arXiv subjects

Lars Bildsten

Publications and source records attributed to Lars Bildsten.

At least 199 records · Page 11Linked to original sources

arXiv2000

Theory and Observations of Type I X-Ray Bursts from Neutron Stars

I review our understanding of the thermonuclear instabilities on accreting neutron stars that produce Type I X-Ray bursts. I emphasize those observational and theoretical aspects that should interest the broad audience of this meeting. The easily accessible timescales of the bursts (durations of tens of seconds and recurrence times of hours to days) allow for a very stringent comparison to theory. The largest discrepancy (which was found with EXOSAT observations) is the accretion rate dependence of the Type I burst properties. Bursts become less frequent and energetic as the global accretion rate increases, just the opposite of what the spherical theory predicts. I present a resolution of this issue by taking seriously the observed dependence of the burning area on the global accretion rate, which implies that as the accretion rate increases, the accretion rate per unit area decreases. This resurrects the unsolved problem of knowing where the freshly accreted material accumulates on the star, equally relevant to the likely signs of rotation during the bursts summarized by Swank at this meeting. I close by highlighting the Type I bursts from GS 1826-238 that were found with BeppoSAX and RXTE. Their energetics, recurrence times and temporal profiles clearly indicate that hydrogen is being burned during these bursts, most likely by the rapid-proton (rp) process.

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arXiv2000

Gravitational Waves from Low-Mass X-ray Binaries: a Status Report

We summarize the observations of the spin periods of rapidly accreting neutron stars. If gravitational radiation is responsible for balancing the accretion torque at the observed spin frequencies of ~300 Hz, then the brightest of these systems make excellent gravitational wave sources for LIGO-II and beyond. We review the recent theoretical progress on two mechanisms for gravitational wave emission: mass quadrupole radiation from deformed neutron star crusts and current quadrupole radiation from r-mode pulsations in neutron star cores.

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arXiv1999

Viscous Boundary Layer Damping of R-Modes in Neutron Stars

Recent work has raised the exciting possibility that r-modes (Rossby waves) in rotating neutron star cores might be strong gravitational wave sources. We estimate the effect of a solid crust on their viscous damping rate and show that the dissipation rate in the viscous boundary layer between the oscillating fluid and the nearly static crust is >10^5 times higher than that from the shear throughout the interior. This increases the minimum frequency for the onset of the gravitational r-mode instability to at least 500 Hz when the core temperature is less than 10^10 K. It eliminates the conflict of the r-mode instability with the accretion-driven spin-up scenario for millisecond radio pulsars and makes it unlikely that the r-mode instability is active in accreting neutron stars. For newborn neutron stars, the formation of a solid crust shortly after birth affects their gravitational wave spin-down and hence detectability by ground-based interferometric gravitational wave detectors.

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arXiv1999

A Method for Distinguishing Between Transiently Accreting Neutron Stars and Black Holes, in Quiescence

We fit hydrogen atmosphere models to the X-ray data for four neutron stars (three from a previous paper, plus 4U 2129+47) and six black hole candidates (A0620-00, GS 2000+25, GS 1124-68, GS 2023+33, GRO J1655-40, and GRO J0422+32). While the neutron stars are similar in their intrinsic X-ray spectra (similar effective temperatures and emission area radii ~10 km), the spectra of two black hole candidates are significantly different, and the spectra of the remaining four are consistent with a very large parameter space that includes the neutron stars. The spectral differences between the neutron stars and black hole candidates favors the interpretation that the quiescent neutron star emission is predominantly thermal emission from the neutron star surface. Our work suggests that an X-ray spectral comparison in quiescence provides an additional means for distinguishing between neutron stars and black holes. The faint X-ray sources in globular clusters are also a class of objects which can be investigated in this manner.

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arXiv1999

The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

We calculate the nucleosynthesis during stable nuclear burning on an accreting neutron star. This is appropriate for weakly magnetic neutron stars accreting at near-Eddington rates in low mass X-ray binaries, and for most accreting X-ray pulsars. We show that the nuclear burning proceeds via the rapid proton capture process (rp process), and makes nuclei far beyond the iron group. The final mixture of nuclei consists of elements with a range of masses between approximately A=60 and A=100. The average nuclear mass of the ashes is set by the extent of helium burning via (alpha,p) reactions, and depends on the local accretion rate. Our results imply that the crust of these accreting neutron stars is made from a complex mixture of heavy nuclei, with important implications for its thermal, electrical and structural properties. A crustal lattice as impure as our results suggest will have a conductivity set mostly by impurity scattering, allowing more rapid Ohmic diffusion of magnetic fields than previously estimated.

