A search for ultra-high energy counterparts to gamma-ray bursts

A small air shower array operating over many years has been used to search for ultra-high energy (UHE) gamma radiation ( 50 TeV) associated with gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each burst are presented for seven GRBs located with zenith angles < 20°. A 4.3 excess over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8%. If this effect is real then cosmological models are excluded for this burst because of absorption of UHE gamma rays by the intergalactic radiation fields.     

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.     

Search for a radio counterpart to GRB 940301 with westerbork

The COMPTEL instrument onboard theCompton Gamma-Ray Observatory imaged the bright gamma-ray burst GRB 940301 within 1.6 hours of the event, with a mean 1 error radius of 1.5°. The error region was subsequently refined by combining the COMPTEL location with the arc derived from differences in the event arrival time at the Ulysses and BATSE detectors. Westerbork observations of the COMPTEL error region began on March 4 1994 at 21 cm, however coverage of the refined position was not obtained until 32 days after the GRB occurrence, by which time the operating wavelength had changed to 92 cm. We have constrained the level of variability of sources within the triangulation arc-COMPTEL 2 error box region to be less than 40 mJy (5 upper limit) at 92 cm 41 days after the burst.     

The evolution of dwarf galaxies

We propose an evolutionary model for dwarf galaxies in which initially metal-poor gas-rich dwarf irregular (dI) galaxies evolve through bursting Blue Compact Dwarf (BCD) stages and eventually fade from the BCD phase to become dwarf ellipticals (dE). During the bursting phase the surface brightness of the galaxy increases rapidly due to enhanced OB star formation. The source of fuel for the intermittent bursts of star formation is assumed to be primordial gas which continues to collapse onto the already formed central structure. The dE galaxies form as a result of eventual gas depletion through star formation.With this proposed dIBCDdE evolutionary sequence we can explain the similar photometric structure of the different dwarf types and the differences in their star formation rates, surface brightnesses,Hi contents and metallicities. A final central BCD burst can account for the nucleation in brighter dEs and their residual star formation, while earlier more widespread star formation bursts would fade to give an irregular dI. Inflow of gas may allow dEs to be less flattened than dIs.Using galaxy fading and metallicity models we can reproduce the observed number ratiosN(dI)N(BCD) andN(dI)N(dE) and also the observed metallicity magnitude relation of local dwarf spheroidal galaxies.     

Search for low energyν e,μ,τ andv e,μ,τ in coincidence with BATSE gamma ray bursts

A search for low energy neutrinos of all flavours in correlation with 553 ray bursts detected by BATSE aboard the Compton Observatory has been performed by the LSD (Liquid Scintillator Detector) neutrino telescope. No excess ofe,, orv _e,, candidate has been detected by LSD during the time interval in which BATSE detected the 90% of the photon flux for any of the GRBs analyzed. Upper limits on the neutrino fluxes are given in the paper.     

UBV photometry of 262 southern galaxies

Multi-aperture photometry of 262 bright southern galaxies in the JohnsonUBV system is given. Most of these are south of =–30°, although some northward to =–10° are included. A total of 169 objects have published radial velocity determinations. These provide distances, and enable construction of colour-magnitude diagrams for this subset of bbjects through a physical diameter of 2.0 kpc (withH _o=100). The two-colour diagrams for the inner regions of the galaxies differ from those of integrated galaxies due to the colour changes towards their centres. Comparison with theoretical models of Larson and Tinsley (1978) suggest that the colours of the inner portions of most ellipticals and lenticulars are consistent with their having all stars formed at nearly one epoch with little subsequent star formation, while for spirals larger amounts of star formation, either in bursts or continuously, are suggested. This simple picture is complicated by the presence of certain objects having peculiar colours indicative of large amounts of recent star formation.     

Some peculiarities of four host galaxies,containing Seyfert nuclei

Star-like objects are found in Seyfert galaxies Markarian 290, Markarian 298, NGC 1275, and NGC 7469, being connected with the structure peculiarities of the galaxies. The absolute magnitudes of these objects are –16 ^m M–19 ^m . It has been supposed that these star formations must stimulate the instability in the disk of the galaxy followed by the matter fall toward the centre of the galaxy. The gas inflow toward the centre will allow the recent star formations and Seyfert nuclei generation.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.     

