CITA hosts a broad program of research on fundamental problems in radiative physics, accretion, and stellar oscillations, in a wide range of astrophysical settings: neutron stars, black holes, AGN, and gamma-ray bursts.
Ballantyne and Strohmayer (NASA/GSFC) analyzed RXTE data of the superburst from the low-mass X-ray binary 4U 1820-30. They employed ionized X-ray reflection models to account for apparent Fe K line and edge features in the X-ray spectra. Time resolved spectral fitting with these models revealed that the accretion disk had a curious response to the superburst: the innermost radius of the reflecting region moved outwards by an order of magnitude about 1000 s into the burst before returning to its original value as the burst was fading. This research was the subject of a NASA and University of Toronto press release.
Font (UVic), McCarthy (UVic) and Johnstone (HIA) joined Ballantyne to produce numerical models of the photoevaporation of disks around young stars. The goal was to attempt to account for the luminosities and velocities of optical emission lines seen toward T Tauri stars. The results found that the simple thermally driven wind models, in contrast to pure magnetically driven winds, could explain the observed data for the S and N lines, but has difficulty with the O lines.
Ballantyne presented properties expected from X-ray reflection from ionized accretion disks illuminated by a neutron star X-ray burst. His models predicted that the soft X-ray band will contain a wealth of information on the accretion disk (density, abundances, temperature, etc.) and should be a goal for future X-ray missions. The equivalent width of the Fe K emission line is correlated with the burst blackbody temperature.
Ballantyne along with Vaughan (Leicester), Fabian (Cambridge), De Rosa (IAS, Roma), Piro (IAS, Roma) and Matt (Roma Tre) studied the XMM-Newton spectrum of the Seyfert 1 galaxy Ark 120. This source is interesting because it shows no signs of optical/UV reddening or warm absorption in the soft X-ray band. Thus, a clear look at the intrinsic X-ray emission from the AGN could be obtained without intervening absorption along the line-of-sight. The X-ray spectrum showed a strong soft excess which indicates that these components originate at the AGN and are not an artefact of absorption. However, the standard emission models (blackbody, free-free, etc.) could not successfully model the soft excess.
Everett and Ballantyne studied the acceleration of highly ionized winds launched from the innermost parts of accretion disks around black holes. The motivation was to understand the origin of several high-energy absorption lines that had been uncovered in XMM-Newton spectra of high-luminosity AGN. These lines had been interpreted as arising from highly-ionized material (such as Fe) that is outflowing from the nucleus at a velocities of 0.1-0.3c. Our models showed that such speeds could only be obtained if the luminosity of the AGN exceeded the Eddington limit, and even in those cases the wind would be so highly ionized that any absorption lines would be undetectable.
Ballantyne, Iwasawa (Cambridge) and Fabian (Cambridge) analyzed a 120 ks XMM-Newton observation of the broad-line radio galaxy 3C 120. The data yielded the highest quality hard X-ray spectrum of a radio-loud object ever taken. The spectrum was well fitted with a moderate amount of reflection and a relatively narrow Fe K emission line. However, due to the small inclination angle into the nucleus the inner radius of the reflecting disk was still about 75 gravitational radii. A more highly ionized reflector inwards of this radius cold not be ruled out.
Ballantyne, Turner (JPL) and Young (MIT) continued a study on the properties of X-ray reflection off disks subject to the photon bubble instability. They found that the rapid time dependence of the instability could cause rapid variability in the observed reflection features such as the Fe K emission line. This has the potential to complicate the interpretation of future Fe K reverberation mapping experiments.