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arXiv1998

The Outbursts and Orbit of the Accreting Pulsar GS 1843-02 = 2S 1845-024

We present observations of a series of 10 outbursts of pulsed hard X-ray flux from the transient 10.6 mHz accreting pulsar GS 1843-02, using the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. These outbursts occurred regularly every 242 days, coincident with the ephemeris of the periodic transient GRO J1849-03 (Zhang et al. 1996), which has recently been identified with the SAS 3 source 2S 1845-024 (Soffitta et al. 1998). Our pulsed detection provides the first clear identification of GS 1843-02 with 2S 1845-024. We present a pulse timing analysis which shows that the 2S 1845-024 outbursts occur near the periastron passage of the neutron star's highly eccentric (e = 0.88+-0.01) 242.18+-0.01 day period binary orbit about a high mass (M > 7 solar masses) companion. The orbit and transient outburst pattern strongly suggest the pulsar is in a binary system with a Be star. Our observations show a long-term spin-up trend, with most of the spin-up occurring during the outbursts. From the measured spin-up rates and inferred luminosities we conclude that an accretion disk is present during the outbursts.

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arXiv1998

Discovery of a Magnetic White Dwarf in the Symbiotic Binary Z Andromedae

We report the first result from our survey of rapid variability in symbiotic binaries: the discovery of a persistent oscillation at P=1682.6 +- 0.6 s in the optical emission from the prototype symbiotic, Z Andromedae. The oscillation was detected on all 8 occasions on which the source was observed over a timespan of nearly a year, making it the first such persistent periodic pulse found in a symbiotic binary. The amplitude was typically 2 - 5 mmag, and it was correlated with the optical brightness during a relatively small outburst of the system. The most natural explanation is that the oscillation arises from the rotation of an accreting, magnetic (B_S > 10^5 G) white dwarf. This discovery constrains the outburst mechanisms, since the oscillation emission region near the surface of the white dwarf was visible during the outburst.

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arXiv1998

Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars

Nuclear reactions occurring deep in the crust of a transiently accreting neutron star efficiently maintain the core at a temperature >5e7 K. When accretion halts, the envelope relaxes to a thermal equilibrium set by the flux from the hot core, as if the neutron star were newly born. For the time-averaged accretion rates typical of low-mass X-ray transients, standard neutrino cooling is unimportant and the core thermally re-radiates the deposited heat. The resulting luminosity has the same magnitude as that observed from several transient neutron stars in quiescence. Confirmation of this mechanism would strongly constrain rapid neutrino cooling mechanisms for neutron stars. Thermal emission had previously been dismissed as a predominant source of quiescent emission since blackbody spectral fits implied an emitting area much smaller than a neutron star's surface. However, as with thermal emission from radio pulsars, fits with realistic emergent spectra will imply a substantially larger emitting area. Other emission mechanisms, such as accretion or a pulsar shock, can also operate in quiescence and generate intensity and spectral variations over short timescales. Indeed, quiescent accretion may produce gravitationally redshifted metal photoionization edges in the quiescent spectra (detectable with AXAF and XMM). We discuss past observations of Aql~X-1 and note that the low luminosity X-ray sources in globular clusters and the Be star/X-ray transients are excellent candidates for future study.

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arXiv1998

Hydrogen Electron Capture in Accreting Neutron Stars and the Resulting g-Mode Oscillation Spectrum

We investigate hydrogen electron capture in the oceans of neutron stars accreting at rates appropriate for most Low-Mass X-Ray Binaries. These stars burn the accreted hydrogen and helium unstably in the upper atmosphere and accumulate material which usually contains some small amount of hydrogen (mass fractions are typically 10%) mixed in with the heavier iron group ashes. The subsequent evolution of this matter is determined by compression towards higher densities until electron capture on the hydrogen occurs. We construct steady-state models of the electron captures and the subsequent neutron recombinations onto the heavy nuclei. The density discontinuity from these captures gives rise to a new g-mode (much like a surface wave), which has a lowest order (l=1) frequency of 35 Hz when X=0.1 on a slowly rotating star. We also discuss, for the first time, a new set of non-radial g-modes unique to these neutron stars. These modes have most of their nodes in the finite thickness layer where the electron captures are occurring. The lowest order mode frequencies are in the 1-10 Hz range for a few radial nodes on a slowly rotating star. We conclude by discussing how the dispersion relations for these modes are modified for a rapidly rotating neutron star. Whether any of these modes are observable depends on their excitation and damping mechanisms and the ability to excite them, issues we will address in a future paper.