A GRB model satisfying stringent observational constraints

The temporal behavior of GRBs is quantified using a power spectrum analysis. The power spectrum of great variety of GRBs is well represented by the simple ^–2 behavior. We then study a cosmological GRB model in which relativistic flows interact with dense radiation fields. This mechanism in the densest stellar regions known to exist, surprisingly yields the correct temporal behavior. Other characteristics are also reproduced, including the duration bimodality and the hardness-duration distribution.     

Analysis of COMPTEL gamma-ray burst locations and spectra

In its first three years of operation, the COMPTEL instrument on theCompton Gamma-Ray Observatory has measured the locations (mean accuracy 1°) and spectra (0.75-30 MeV) of 18 gamma-ray bursts and continues to observe new events at a rate of 1/month. With good angular resolution and sensitivity at MeV energies, the growing COMPTEL burst catalog is an important new piece of evidence in the on-going GRB mystery. The COMPTEL burst locations are consistent with an isotropic distribution of sources, yet the spatial coincidence of two of the bursts indicates the possibility of repetition. The COMPTEL burst spectra are in most cases consistent with a single power law model with spectral index in the range 2–3. However, two bursts show evidence of a spectral break in the MeV range. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed. We present an overview of analysis results obtained from the COMPTEL burst catalog concentrating on the search for burst repetition and the implications of highly variable MeV emission.     

AGNs,X-ray background and the possible candidates for great attractor

In the five last years, different structures (density excess 1) have been proposed as the direct cause of our infall toward the direction of Hydra—Centaurus with a velocity of 500 km s^–1. The direct effect of the mentioned matter accumulations on the X-ray background (XRB) can be estimated as a function of the geometry of the structures and of the cosmological evolution of the sources emitting in the X-ray band (2–10 keV) for different universes (₀1). If the XRB comes mostly from AGNs with low luminosity (L _X <10⁴³ erg s^–1 and, therefore, they will have a weak cosmological evolution) and we consider the difference between the intensities coming from both hemispheres (that oriented toward the direction of our motion and the opposite one) obtained by means of different satellites, we can conclude that some candidates are highly unlikely.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     

A maximal background spectrum of gravitational radiation

A maximal spectrum of gravitational radiation from sources outside our galaxy is calculated. The sources are galaxies, quasars and events that occur in the early history of the universe. The major contribution is from galaxies whose effect extends over the frequency region 10^–810⁺⁴Hz, peaking at 10^–110 Hz, with a spectral flux of 10 erg cm^–2, s^–1. The main processes of gravitational radiation in the galaxies are stellar collapse into a black hole and dying binary systems. In the region 10^–410⁴ Hz the background spectrum is well above the detection levels of currently proposed detectors. FromMinimal considerations of this spectrum it is determined that the density of gravitational radiation is 10^–39g cm^–3. This background spectrum is sensitive to galactic evolution and especially sensitive to the upper mass limits and mass distribution of stars in galactic models. Therefore, the spectrum could provide information about galactic evolution complementary to that obtained by electromagnetic investigations.     

On the interior of the neutron star (II)

With the assumption, the physical 3-spacet = constant in a superdense star is spheroidal and the matter-density on the boundary surface of the configurationa = 2 × 10¹⁴ g cm^–3( the average matter density in a neutron star) Vaidya and Tikekar (1982) proposed an exact relativistic model for a neutron star. They suggested that their model can describe the hydrostatic equilibrium conditions in such a superdense star with densities in the range of 10¹⁴-10¹⁶ g cm^–3. Based on this model Parui and Sarma (1991) estimated the maximum limit of the density variation parameter for a stable neutron star (both for charged and uncharged) which is equal to 0.68, i.e. _max = 0.68.In this paper we have shown variation of central density per unit equilibrium radius (0/a), variation of mass, upper limit of density variation parameter both for charged and uncharged neutron stars at densities 10¹⁵ and 10¹⁶ g cm^–3, respectively. We have obtained _max = 0.68, i.e. the same as before. The important is that the duration of stability among the neutron star's constituents around _max will be shorter and shorter at higher densities as we proceed near the centre of the neutron star. In case of a charged neutron star, once stability among the constituents has been established, then unstability appears gradually maintaining linear relation between change in central density per unit equilibrium radius and change in mass whereas in case of uncharged neutron star, linear relation does not maintain.     

