Showing papers by "Ronald E. Taam published in 2007"

On the rarity of double black hole binaries : Consequences for gravitational wave detection

Abstract: Double black hole binaries are among the most important sources of gravitational radiation for ground-based detectors such as LIGO or VIRGO. Even if formed with lower efficiency than double neutron star binaries, they could dominate the predicted detection rates, since black holes are more massive than neutron stars and therefore could be detected at greater distances. Here we discuss an evolutionary process that could very significantly limit the formation of close double black hole binaries: the vast majority of their potential progenitors undergo a common-envelope (CE) phase while the donor, one of the massive binary components, is evolving through the Hertzsprung gap. Our latest theoretical understanding of the CE process suggests that this will probably lead to a merger, inhibiting double black hole formation. Barring uncertainties in the physics of CE evolution, we use population synthesis calculations and find that the corresponding reduction in the merger rate of double black holes formed in galactic fields is so great (by ~500) that their contribution to inspiral detection rates for ground-based detectors could become relatively small (~1 in 10) compared to double neutron star binaries. A similar process also reduces the merger rates for double neutron stars, by a factor of ~5, eliminating most of the previously predicted ultracompact NS-NS systems. Our predicted detection rates for Advanced LIGO are now much lower for double black holes (~2 yr-1), but are still quite high for double neutron stars (~20 yr-1). If double black holes were found to be dominant in the detected inspiral signals, this could indicate that they mainly originate from dense star clusters (not included here) or that our theoretical understanding of the CE phase requires significant revision.

The Existence of Inner Cool Disks in the Low/Hard State of Accreting Black Holes

Abstract: The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low/hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low/hard state of black hole transient systems for a range of luminosities that depends on the magnitude of the viscosity parameter. For α ~ 0.1-0.4, an inner disk can exist for luminosities in the range ~(0.001-0.02)LEdd. The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and SWIFT J1753.5-0127 in their low/hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low-luminosity active galactic nuclei.

Constraints on Thermal X-Ray Radiation from SAX J1808.4–3658 and Implications for Neutron Star Neutrino Emission*

Abstract: Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars and thus on the interior composition and properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power law of photon index 1.83 ± 0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of 68% of that inferred from the 2001 observation. Simultaneous fitting of all available XMM-Newton data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of LNS < 1.1 × 1031 ergs s-1. This limit excludes some current models of neutrino emission mediated by pion condensates and provides further evidence of additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars.

The thermal evolution of the donors in AM Canum Venaticorum binaries

Abstract: We calculate the full stellar structural evolution of donors in AM CVnAM Canum Venaticorum (AM CVn) systems formed through the white dwarf (WD) channel coupled to the binary's evolution. Contrary to assumptions made in prior modelling, these donors are not fully convective over much of the AM CVn phase and do not evolve adiabatically under mass loss indefinitely. Instead, we identify three distinct phases of evolution: a mass-transfer turn-on phase (during which P orb continues to decrease even after contact, the donor contracts, and the mass-transfer rate accelerates to its maximum), a phase in which the donor expands adiabatically in response to mass loss, and a cooling phase beginning at P orb ≈ 45-55 min during which the donor contracts. The physics that determines the behaviour in the first and third phases, both of which are new outcomes of this study, are discussed in some detail. We find the overall duration of the turn-on phase to be between ∼10 4 and ∼10 6 yr, significantly longer than prior estimates. We predict the donor's luminosity, L, and effective temperature, T eff . During the adiabatic expansion phase (ignoring irradiation effects), L ≈ 10 -6 -10 -4 L ⊙ and T eff ≈ 1000-1800 K. However, the flux generated in the accretion flow dominates the donor's intrinsic light at all times. The impact of irradiation on the donor extends the phase of adiabatic expansion to longer P orb , slows the contraction during the cooling phase, and alters the donor's observational characteristics. Irradiated donors during the adiabatic phase can attain surface luminosities up to ≈10 -2 L ⊙ . We argue that the turn-on and cooling phases both will leave significant imprints on the AM CVn population's P orb -distribution. Finally, we show that the eclipsing AM CVn system SDSS J0926+3624 provides evidence that WD channel systems with non-zero entropy donors contribute to the AM CVn population, and we discuss the observational signature of the donor in this system.

