A theory for gamma ray and X-ray burst sources

A common mechanism for both X-ray and-ray bursters is proposed on the basis that a window can be created transiently in the polar cap of a degenerate star, a white dwarf for X-ray bursts and a neutron star for-ray bursts. The window exposes transiently a hot degenerate sub-layer of the star at shallow depth, from which escapes blackbody flux for a source at temperature 3 kev with window radius 10 km in the case of X-ray bursts and for a source at temperature 300 kev with window radius 0.5 km in the case of-ray bursts.     

Observation of MKN 501 at gamma-ray energies

During a balloon flight in September 1979 of the MISO low-energy -ray telescope, the BL Lac-object MkN 501 was studied in the hard X-ray range above 30 keV and in the low energy -ray range up to 19 MeV. No statistically significant X- and -ray fluxes were detected. The implications of the upper limits obtained are discussed in the light of the relativistic jet theories recently proposed.     

THE RELATIONSHIP BETWEEN SOLAR FLARE GAMMA-RAY EMISSION AND NEUTRON PRODUCTION

We have examined six solar neutron events measured by satellite instruments and/or neutron monitors (NM) to understand the relationship between the intensity–time profiles of the -ray lines, the pion-related -rays, and the neutron production. In all six events the solar neutron production was clearly time-extended. We find that neutron emission as detected by NMs most closely follows the emission of pion-related -rays, whereas lower energy neutron production may follow that of nuclear -ray line emissions. Although this distinction is not unexpected, it is safe to say that the 2.223 MeV -ray line from neutron capture on hydrogen is a poor measure of the neutron production at energies >200 MeV. During the three events on 1982, June 3, 1990, May 24 and 1991, June 4 solar neutrons with energies greater than 200 MeV were recorded by NMs. The NM increases on 1982, June 3 and 1990, May 24 can be modeled using the time profile of the pion-related -rays. For the 1991, June 4 event the NM signal was small but lasted for 60 min and the high-energy -ray data available to us are insufficient to conclude unambiguously that the high-energy neutron production followed the pion-related -rays. In the other three events on 1991, June 9, 11, and 15 solar neutrons with energies 10–100 MeV were observed by the COMPTEL -ray instrument on the Compton Gamma Ray Observatory. The duration of the low-energy neutron production on 1991, June 9 corresponded clearly to the high-energy and not to the low-energy -ray emission.     

Immediate reports of GRBs in progress from HETE

The High Energy Transient Experiment (HETE) will be able to perform multiwavelength observations of-ray and X-ray bursts. HETE will potentially be able to localize-ray bursts to a precision of 20 arc-minutes if significant X-ray flux is detected from the burst; a precision of 20 arc-seconds is possible if there is also significant UV radiation from the burst. HETE will broadcast information about bursts detected within seconds of their detection. This VHF-band broadcast will be received at a number of secondary ground stations (SGS) dedicated to HETE, and forwarded to a central distribution site at MIT, from which it is sent to interested observers via Internet.     

Plasma acceleration by radiation in X-ray pulsars

Charged particle acceleration is considered by a radiation flux from a star or hot spot in X-ray pulsars. It is shown that for any distance from the star there exists the upper velocity limit up to which a particle can be accelerated by radiation. This critical velocity does not depend on the luminosity of the spot. Near the hot spot surface the critical velocityv0.65c. These results are applied to plasma acceleration inX-ray pulsars. The mechanism is advanced, of -ray generation in the course of plasma accretion, onto a neutron star. It is shown that in the presence of a large magnetic field and high luminosity of the spot the relativistic electron-position avalanche may appear. The optical depth of the electron-positron cloud achieves the value of order one. The X-ray quanta emitted by the spot are scattered by relativistic (2.6) electron-positron pairs and are transformed into -radiation. Hard quanta with energy 1 MeV leave the generation region in the narrow cone 0.25.     

The Coronal Mass Ejection Waiting-Time Distribution

The distribution of times t between coronal mass ejections (CMEs) in the Large Angle and Spectrometric Coronagraph (LASCO) CME catalog for the years 1996–2001 is examined. The distribution exhibits a power-law tail (t) with an index –2.36±0.11 for large waiting times (t>10 hours). The power-law index of the waiting-time distribution varies with the solar cycle: for the years 1996–1998 (a period of low activity), the power-law index is –1.86±0.14, and for the years 1999–2001 (a period of higher activity), the index is –2.98±0.20. The observed CME waiting-time distribution, and its variation with the cycle, may be understood in terms of CMEs occurring as a time-dependent Poisson process. The CME waiting-time distribution is compared with that for greater than C1 class solar flares in the Geostationary Operational Environmental Satellite (GOES) catalog for the same years. The flare and CME waiting-time distributions exhibit power-law tails with very similar indices and time variation.     