Optical identification of the IRAS point sources based on the first Byurakan survey

Optical identification of infrared sources from the IRAS Point Source Catalogue (PSC) is made by means of low-dispersion spectra of the First Byurakan Survey (FBS) and Palomar Observatory Sky Survey (POSS) red and blue images. The purpose of this work is to examine the composition of the PSC sample of fainter sources at high galactic latitudes and to reveal QSOs, infrared galaxies, red stars (C and M), planetary nebulae, for their further investigation at the optical range. 100 of 108 unknown IRAS sources in the region with 3^h50^m 7^h40^m and + 69° + 73° are optically identified. Optical coordinates, V magnitudes, color indices, and preliminary classes are determined. According to preliminary classification 3 objects turned out to be QSOs, 36 are galaxies with very interesting morphology, 5 are faint planetary nebulae, 9 are carbon stars, and 47 are late M-type stars.Published in Astrofizika, Vol. 38, No. 4, pp. 625–629, October–December, 1995.     

Comparison of large-scale structures and velocities in the local universe

Comparison of the large-scale density and velocity fields in the local universe shows detailed agreement, strengthening the standard paradigm of the gravitational origin of these structures. Quantitative analysis can determine the cosmological density parameter, , and biasing factor,b; there is virtually no sensitivity in any local analyses to the cosmological constant,. Comparison of the dipole anisotropy of the cosmic microwave background with the acceleration due to theIRAS galaxies puts the linear growth factor in the range ^0.6 /b = 0.6 _–0.3 ^+0.7 (95% confidence). A direct comparison of the density and velocity fields of nearby galaxies gives = 1.3 _–0.6 ^+0.7 , and from nonlinear analysis the weaker limit > 0.45 forb > 0.5 (again 95% confidence). A tighter limit, > 0.3 (4–6), is obtained by a reconstruction of the probability distribution function of the initial fluctuations from which the structures observed today arose. The last two methods depend critically on the smooth velocity field determined from the observed velocities of nearby galaxies by thePOTENT method. A new analysis of these velocities, with more than three times the data used to obtain the above quoted results, is now underway and promises to tighten the uncertainties considerably, as well as reduce systematic bias.     

Generalized polytropes: Stellar structure with a boltzmann factor

In order to study the structure of a chemically homogeneous star in equilibrium, a density profile of the form T ^N exp(–µm(–)/kT) is suggested. As for polytropes, qualitative aspects of the resulting stellar model can be discussed analytically. In particular it is shown that one reobtains forN=3 Eddington's standard model, whereas forN<3 nearly polytropic models result. WhenN>3, the effective polytropic index does vary appreciably over the star. Numerical results indicate that the proposed density profile is quite reasonable in view of the simplicity of the model. From a comparison of the degree of precision of a polytropic approximation with that of the newly proposed model it follows that the new approximation is definitely better than the polytropic one. It is suggested that the model may be useful to study the structure of stellar clouds, clusters and (spherical) galaxies.Now at Department of Applied Mathematics, Queen Mary College, University of London, England.     

Microscopic spectral features in solar decametric bursts and coronal irregularities

A crossed Yagi antenna array at 35 MHz was employed in conjunction with a polarization switch so as to enable spectral observations of solar noise storm activity in R and L polarizations. Intense decametric solar noise storms were recorded during the third week of November 1975 and fourth week of March 1976 with the help of a high resolution spectroscope operating near 35 MHz.The paper describes some of the new microscopic spectral features observed during these two noise storms. Three sets of high resolution dynamic spectra of decametric solar bursts, two of which are explained in terms of induced scattering of Langmuir waves by thermal ions and the third in terms of additional propagation effects through dense coronal irregularities, are presented. The microscopic bursts, classified as inverted U U and dots, represent small-scale (10⁴ km) phenomena with durations of less than a second.Some burst spectra appear as chain of dots with individual bandwidths 40 kHz and durations 0.3 sec. It is suggested that the bandwidth of such dot emissions (40 kHz) provides an evidence that they might indeed be generated by the process of induced scattering of plasma waves which predicts emission bandwidth f × 10^–3, where f is the center frequency.Some bursts are observed as a chain of striations showing curvature along the frequency axis which is attributed to dispersion in propagation delays through the dense coronal irregularities.     