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arXiv1998

Gravitational Radiation and Rotation of Accreting Neutron Stars

Recent discoveries by the Rossi X-Ray Timing Explorer indicate that most of the rapidly accreting and weakly magnetic neutron stars in the Galaxy are rotating at spin frequencies greater than 250 Hz. Remarkably, they all rotate in a narrow range of frequencies. I suggest that these stars rotate fast enough so that, on average, the angular momentum added by accretion is lost to gravitational radiation. The strong spin frequency dependence of the angular momentum loss rate from gravitational radiation then provides a natural reason for similar spin frequencies. Provided that the interior temperature has a large scale asymmetry misaligned from the spin axis, then the temperature sensitive electron captures in the deep crust can provide the quadrupole needed to reach this limiting situation at 300 Hz. This quadrupole is only present during accretion and makes it difficult to form radio pulsars spinning more rapidly than 600-800 Hz via rapid accretion. The gravity wave strength is <10^{-26} from most neutron stars and >2 X 10^{-26} for Sco X-1. Prior knowledge of the position, spin frequency and orbital periods will allow for deep searches for these periodic signals with gravitational wave interferometers (LIGO, VIRGO and the dual-recycled GEO 600 detector) and experimenters need to take such sources into account. Sco X-1 will most likely be detected first.

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arXiv1998

Rapid Rotation and Nonradial Pulsations: Kappa-Mechanism Excitation of G-Modes in B Stars

Several classes of stars (most notably O and B main-sequence stars, as well as accreting white dwarfs and neutron stars) rotate quite rapidly, at spin frequencies greater than the typical g-mode frequencies. We discuss how rapid rotation modifies the $κ$-mechanism excitation and observability of g-mode oscillations. We find that, by affecting the timescale match between the mode period and the thermal time at the driving zone, rapid rotation stabilizes some of the g-modes that are excited in a non-rotating star, and, conversely, excites g-modes that are damped in absence of rotation. The fluid velocities and temperature perturbations are strongly concentrated near the equator for most g-modes in rapidly rotating stars, which means that a favorable viewing angle may be required to observe the pulsations. Moreover, the stability of modes of the same $l$ but different $m$ is affected differently by rotation. We illustrate this by considering g-modes in Slowly Pulsating B-type stars as a function of the rotation rate.

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arXiv1998

The Ocean and Crust of a Rapidly Accreting Neutron Star: Implications for Magnetic Field Evolution and Thermonuclear Flashes

We investigate the atmosphere, ocean, and crust of neutron stars accreting at rates sufficiently high (typically in excess of the local Eddington limit) to stabilize the burning of accreted hydrogen and helium. For hydrogen-rich accretion at global rates in excess of 10^-8 solar masses per year (typical of a few neutron stars), we discuss the thermal state of the deep ocean and crust and their coupling to the neutron star core, which is heated by conduction (from the crust) and cooled by neutrino emission. We estimate the Ohmic diffusion time in the hot, deep crust and find that it is noticeably shortened (to less than 10^8 yr) from the values characteristic of the colder crusts in slowly accreting neutron stars. We speculate on the implications of these calculations for magnetic field evolution in the bright accreting X-ray sources. We also explore the consequences of rapid compression at local accretion rates exceeding ten times the Eddington rate. This rapid accretion heats the atmosphere/ocean to temperatures of order 10^9 K at relatively low densities; for stars accreting pure helium, this causes unstable ignition of the ashes (mostly carbon) resulting from stable helium burning. This unstable burning can re-occur on timescales as short as hours to days, and might be the cause of some flares on helium accreting pulsars, in particular 4U~1626--67. Such rapid local accretion rates are common on accreting X-ray pulsars, where the magnetic field focuses the accretion flow onto a small fraction of the stellar area.

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arXiv1998

The Angular Momentum of Accreting Neutron Stars

I review the rotation measurements of accreting neutron stars. Many of the highly magnetic accreting X-ray pulsars have been continuously observed with the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory (CGRO) since April 1991. These observations show that the accretion torque exerted on many disk-fed accreting X-ray pulsars changes sign on a monthly to yearly timescale. This results in alternating periods of spin-up and spin-down with nearly the same torques, leading to little net angular momentum gained by accretion. I also summarize recent discoveries with the Rossi X-Ray Timing Explorer (RXTE) of periodicities during Type I X-ray bursts. These seem to indicate that many of the rapidly accreting and weakly magnetic neutron stars in our galaxy are rotating at frequencies greater than 250 Hertz. Most remarkable is that they all rotate within a rather narrow range of frequencies.

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arXiv1997

Oceanography of Accreting Neutron Stars: Non-Radial Oscillations and Periodic X-Ray Variability

Observations of quasi-periodic oscillations (QPOs) in the luminosity from many accreting neutron stars (NS) have led us to investigate a source of periodicity prevalent in other stars: non-radial oscillations. After summarizing the structure of the atmosphere and ocean of an accreting NS, we discuss the various low l g-modes with frequencies in the 1-100 Hz range. Successful identification of a non-radial mode with an observed frequency would yield new information about the thermal and compositional makeup of the NS, as well as its radius. We close by discussing how rapid rotation changes the g-mode frequencies.