The Thermal Evolution of the Donors in AM CVn Binaries

Abstract: We calculate the full stellar-structural evolution of donors in AM CVn systems formed through the WD channel coupled to the binary’s evolution. Contrary to assumptions made in prior modelling, these donors are not fully convective over much of the AM CVn phase and do not evolve adiabatically under mass loss indefinitely. Instead, we identify three distinct phases of evolution: a mass transfer turn-on phase (during which Porb continues to decrease even after contact, the donor contracts, and the mass transfer rate accelerates to its maximum), a phase in which the donor expands adiabatically in response to mass loss, and a cooling phase beginning at Porb � 45–55 minutes during which the donor contracts. The physics that determines the behaviour in the first and third phases, both of which are new outcomes of this study, are discussed in some detail. We find the overall duration of the turn-on phase to be between � 10 4 -10 6 yrs, significantly longer than prior estimates. We predict the donor’s luminosity, L, and effective temperature, Teff. During the adiabatic expansion phase (ignoring irradiation effects), L � 10 −6 –10 −4 L⊙ and Teff � 1000–1800 K. However, the flux generated in the accretion flow dominates the donor’s intrinsic light at all times. The impact of irradiation on the donor extends the phase of adiabatic expansion to longer Porb, slows the contraction during the cooling phase, and alters the donor’s observational characteristics. Irradiated donors during the adiabatic phase can attain surface luminosities up to � 10 −2 L⊙. We argue that the turn-on and cooling phases both will leave significant imprints on the AM CVn population’s Porb-distribution. Finally, we show that the eclipsing AM CVn system SDSS J0926+3624 provides evidence that WD-channel systems with non-zero entropy donors contribute to the AM CVn population, and we discuss the observational signature of the donor in this system.

Accretion Disk Spectra of the Brightest Ultraluminous X-Ray Source in M82

Abstract: Emission spectra of hot accretion disks characteristic of advection-dominated accretion flow (ADAF) models are investigated for comparison with the brightest ultraluminous source, X-1, in the galaxy M82. If the spectral state of the source is similar to the low-luminosity hard state of stellar mass black holes in our Galaxy, a fit to the Chandra X-ray spectrum and constraints from the radio and infrared upper limits requires a black hole mass in the range of 9 x 10(4)-5; 10(5) M-circle dot. Lower black hole masses (<= 10(4) M-circle dot) are possible if M82 X-1 corresponds to the highluminosity hard state of Galactic black hole X-ray binary sources. Both of these spectrally degenerate hot accretion disk solutions lead to an intermediate-mass black hole interpretation for M82 X-1. Since these solutions have different spectral variability with X-ray luminosity and predict different radio/infrared emission, they could be distinguished by future off-axis Chandra observations or simultaneous sensitive radio/infrared detections.

Theoretical Orbital Period Distributions of Cataclysmic Variables above the Period Gap: Effects of Circumbinary Disks

Abstract: Population synthesis tools are used to investigate the population of nonmagnetic cataclysmic variables with unevolved main-sequence-like donors at orbital periods greater than 2.75 hr. In addition to the angular momentum losses associated with gravitational radiation, magnetic braking, and mass loss from the system, we also include the effects of circumbinary disks on the evolution. For a fractional mass input rate into the disk, corresponding to 3×10-4 of the mass transfer rate, the model systems exhibit a bounce at orbital periods greater than 2.75 hr. The simulations reveal that (1) some systems can exist as dwarf novae throughout their lifetime, (2) dwarf novae can evolve into novalike systems, and (3) novalike systems can evolve back into dwarf novae during their postbounce evolution to longer orbital periods. Among these subclasses, novalike cataclysmic variables would be the best candidates to search for circumbinary disks at wavelengths >~10 μm. The theoretical orbital period distribution is in reasonable accord with the combined population of dwarf novae and novalike systems above the period gap, suggesting the possibility that systems with unevolved donors need not detach and evolve below the period gap as in the disrupted magnetic braking model. The resulting population furthermore reveals the possible presence of systems with small mass ratios and a preference of O/Ne/Mg white dwarfs in dwarf nova systems in comparison to novalike systems. The novalike population furthermore shows a lack of systems with high-mass white dwarfs. The importance of observational bias in accounting for the differing populations is examined, and it is shown that an understanding of these effects is necessary in order to confront the theoretical distributions with the observed ones in a meaningful manner.