Astron observations of the Rapid Burster MXB 1730-335 and constraints on burster parameters from spectra of trailing bursts

We report on eight X-ray bursts detected by ASTRON from the Rapid Burster (RB) on 13 and 28 April and 16 August, 1983. Six of them (trailing bursts), with durations of 1.5–2 min, rise times of 5–10 s and intervals of 1–1.5 hours, exhibit spectral softening during the burst decay and may be related to the type I bursts. Two of the bursts (triangle bursts) observed on 28 April at interval of 28 min with much longer rise times (30–50 s) and longer durations (3 min), do not show distinct spectral softening. Persistent flux from RB on 16 August was estimated asF _p(2.0–2.4)×10^–9 erg cm^–2 s^–1. Spectral evolution of two trailing bursts was investigated by fitting their spectra in consecutive time intervals with the blackbody (BB), isothermal scattering photosphere (SP) and thermal bremsstrahlung (TB) models. Around the burst maxima the SP model fits the data best whereas in the burst tails the TB model is generally better. The BB model is worse than at least one of the two others. Interpretation of the burst spectra in terms of the BB radiation leads to improbably small neutron star mass and radius (M<0.86M ,R _NS<5 km) if the peak luminosity does not exceed the Eddington limit. Interpretation of the spectra around the burst maxima (3–15 s from the burst onset) in terms of an isothermal SP yields reasonable constraints onM,R _NS, and distanceD. For instance, for the hydrogen photosphere we obtainedM=(1.0–2.1)M R _NS=(7.1–16.4) km ifD=11 kpc. If one postulatesM=1.4M , thenD=(8.5–13) kpc for hydrogen photosphere; if, besides,D=11 kpc, thenR _NS=(8.1–13.3) km. It follows also from the SP-interpretation that the photosphere radius may increase up to 20–30 km in maxima of the trailing bursts when the luminosity becomes close to the Eddington luminosity.     

Absolute and relative amplitudes of variations in radius of classical cepheids

On the basis of observational data for the absolute R and relative R/R amplitudes of variations in radius of galactic classical cepheids (55 stars from Balona and Stobie (1979) and 30 stars from Sollazzoet al. (1981)), four kinds of empirical linear relations are obtained: log(P V)–logR, logP–logR, log(P V)–log(R/R), and logP–log(R/R);P, R, and V are the pulsation periods, the mean stellar radii, and the amplitudes of light variations, respectively. Three groups of stars are considered: short-period cepheids (SPC)-with logP1.1; long-period cepheids (LPC)-with logP>1.1; and s-cepheids (sC). Both the R values and the R/R values increase withP andP V, for a given group of variables. A comparison is performed with our results obtained from data in other sources (Kurochkin, 1966; Gieren, 1982; etc.). The investigated relations can be applied for determining R and R/R of galactic classical cepheids, by using their observedP and V. All studied galactic classical cepheids have R/R<0.35, R<10R for SPC and 10R <R60R for LPC. The sC have smaller R and R/R values than other classical cepheids, at the same periods (the difference is about 2 times for R and 1.4–2.8 times for R/R); the studied sC have R/R in the range 0.025–0.075 and R in the range 1–3R (only Y Oph has R8R ).     

X-ray of gamma-ray bursts observable by HEAO-B

On grounds of Mazetset al. (1980a) conclusion that all gamma-ray bursts recorded were emitted at the distance 1 kpc from the Sun it is shown that bursts from the Galaxy and from another galaxies may be observable byEinstein Observatory (HEAO-B). The HEAO-B observing programs are considered and the numbers of possibly recorded events are estimated. It is concluded that the important information about sources of gamma-bursts may be obtained in this way.     

The distribution law of stellarp-modes

We show that the overall densityg() of asymptotic acoustic frequencies of a star obeys a Weyl lawg() ^D–1, whereD is the dimensionality of the oscillating stellar configuration. For realistic stars with a finite non-zero surface sound speed,D is equal to the actual dimensionality of the star,D=3. For formal models with a vanishing sound velocity at the surface, heuristic arguments lead to a dimensionality parameterD=4.5. The empirical frequencies of Eddington's standard model are found to be consistent with the latter distribution, with reasonable agreement already occurring in the low-frequency range > _i 2× fundamental radial mode. We argue that real stars obey this 3.5-power law in some finite frequency interval _i << _f , _f being a very high frequency critically depending on the surface sound velocity, while the full asymptotic law, withD=3, holds for > _f .