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arXiv1997

Light Element Depletion in Contracting Brown Dwarfs and Pre--Main-Sequence Stars

We present an analytic calculation of the thermonuclear depletion of the light elements lithium, beryllium, and boron in fully convective, low-mass stars. Under the presumption that the pre--main-sequence star is always fully mixed during contraction, we find that the burning of these rare light elements can be computed analytically, even when the star is degenerate. Using the effective temperature as a free parameter, we constrain the properties of low-mass stars from observational data, independently of the uncertainties associated with modeling their atmospheres and convection. Our analytic solution explains the dependence of the age at a given level of elemental depletion on the stellar effective temperature, nuclear cross sections, and chemical composition. Most importantly, our results allow observers to translate lithium non-detections in young cluster members into a model-independent minimum age for that cluster. Using this procedure, we have found lower limits to the ages of the Pleiades (100 Myr) and Alpha Persei (60 Myr) clusters. Recent experimental work on the low energy resonance in the ^10B(p,α)^7Be reaction has greatly enhanced estimates of the destruction rate of ^10B, making it possible for stars with M>0.1 M_sun to deplete both ^10B and ^11B before reaching the main sequence. Moreover, there is an interesting range of masses, 0.085 M_sun < M < 0.13 M_sun, where boron depletion occurs on the main sequence in less than a Hubble time, providing a potential ``clock'' for dating low-mass stars.

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arXiv1997

Thermonuclear Burning on Rapidly Accreting Neutron Stars

Neutron stars in mass-transferring binaries are accreting the hydrogen and helium rich matter from the surfaces of their companions. This article simply explains the physics associated with how that material eventually fuses to form heavier nuclei and the observations of the time dependent phenomena (such as Type I X-ray bursts) associated with the thermally unstable thermonuclear reactions. We explain how the outcome depends on the composition of the accreting matter, the accretion rate and the mass, radius and thermal state of the neutron star. We also introduce many new analytic relations that are convenient for comparisons to both observations and computational results. After explaining nuclear burning for spherically symmetric accretion onto neutron stars, we discuss the possibility of asymmetric burning. In particular, we discuss some of the mysteries from EXOSAT observations of Type I X-Ray bursts and how the solution to these puzzles may lie in considering the lateral propagation of nuclear burning fronts around the star. Fully understanding this problem requires knowledge of parameters previously neglected such as the distribution of fresh fuel on the star, the magnetic field strength, and the stellar rotation. Recent RXTE observations of bursters may finally tell us some of these parameters.

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arXiv1997

On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars

Dramatic torque reversals between spin up and spin down have been observed in half of the persistent X-ray pulsars monitored by the BATSE all-sky monitor on CGRO. Theoretical models developed to explain early pulsar timing data can explain spin down torques via a disk-magnetosphere interaction if the star nearly corotates with the inner accretion disk. To produce the observed BATSE torque reversals, however, these equilibrium models require the disk to alternate between two mass accretion rates, with $\dot M_{\pm}$ producing accretion torques of similar magnitude, but always of opposite sign. Moreover, in at least one pulsar (GX 1+4) undergoing secular spin down the neutron star spins down faster during brief ($\sim 20$ day) hard X-ray flares -- this is opposite the correlation expected from standard theory, assuming BATSE pulsed flux increases with mass accretion rate. The $10$ day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or viscous time scale near the inner disk boundary and are more suggestive of a global disk phenomenon. We discuss possible explanations of the observed torque behavior. Despite the preferred sense of rotation defined by the binary orbit, the BATSE observations are surprisingly consistent with an earlier suggestion by Makishima \etal (1988) for GX~1+4: the disks in these systems somehow alternate between episodes of prograde and retrograde rotation. We are unaware of any mechanism that could produce a stable retrograde disk in a binary undergoing Roche-lobe overflow, but such flip-flop behavior does occur in numerical simulations of wind-fed systems. One possibility is that the disks in some of these binaries are fed by an X-ray excited wind.

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arXiv1997

Observations of Accreting Pulsars

We summarize five years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered 5 new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A 0535+26, GRO J2058+42, 4U 1145-619 and A 1118-616), and also measured the accretion torque history of during outbursts of 6 of those transients whose orbital parameters were also known. We have also continuously measured the pulsed flux and spin frequency for eight persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long time scales, blurring the conventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars, but uncorrelated, or even anticorrelated, in persistent sources.

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