Spitzer Space Telescope Observations of the Magnetic Cataclysmic Variable AE Aquarii

Abstract: The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white dwarf that is thought to expel most of the material streaming onto it. Observations of AE Aqr have been obtained in the wavelength range 5-70 μm with the IRS, IRAC, and MIPS instruments on board the Spitzer Space Telescope. The spectral energy distribution reveals a significant excess above the K4 V spectrum of the donor star, with the flux increasing with wavelength above 12.5 μm. Superposed on the energy distribution are several hydrogen emission lines, identified as Pfα and Huα, Huβ, and Huγ. The infrared spectrum above 12.5 μm can be interpreted as synchrotron emission from electrons accelerated to a power-law distribution dN ∝ E-2.4 dE in expanding clouds, with an initial evolutionary timescale of seconds. However, too many components must then be superposed to explain satisfactorily both the mid-infrared continuum and the observed radio variability. Thermal emission from cold circumbinary material could contribute, but it requires a disk temperature profile intermediate between that produced by local viscous dissipation in the disk and that characteristic of a passively irradiated disk. Future high time resolution observations spanning the optical-to-radio regime could shed light on the acceleration process and the subsequent particle evolution.

The Interaction of Stellar Objects within a Common Envelope

Abstract: We use high-resolution, three-dimensional hydrodynamic simulations to study the hydrodynamic and gravitational interaction between stellar companions embedded within a differentially rotating common envelope. Specifically, we evaluate the contributions of the nonaxisymmetric gravitational tides and ram pressure forces to the drag force and, hence, to the dissipation rate and the mass accumulated onto the stellar companion. We find that the gravitational drag dominates the hydrodynamic drag during the inspiral phase, leading to the result that a simple prescription based on a gravitational capture radius formalism significantly underestimates the dissipation rate and overestimates the inspiral decay timescale. Although the rate of mass accretion fluctuates significantly, we observe a secular trend leading to an effective rate of mass accretion which is significantly less than the rate based on a gravitational capture radius. The implications of these results are discussed within the context of accretion of compact objects in the common-envelope phase.

Black Hole Spin Evolution: Implications for Short-hard Gamma Ray Bursts and Gravitational Wave Detection

Abstract: The evolution of the spin and tilt of black holes in compact black hole - neutron star and black hole - black hole binary systems is investigated within the framework of the coalescing compact star binary model for short gamma ray bursts via the population synthesis method. Based on recent results on accretion at super critical rates in slim disk models, estimates of natal kicks, and the results regarding fallback in supernova models, we obtain the black hole spin and misalignment. It is found that the spin parameter, a_spin}, is less than 0.5 for initially non rotating black holes and the tilt angle, i_tilt, is less than 45 deg for 50% of the systems in black hole - neutron star binaries. Upon comparison with the results of black hole - neutron star merger calculations we estimate that only a small fraction (~ 0.01) of these systems can lead to the formation of a torus surrounding the coalesced binary potentially producing a short-hard gamma ray burst. On the other hand, for high initial black hole spin parameters (a_spin>0.6) this fraction can be significant (~ 0.4). It is found that the predicted gravitational radiation signal for our simulated population does not significantly differ from that for non rotating black holes. Due to the (i) insensitivity of signal detection techniques to the black hole spin and the (ii) predicted overall low contribution of black hole binaries to the signal we find that the detection of gravitational waves are not greatly inhibited by current searches with non spinning templates. It is pointed out that the detection of a black hole - black hole binary inspiral system with LIGO or VIRGO may provide a direct measurement of the initial spin of a black hole.

