comparison of mode frequencies for specific WDs should help constrain the still uncertain ^{22}Ne diffusion coefficient for the liquid interior.

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arXiv2002

Transient X-ray Binaries in Elliptical Galaxies

Chandra observations of elliptical galaxies have revealed large numbers of Low Mass X-ray Binaries (LMXBs) accreting at rates above 10^-9 solar masses per year. One scenario which generates this transfer rate from an old stellar population is nuclear driven mass transfer onto a neutron star or black hole from a Roche lobe filling red giant. However, in our Galaxy, most of these systems accrete sporadically as transients due to a thermal instability in the accretion disk. Using the common criterion for disk instability (including irradiation), we find that this mode of mass transfer leads to transient accretion for at least 75% of the binary's life. Repeated Chandra observations of elliptical galaxies should reveal this population. The recurrence times might exceed 1000 years (depending on the orbital period at the onset of mass transfer), and outbursts might last for 1-100 years. Mass transfering binaries can also be formed in old populations via interactions in dense stellar environments, such as globular clusters. These tend to have shorter orbital periods and are more likely stable accretors, making them apparently a large fraction of the elliptical's LMXB population.

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arXiv2002

Variable Thermal Emission from Aql X-1 in Quiescence

We obtained four Chandra/ACIS-S observations beginning two weeks after the end of the November 2000 outburst of the neutron star (NS) transient Aql X-1. Over the five month span in quiescence, the X-ray spectra are consistent with thermal emission from a NS with a pure hydrogen photosphere and R_{\infty}=15.9+{0.8}-{2.9} (d/5 kpc) km at the optically implied X-ray column density. We also detect a hard power-law tail during two of the four observations. The intensity of Aql X-1 first decreased by 50+/-4% over three months, then increased by 35+/-5% in one month, and then remained constant (<6% change) over the last month. These variations in the first two observations cannot be explained by a change in the power-law spectral component, nor in the X-ray column density. Presuming that R_{\infty} is not variable and a pure hydrogen atmosphere, the long-term changes can only be explained by variations in the NS effective temperature, from kT_{eff, \infty}=130+3-5 eV, down to 113+3-4 eV, finally increasing to 118+9-4 eV for the final two observations. During one of these observations, we observe two phenomena which were previously suggested as indicators of quiescent accretion onto the NS: short-timescale (<1e4 sec) variability (at 32+8-6% rms), and a possible absorption feature near 0.5 keV. The possible absorption feature can potentially be explained as due to a time-variable response in the ACIS detector. Even so, such a feature has not been detected previously from a NS, and if confirmed and identified, can be exploited for simultaneous measurements of the photospheric redshift and NS radius.

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arXiv2002

Variability in the Thermal Emission from Accreting Neutron Star Transients

The composition of the outer 100 m of a neutron star sets the heat flux that flows outwards from the core. For an accreting neutron star in an X-ray transient, the thermal quiescent flux depends sensitively on the amount of hydrogen and helium remaining on the surface after an accretion outburst and on the composition of the underlying ashes of previous H/He burning. Because H/He has a higher thermal conductivity, a larger mass of H/He implies a shallower thermal gradient through the low density envelope and hence a higher effective temperature for a given core temperature. The mass of residual H and He varies from outburst to outburst, so the thermal quiescent flux is variable even though the core temperature is constant for timescales < 10 000 yr. Heavy elements settle from a H/He envelope in a few hours; we therefore model the quiescent envelope as two distinct layers, H/He over heavier elements, and treat the mass of H/He as a free parameter. We find that the emergent thermal quiescent flux can vary by a factor of 2 to 3 between different quiescent epochs. The variation is more pronounced at lower interior temperatures, making systems with low quiescent luminosities and frequent outbursts, such as SAX J1808.4-3658, ideal candidates from which to observe this effect. We compute, for different ash compositions, the interior temperatures of Cen X-4, Aql X-1, and SAX J1808.4-3658. In the case of Aql X-1, the inferred high interior temperature suggests that neutrino cooling contributes to the neutron star's thermal balance.

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arXiv2002

Four New Delta Scuti Pulsators from a Variability Survey of 131 Stars

In a photometric variability survey of 131 stars with B < 14 mag, we have found four new Delta Scuti stars. We were sensitive to oscillation amplitudes as low as a few mmag. The detection rate of short-period (P < 0.1 d) pulsating variable stars, which may be relevant for planned large variability surveys such as GAIA, POI, and even the LSST, was therefore 3%. All four new variable stars have low pulsation amplitude (tens of mmag), and one has a very short period (0.0198 d). This star is one of the fastest known Delta Scuti pulsators. The short period of this variable star makes it observationally tractable, and it may therefore be a particularly good candidate for asteroseismological studies. All four new variable stars will add to the cadre of low-amplitude and relatively short-period Delta Scuti stars that are potentially useful for learning about the structure of stars on or near the main sequence, slightly more massive than the Sun.

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