Astrophysical Flux Research Articles

Angular dependence of neutrino flux inKM3detectors in low scale gravity models

Cubic kilometer neutrino telescopes are capable of probing fundamental questions of ultra-high energy neutrino interactions. There is currently great interest in neutrino interactions caused by low-scale, extra dimension models. Above 1 PeV the cross section in low scale gravity models rises well above the total Standard Model cross section. We assess the observability of this effect in the 1 PeV - 100 PeV energy range of kilometer-scale detectors with several new points of emphasis that hinge on enhanced neutral current cross sections. A major point is the importance of ``feed-down'' regeneration of upward neutrino flux, driven by new-physics neutral current interactions in the flux evolution equations. Feed-down is far from negligible, and it is essential to include its effect. We then find that the angular distribution of events has high discriminating value in separating models. In particular the ``up-to-down'' ratio between upward and downward-moving neutrino fluxes is a practical diagnostic tool which can discriminate between models in the near future. The slope of the angular distribution, in the region of maximum detected flux, is also substantially different in low-scale gravity and the Standard Model. These observables are only weakly dependent on astrophysical flux uncertainties. We conclude that angular distributions can reveal a breakdown of the Standard Model and probe the new physics beyond, as soon as data become available.

Critical radiation fluxes and luminosities of black holes and relativistic stars

The critial luminosity at which the outward force of radiation balances the inward force of gravity plays an important role in many astrophysical systems. We present expressions for the radiation force on particles with arbitrary cross sections and analyze the radiation field produced by radiating matter, such as a disk, ring, boundary layer, or stellar surface, that rotates slowly around a slowly rotating gravitating mass. We then use these results to investigate the critical radiation flux and, where possible, the critical luminosity of such a system in genral relativity. We demonstrate that if the radiation source is axisymmetric and emission is back-front symmetric with repect to the local direction of motion of the radiating matter, as seen in the comoving frame, then the radial component of the radiation flux and the diagonal components of the radiation stress-energy tensor outside the source are the same, to first order in the rotation rates, as they would be if the radiation source and gravitating mass were not rotating. We argue that the critical radiation flux for matter at rest in the locally nonrotating frame is often satisfactory as an astrophysical benchmark flux and show that if this benchmark is adopted, many of the complications potentially introduced by rotation of the radiation source and the gravitating mass are avoided. We show that if the radiation field in the absence of rotation would be spherically symmetric and the opacity is independent of frequency and direction, one can define a critical luminosity for the system that is independent of frequency and direction, one can define a critical luminosity for the system that is independent of the spectrum and angular size of the radiation source and is unaffected by rotation of the source and mass and orbital motion of the matter, to first order. Finally, we analyze the conditions under which the maximum possible luminosity of a star or black hole powered by steady spherically symmetric radial accretion is the same in general relativity as in the Newtonian limit.

A reinterpretation of millimeter observations of nearby IRAS excess stars

We analyze new and previously published 1300, 870, and 800 micrometers, single-element bolometer observations of Vega (alpha Lyr), Formalhaut (alpha PsA), epsilon Eri, tau(sup1) Eri, and Beta Leo. We show that these data are consistent with models in which the dust disks around these stars are larger than the radio telescope beams with which they were observed; thus, these disks may be many hundreds of AU in radius or larger. Our interpretation of submillimeter/millimeter measurements of these stars also indicates that for some Infrared Astronomy Satellite (IRAS) IR excess stars the assumption that there is no astrophysical flux in the OFF beams, when using the standard ON-minus-OFF chopping technique and the chop distance is less than 1000AU, may be incorrect. Therefore, for IRAS IR excess stars within approximately 20 pc, virtually all submillimeter/millimeter chopping observations with chop throws less than may have subtracted away some or most of the flux associated with their circumstellar disks. Finally, we present new 1300 micrometers continuum observations of Vega made with chop throws of 500 and 1000 AU. These data are consistent with an interpretation in which Vega has a disk that is at least 1000AU in radius; this disk could have a region with much less material per beam area near 500 AU than at both 100 AU and 1000 AU, corresponding to a gap at the orbital distance of alpha Lyr B. The observations of Vega are also consistent with the assumption that the circumVega dust structure is unresolved. Thus, these new data very effectively illustrate that the 'standard' model of small, unresolved, dust structures around main-sequence IRAS IR excess stars is not unique. Maps of IRAS IR excess stars, which soon will be available from submillimeter/millimeter array detectors, will determine whether the paradigm shift we propose will occur.