Everett carried out several related programs studying mass outflows from accretion disks of Active Galactic Nuclei (AGN). Specifically, three different research projects all aimed to constrain and illuminate the acceleration processes of observed outflows on different scales within AGN. On the scale of the Broad Line Region of AGN (10^16 to 10^17 cm from the central black hole), ongoing work was conducted with Arieh Konigl (U. of Chicago) on radiative transfer through magnetohydrodynamic winds as well as on how radiative and magnetic acceleration interact. Specifically, this work focuses on how absorption of photons in an inner magnetic wind would aid radiative acceleration; such an MHD "shield" would help explain why radiatively-driven wind models may function efficiently when illuminated by the highly-ionizing, X-ray rich central continuum. More general than this, examining the radiative transfer through such winds and their opacities may also help to explain "the torus": the obscuring material that is central to the AGN Unification scheme. This work was later submitted to ApJ in January, 2005.
Everett investigated tens of parsec scale outflows from AGN. At such distances, ionized gas emits narrow emission lines, whose outflow velocities are seen to vary as a function of distance from the nucleus. This is especially apparent in the best studied narrow-emission line AGN: NGC 1068. There is no dynamical model for the velocities observed in this galaxy, despite numerous detailed Hubble Space Telescope studies of the narrow line region. Everett & Norm Murray (also at CITA) have used these observational results to constrain many of the possible dynamical models that aim to explain these outflows. Such studies will help us understand the the mass outflow rates, and understand the balance of forces at large distances from the AGN.
M. Liebendoerfer, M. Rampp (Max-Planck Institute for Astrophysics, Garching, MPA), H.-T. Janka (MPA), and A. Mezzacappa (Oak Ridge National Laboratory, ORNL) have compared in detail their supernova simulations implementing an implicit general relativistic solution of the Boltzmann transport equation or a variable Eddington factor method for the neutrino transport and found satisfactory agreement in spherical symmetry. Machine-readable data from these reference simulations have been published to gauge neutrino transport approximations.
In collaboration with K. Langanke (Univ. of Aarhus), G. Martinez-Pinedo (Institut d'Estudis Espacials de Catalunya, Barcelona, IEEC), J. M. Sampaio (Univ. of Aarhus), D. J. Dean (ORNL), W. R. Hix (University of Tennessee, Knoxville, UTK), O. E. B. Messer (UTK), A. Mezzacappa (ORNL), H.-T. Janka (MPA), and M. Rampp (MPA), Liebendoerfer investigated the electron capture rates on heavy nuclei. They found that Pauli blocking does not occur to the extent assumed in previous core collapse simulations. Simulations with the improved rates showed that these ignored reactions actually dominate throughout core collapse.
D. Richmond (Univ. of Victoria) and M. Liebendoerfer have published an extension of the adaptive grid in the hydrodynamics code AGILE that would allow to dynamically excise the singularity from a supernova simulation when a black hole forms at the center of the event.
C. Froehlich (Univ. of Basel), P. Hauser (Univ. of Basel), M. Liebendoerfer, G. Martinez-Pinedo (IEEC), and F.-K. Thielemann (Univ. of Basel) et al. investigated the impact of weak interactions in the vicinity of the mass cut in nucleosynthesis calculations for supernova explosions. Previous calculations assumed an unchanged progenitor composition. It turns out that the large neutrino fluxes drive the electron fraction to higher values, sometimes even above 0.5 and the correspondingly enhanced abundances of 45Sc, 49Ti, and 64Zn agree better with observation.
J. L. Fisker (Univ. of Notre Dame), E. Brown (Michigan State Univ.), M. Liebendoerfer, F.-K. Thielemann (Univ. of Basel), and M. Wiescher (Univ. of Notre Dame) investigated the rp-process reaction flow on an accreting neutron star and the resulting ashes of a type 1 X-ray burst based on a specialized version of the general relativistic hydrodynamics code AGILE.
M. Liebendoerfer, U.-L. Pen, and C. Thompson performed preliminary three-dimensional simulations of stellar core collapse with magnetic fields on the McKenzie cluster. The simulations are based on the realistic Lattimer-Swesty equation of state and a parameterization of the relevant neutrino physics. More realistic initial data and improved boundary conditions will make the simulations more definitive.