On the formation of energy spectra in synchrotron sources

The observed energy spectra in synchrotron sources are power laws,N (E)=KE ^–, with the distribution in peaked around 2.5. These are consistent with initially injected spectra with between 1 and 2, subsequently steepened by synchrotron losses. Contrary to the results of Kardashev (1962), it is shown that statistical acceleration when coupled with synchrotron losses lead naturally to the formation of flat stationary spectra with 1. These stationary spectra have bends near the energy at which synchrotron losses balance the energy gains by acceleration. Above this bend the spectrum tends to =2. The time evolution of an initial spectrum towards the stationary spectrum is investigated. It is suggested that the initially flat spectra with 1 to 1.5 observed in some variable sources result from an incomplete approach to the stationary spectrum, and that in sources with constant acceleration spectra with 2 are to be expected.     

Constraints on the high-energy properties of active galactic nuclei

Number-flux relations at -ray energies for active galactic nuclei have been computed, starting from their X-ray luminosity function and making different assumptions on the spectral shape and cosmological evolution of these sources.The radiation resulting from their integrated emission has been computed and compared to the observed intensity and spectrum of the extragalactic -ray diffuse background (0.3–20 MeV), in order to derive quantitative constraints on the evolution and spectral properties of active galactic nuclei at -ray energies.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Present Observational Status of the Intermediate Mass Stars: δ Sct Stars, γ Dor Stars and roAp Stars

The present observational status of the Sct stars, Dor stars and roAp stars is discussed. The Sct stars are the most intensively observed of the three groups, but it has become clear that there are severe problems in extracting asteroseismic information from them. Dozens of frequencies are observed, but hundreds of frequencies are predicted from the models; unique matches of observation and theory still elude us. The Sct stars are observationally complex – some recent `best case' campaigns are discussed. It is possible that substantial observational advances for Sct stars may need to await upcoming satellite missions. New Dor stars are beingdiscovered frequently, and new behaviour is being found for them. They constitutean observationally young field. Their pulsational frequency range is being expanded, their position in the HR diagram is becoming better known (but is yet to be fully constrained), and the possibility exists of hybrid Dor – Sct stars that have greatasteroseismic promise, although it is clear such stars are rare, if they do exist. It has been observationally challenging to extract more than a fewfrequencies for any Dor star so far. Exciting spectroscopic discoveries of new behaviour in roAp stars promise unprecedented information about the structure of the peculiar atmospheres ofthose stars – pulsation amplitude and phase in 3D, magnetic field structurein 3D, abundance stratification in 3D, realistic T- for the most peculiarstars – as well as entirely new information about the interaction of pulsation,rotation and magnetic fields. Recent theoretical work has led to new understandingof the previously inexplicable frequency spacing of HR 1217 with new Whole Earth Telescope observations supporting this theory. An `improved oblique pulsator model' has been developed in which the pulsationaxis is not the magnetic axis; this model has passed several observationaltests and new ones are being devised to examine it further.     

Stimulated comption effect in very compact radio sources

Expressions for the stimulated Compton effect are derived that are complete to order /ge/, where is the photon energy in the laboratory system, and =m ₀ C ² is the electron energy. Explicit formulas are given for the energy flow between a relativistic electron and a radiation field that obeys a power law so that the number of photons is proportional to ^–m–1. The amount of energy gained by an electron per second is then numerically calculated for conditions suggested by very compact radio sources as a function of the width of the spectrum, the spectral index, and the electron energy.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A coronal loop model for the production of X-ray and radio flares in the RS CVn binary HR 1099

The RS CVn binary stellar system HR 1099 is a source of both X-ray and radio flares. We present here a model of the system in which the two types of flare are produced by the same population of mildly-relativistic ( 10) electrons, injected into a coronal loop. After reviewing possible radiation mechanisms we conclude that, given the probable conditions in the flaring region, the radio emission is gyrosynchrotron radiation and the X-ray emission is thermal bremsstrahlung. The thermal X-ray source must lie in the stellar chromosphere, but the apparent absence of plasma absorption at radio frequencies indicates that the radio source is located high in the coronal loop. Using the relationships given by Dulk and Marsh (1982) for the radio emission from a power-law electron energy spectrum,N() ( - 1)^–, we conclude that 3 7, with 30% of the electron population trapped in the radio source. Some implications of these results for one particular version of the model are discussed.     

Gamma-ray bursts from high velocity neutron stars

We investigate the viability of the Galactic corona model of -ray bursts by calculating the spatial distribution of neutron stars born with high velocities in the Galactic disk, and comparing the resulting brightness and angular distribution with the BATSE data. We find that the Galactic corona model can reproduce the BATSE peak flux and angular distribution data for neutron star kick velocities 800 km s^–1, source turn-on ages 10 Myrs, and sampling depths 100 kpc d _max 400 kpc.     