Magnetohydrodynamic spectroscopy: Large scale computation of the spectrum of waves in plasmas

A systematic study of the spectrum of magnetohydrodynamic (MHD) waves in thermonuclear and astrophysical plasmas is presently undertaken at the FOM Institute for Plasma Physics. This study is based on the fact that the macroscopic waves and instabilities in both thermonuclear confinement machines and in astrophysical magnetic flux loops are described by the MHD equations. The linearised interaction between plasma and magnetic field leads to large-scale non-symmetric eigenvalue problems for free oscillations and to temporal evolution problems for forced (externally driven) oscillations. These problems have been cast in the form of a set of two modular computer programs, called CASTOR for tokamaks and POLLUX for coronal flux tubes, exploiting identical solvers for the resulting very large systems. CASTOR is extensively used at JET (the Joint European Torus experiment at Culham) for the interpretation of measured MHD spectra, whereas POLLUX is presently used for the study of Alfvén wave heating of the solar corona and will be used in the near future for the interpretation of observed x-ray emissions from the SOHO satellite which will be launched by ESA and NASA in 1995. For the interpretation of actual plasma dynamics in tokamaks and solar coronal loops it is crucial to understand the non-linear phase of the evolution of MHD phenomena as well. This adds two complications: non-linear mode coupling of hundreds of small-scale modes and enormous discrepancies between the different dynamical time scales. We have developed numerical codes that simulate the non-linear temporal evolution of plasma columns under external excitation. These codes have been modelled after the linear ones, with a finite difference discretisation in the radial direction and a spectral discretisation in the two other directions. A relatively simple and flexible semi-implicit predictor-corrector scheme has been used for the time stepping. This allows time steps up to a factor 1000 larger than the largest time steps allowed for explicit methods. Because of the many modes involved, memory requirements and CPU time requirements soon become prohibitive. Hence, the new physics in the non-linear plasma dynamics regime requires parallel computing.

Limits on astrophysical nu-e flux at E-nu greater than 10 to the 19th eV

We report on a search for upward-going extensive air showers using the University of Utah Fly's Eye detector. No eVents hae been found in 3.9 x 10/sup 6/ s of running time. The resultant ..nu.. flux limit at 10/sup 20/ eV varies from 5.8 x 10/sup -16/ ..nu.. cm/sup -2/ s/sup -1/-sr/sup -1/ to 3.0 x 10/sup -14/ ..nu.. cm/sup -2/ s/sup -1/ sr/sup -1/ for sigma/sub ..nu../ between 10/sup -33/ cm/sup 2/ and 10/sup -3/ cm/sup 2/. We also present flux limits for large sigma/sub ..nu../ using near-horizontal events originating in the atmosphere. The implications of the flux limit at E/sub ..nu../>10/sup 20/ ev are discussed.

Energy dynamics in stressed magnetic fields - The filamentation and flare instabilities

The thermal and tearing instabilities are believed to be the two primary temperature modification mechanisms in sheared astrophysical magnetic fields. The former gives rise to the formation of cool filaments and the latter to the release of magnetic energy. It has long been known that these processes are interrelated, most conspicuously in the case of the solar corona where prominences often precede flares within the same magnetic structure. It is also clear, from first principles, that the energy transport underlying the thermal instability should have a strong effect on the resistivity which facilitates magnetic tearing, and that the energy release of the latter should affect the temperature drop of the former. This paper describes some results of the first calculations which attempt to unify the dynamic treatment of these two coexisting instabilities. Growth rates as a function of resistivity, and examples of the primary mode structures are provided, along with a discussion of some critical aspects of the interaction of these two astrophysical energy flux mechanisms.