M. Liebendoerfer, U.-L. Pen, and C. Thompson performed preliminary three-dimensional simulations of stellar core collapse with magnetic fields on the McKenzie cluster. The simulations are based on the realistic Lattimer-Swesty equation of state and a parameterization of the relevant neutrino physics. More realistic initial data and improved boundary conditions will make the simulations more definitive.
Kaya Mori, D. Lai (Cornell) and A. Potekhin (Ioffe Physico-Technical Institute) have continued modeling non-hydrogenic atmospheres of neutron stars. The modeling involves atomic physics, non-ideal effects and radiative transfer in strongly-magnetized dense plasma. This work is motivated by recent discovery of spectral feature(s) from several isolated neutron stars.
Kaya Mori, J. Chonko (Columbia) and C. Hailey (Columbia) have analyzed the 260 ksec XMM-Newton data of the neutron star 1E1207.4-5209. 1E1207 is unique in showing more than one spectral feature in its thermal spectrum. We have found that two absorption features at 0.7 and 1.4 keV are significant. Using our atmosphere models, we will probe surface composition, magnetic field strength and nuclear equation of state via mass and radius measurement.
Kaya Mori, M.H. van kerkwijk (Toronto), G. Pavlov (Penn state) and D. Kaplan (MIT) have been analyzing recent X-ray data of the nearby radio-quiet isolated neutron star RXJ0720.4-3125 which has exhibited puzzling year-scale X-ray variability. We are investigating an origin of the variability using both timing and spectroscopic data obtained by Chandra and XMM-Newton observatory.
Kaya Mori has been involved in a NASA-funded project called the gaseous antiparticle spectrometer (GAPS). GAPS is a novel approach to the detection of antimatter which exploits the characteristic decay X-rays formed when antimatter is captured in conventional matter, making exotic atoms. Using GEANT4 simulation software, he has helped designing a prototype detector which was tested at the KEK research facility in Japan.
Thompson in collaboration with Phil Arras (KITP) and Andrew Cumming (UCSC) published detailed calculations using relativistic neutron star models with realistic nuclear equations of state; magnetic envelopes of both light (H/He) and heavy (Fe) composition; and a proper account of the effects of superfluidity and superconductivity on the rates of neutrino cooling. These results demonstrated in detail the turn-off of magnetic activity in an active SGR/AXP, as a function of pair energy of the core neutrons and the presence or absence of continuous magnetic field decay. Internal heating can lead to a significant delay in the onset of core neutron pairing, if the critical temperature lies in the range 3-6x10^8 K. The combination of fast timing observations and X-ray flux measurements place strong constraints on the presence of a core superfluid.
Parker Troischt, a graduate student at UNC-Chapel Hill, performed a series of calculations under the supervision of Thompson. He demonstrated how the exact, non-linear wave solutions of force-free magnetohydrodynamics are modified by a strong gravitational field. Beyond the WKB limit, there is a non-linear coupling between torsional and compressive waves. The effect of zero-frequency components of the wave on the non-linear coupling between force-free waves was also worked out, taking into account the finite compressibility of a relativistic magnetofluid.
Thompson and Beloborodov considered the 10-100 keV X-ray emission that is expected in the non-potential magnetosphere of a Soft Gamma Repeater or Anomalous X-ray Pulsar. They demonstrated two simple mechanisms that could explain the observation of a high-energy excess in the spectra of several AXPs: thermal bremssatrahlung emission from a current-heated atmosphere at the surface of the star; and synchrotron emission in the outer magnetosphere (at ~ 100 km) where a large electrostatic potential develops in reponse to the strong resistive force acting on current-carrying electrons as they scatter off the keV photons.
Thompson and Peter Woods (NSTCC) completed a major review of the observational properties of the Soft Gamma Repeaters and Anomalous X-ray Pulsars, and on the theoretical properties of magnetars. This review will be published in a book on Compact Stellar X-ray Sources by Cambridge University Press.
[ Back to CITA Research 2004 | CITA Annual Report 2004 ]