Взаимооействие Излучения Рентгеновскооо Истооника с АтмосФерой Нормальной Звезды в Тесных Двойных Системах

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A momentum distribution of high energy particles around Crab pulsar

We determine the momentum distribution of the relativistic particles near the Crab pulsar from the observed X- and -ray spectra (10³10⁹ eV), provided that the curvature radiation is responsible for it. The power law spectrum for the relativistic electrons,f() ^–5, reproduces a close fit to the observed high-energy photon spectrum. The theoretically determined upper limit to the momentum (due to radiation damping), _M 8×10⁶, corresponds to the upper cut-off energy of the -ray spectrum, 10⁹ eV. The lower limit to the momentum, _m 1.8×10⁵, is chosen such that flattening of the X-ray spectrum below 10 keV is simulated. The number density of these electrons is found to be much higher than the Goldreich-Julian density. We also discuss pulse shape and polarization of high-energy photons. The extremely high density of particles and the steep momentum spectrum are difficult to understand. This may imply that another, more efficient, mechanism is in operation.     

Binary star model of the γ-ray burst with special reference to that on 5 March, 1979 and subsequent bursts on 6 March, 4 April,and 24 April, 1979

The peculiar -ray burst phenomenon of 5 March, 1979, and the other subsequent bursts on 6 March, 4 April, and 24 April, 1979, are studied, using the physically more realistic exponentially increasing accretion rate on a highly magnetized neutron star from its companion, and the conclusions that pycnonuclear reaction flash for the first and thermonuclear flashes for the subsequent bursts as the most probable model for this series of bursts, are made.We further conclude that a huge -ray burst is a sequel to rapid X-ray transient or type-I X-ray bursts, i.e., an almost exactly similar burst as on 5 April, 1979 will never repeat from the same source, instead rapid X-ray transient burst, or type-I X-ray burst will be occured. A rough estimate gives that the next burst will occur within 0.5 yr since 24 April, 1979.     

Equilibrium spectra of secondary cosmic-ray positrons in the galaxy and the spectrum of cosmic gamma-rays resulting from their annihilation

A general formula is derived for calculating the -ray spectrum resulting from the annihilation of cosmic-ray positrons. This formula is used to calculate annihilation--ray spectra from various equilibrium spectra of secondary galactic positrons. These spectra are then compared with the -ray spectra produced by other astrophysical processes.Particular attention is paid to the form of the -ray spectrum resulting from the annihilation of positrons having kinetic energies below 5 keV. It is found that for mean leakage times out of the galaxy of less than 400 million years, most of the positrons annihilating near rest come from the -decay of unstable nuclei produced in cosmic-ray p-C¹², p-N¹⁴, and p-O¹⁶ interactions, rather than from pi-meson decay. It is further found that the large majority of these positrons will annihilate from an S state of positronium and that 3/4 of these will produce a three-photon annihilation continuum rather than the two-photon line spectrum at 0.51 MeV. The results of numerical calculations of the -ray fluxes from these processes are given. It is concluded that annihilation -rays from the galactic halo may remain forever masked by a metagalactic continuum. However, an 0.51 MeV line from the disk may well be detectable. It is most reasonable to assume that this line is formed predominantly by the annihilation of the CNO -decay positrons. Under this assumption, the intensity of the line becomes a sensitive measure of the galactic cosmic-ray flux below 1000 MeV/nucleon.     

Time variation of the spectrum of Gyro-synchrotron emission from the Sun

The emission spectra and their time variations of gyro-synchrotron emission from an ensemble of energetic electrons are computed for some initial power-law distributions of the electron energies N()d= ^– with =2 or 4. The spectra and decay curves of the emission are compared with solar microwave bursts in order to separately estimate the magnetic field H and . From a limited number of observations, we have 3 and H 10³ gauss for the microwave impulsive bursts, and 2 and H (500–1000) gauss for the microwave type-IV bursts.     

Estimates of proton energies in extragalactic radio sources

It is shown that inelastic proton collisions in extragalactic radio sources can account for their radio and -ray emissions. The proton Lorentz-factor _p responsible for -ray emission is estimated to lie between 1.4 and 86. But for the radio emission (with _e ~ 10³-10⁴) the estimated _p values lie between 7 to 300. The estimates of total particle and magnetic energy for a typical radio source is in agreement with equipartition theory.     

An Improved Method of Photometric Mode Identification: Applications to Slowly Pulsating B, β Cephei, δ Scuti and γ Doradus Stars

We present an improved version of the method of photometric mode identification based upon the inclusion of non-adiabatic eigenfunctions determined in the stellar atmosphere, according to the formalism recently proposed by Dupret et al. (2002).We apply our method to Cephei, Slowly Pulsating B, Scuti and Doradus stars. Besides identifying the degree of the pulsatingstars, our method is also a tool for improving the knowledge of stellar interiors and atmospheres, by imposing constraints on the metallicity for Cephei and SPBs, the characteristics of the superficial convection zone for Scuti and Doradus starsand the limb-darkening law.     

Symmetric periodic orbits in Størmer's problem

The hypothesis on the -ray burst generation in the process of the collapse of surpermassive bodies in the nuclei of active galaxies is considered. It is shown that -ray burst properties observed may be interpreted within the frames of the given model. A statistical test for choosing a hypotheses on -ray burst nature is discussed.