Spitzer Space Telescope Observations of the Magnetic Cataclysmic Variable AE Aqr

Abstract: The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white dwarf which is thought to expel most of the material streaming onto it. Observations of AE Aqr have been obtained in the wavelength range of 5 - 70 microns with the IRS, IRAC, and MIPS instruments on board the Spitzer Space Telescope. The spectral energy distribution reveals a significant excess above the K4V spectrum of the donor star with the flux increasing with wavelength above 12.5 microns. Superposed on the energy distribution are several hydrogen emission lines, identified as Pf alpha and Hu alpha, beta, gamma. The infrared spectrum above 12.5 microns can be interpreted as synchrotron emission from electrons accelerated to a power-law distribution dN=E^{-2.4}dE in expanding clouds with an initial evolution timescale in seconds. However, too many components must then be superposed to explain satisfactorily both the mid-infrared continuum and the observed radio variability. Thermal emission from cold circumbinary material can contribute, but it requires a disk temperature profile intermediate between that produced by local viscous dissipation in the disk and that characteristic of a passively irradiated disk. Future high-time resolution observations spanning the optical to radio regime could shed light on the acceleration process and the subsequent particle evolution.

The Existence of Inner Cool Disks in the Low Hard State of Accreting Black Holes

Abstract: The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low hard state of black hole transient systems for a range of luminosities which depend on the magnitude of the viscosity parameter. For $\alpha \sim 0.1-0.4$ an inner disk can exist for luminosities in the range $\sim 0.001- 0.02$ Eddington value. The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and Swift J1753.5-0127 in their low hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low luminosity active galactic nuclei.

The lowest-mass stellar black holes: catastrophic death of neutron stars in gamma-ray bursts

Abstract: Mergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts. Indeed such a merger can produce a black hole with a transient accreting torus of nuclear matter (Lee & Ramirez-Ruiz 2007, Oechslin & Janka 2006), and the conversion of a fraction of the torus mass-energy to radiation can power a gamma-ray burst (Nakar 2006). Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation, we demonstrate that the fraction of mergers that can form a black hole -- torus system depends very sensitively on the (largely unknown) maximum neutron star mass. We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that a black hole formation is required to produce a short gamma-ray burst in a double neutron star merger. Specifically, we find that the maximum neutron star mass must be within 2 - 2.5 Msun. Moreover, a single unambiguous measurement of a neutron star mass above 2.5 Msun would exclude a black hole -- torus central engine model of short gamma-ray bursts in double neutron star mergers. Such an observation would also indicate that if in fact short gamma-ray bursts are connected to neutron star mergers, the gamma-ray burst engine is best explained by the lesser known model invoking a highly magnetized massive neutron star (e.g., Usov 1992; Kluzniak & Ruderman 1998; Dai et al. 2006; Metzger, Quataert & Thompson 2007).

Thermal Evolution of AM CVn Binary Donors

Abstract: We discuss results of our study on AM CVn binaries formed with donors that never ignited He before contact. For the first time, we treat the donor's in these systems in the context of a full stellar structure evolution theory and find that the binary's evolution can described in terms of 3 phases: contact, adiabatic donor expansion, and late-time donor cooling. Details of the first and third phase are new results from this study and we focus on generally characterizing these two phases. Finally, we present our predictions for the donor's light in these systems.

SAX J1808.4-3657 in Quiescence: A Keystone for Neutron Star Science

Abstract: The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object between accreting X-ray binaries and millisecond radio pulsars. We have constrained the thermal radiation from its surface through XMM-Newton X-ray observations, providing strong evidence for neutrino cooling processes from the neutron star core. We have also undertaken simultaneous X-ray and optical (Gemini) observations, shedding light on whether the strong heating of the companion star in quiescence may be due to X-ray irradiation, or to a radio pulsar turning on when accretion stops.