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.     

Relativistic Beaming and Orientation Effects in Lobe-Dominated Quasars

We examine a simple statistical consequence of relativistic beaming model using the core-to-lobe flux density ratio (R) and projected linear size (D) as orientation indicators. It is shown that the observed R-D intercept for the 3CRR source sample provides values of the bulk Lorentz factor () which are consistent with the beaming scenario. The result also shows that if the lobe-dominated quasars are to be unified with the core-dominated quasars, they should be oriented at a median angle, _{_med} 40° with respect to the line of sight, but if with powerful radio galaxies, then _{_med} 24°. Application of this result to the quasar-galaxy unification scheme using the observed angular size-redshift relation shows that the scheme is consistent with the observed data.     

Γ-rays from 3C273: scaling the orion cloud complex

The Orion cloud complex in the Milky Way is a source of-rays attributable to cosmic-ray interactions with ambient thermal gas. Quantities upon which the-ray flux depends can be scaled to the QSO 3C273. This result supports the hypothesis that at least some of the-rays from this object and other QSOs may also be attributed to a similarpp-interaction scenario.     

Gamma-ray astrophysics: parsecs to megaparsecs

A rapidly spinning, slowly accreting magnetic white dwarf (or X-ray pulsar) in hibernation is expected to result in rapid spindown as a result of the stretching and reconnection of magnetic field lines, leading to particle acceleration at the magnetospheric radiusoutside the corotation radius, and the propeller type ejection of magnetized synchrotron-emitting clouds. This may explain the non-thermal (radio and-rays) emission seen from the unique nearby AE Aquarii. Moving to Galactic distances we show how TeV-ray observations of pulsar-driven supernova remnants (with well-measured synchrotron X-ray spectra) allow us to obtain a direct measurement of the average magnetic field strength in the nebula. Finally, GeV to TeV observations of-ray blazars out to redshifts of 2 allow us to probe the intergalactic infrared radiation field, the Hubble constant and possibly the parameter of the Universe.     

Seyfert galaxies and the cosmic γ-ray diffuse background

Three active galaxies, generally classified as Seyferts, have been discovered recently to be powerful, low energy -ray sources. The similarity between their spectral characteristics and those of the cosmic background at -ray energies suggests that these objects could make a significant contribution to this diffuse flux. This contribution has been assessed using two different number densities of -ray-emitting Seyfert galaxies based on optical and X-ray data. The comparison of the estimated and measured diffuse -ray background intensities is used to gain a deeper understanding of the metabolism of Seyfert galaxies.     

Continuum radiation from active galactic nuclei

Summary Active galactic nuclei (AGN) can be divided into two broad classes, where the emitted continuum power is dominated either by thermal emission (radio-quiet AGN), or by nonthermal emission (blazars). Emission in the 0.01–1 m range is the primary contributor to the bolometric luminosity and is probably produced through thermal emission from an accretion disk, modified by electron scattering and general relativistic effects. The 1–1000 m continuum, the second most important contributor to the power, is generally dominated by thermal emission from dust with a range of temperatures from 40 K to 1000–2000 K. The dust is probably reemitting 0.01–0.3 m continuum emission, previously absorbed in an obscuring cone (or torus) or an extended disk. The 1–10 keV X-ray emission is rapidly variable and originates in a small region. This emission may be produced through Compton scattering by hot thermal electrons surrounding an accretion disk, although the observations are far from being definitive. The weak radio emission, which is due to the nonthermal synchrotron process, is usually elongated in the shape of jets and lobes (a core may be present too), and is morphologically distinct from the radio emission of starburst galaxies.In the blazar class, the radio through ultraviolet emission is decidedly non-thermal, and apparently is produced through the synchrotron process in an inhomogeneous plasma. The plasma probably is moving outward at relativistic velocities within a jet in which the Lorentz factor of bulk motion (typically 2–6) increases outward. This is inferred from observations indicating that the opening angle becomes progressively larger from the radio to the optical to the X-ray emitting regions. Shocks propagating along the jet may be responsible for much of the flux variability. In sources where the X-ray continuum is not a continuation of the optical-ultraviolet synchrotron emission, some objects show variability consistent with Compton scattering by relativistic electron in a large region (in BL Lacertae), while other objects produce their X-ray emission in a compact region, possibly suggesting pair production.When orientation effects are included, all AGN may be decomposed into a radio-quiet AGN, a blazar, or a combination of the two. Radio-quiet AGN appear to have an obscuring cone or torus containing the broad emission line clouds and an ionizing source. Most likely, the (non-relativistic) directional effects of this obscuring region give rise to the difference between Seyfert 1 and 2 galaxies or narrow and broad line radio galaxies. For different orientations of the nonthermal jet, relativistic Doppler boosting can produce BL Lacertae objects or FR I radio galaxies, or at higher jet luminosities, flat-spectrum high-polarization quasars or FR II radio galaxies.     

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.     

Spectrophotometry of some Be stars

The continuum energy distribution of six Be stars, namely 25 Cyg, 31 Peg, HR 8758, 14 Lac, 12 Vul, and Psc, in the wavelength region 3200–7800 Å, are presented. Comparing the observed energy distributions with those of theoretical models given by Kurucz (1979), their effective temperatures are determined.A near-infrared excess emission at wavelengths above 6000 Å is seen in most of the stars.     

Pregalactic-primordial low-mass stars

The Main-Sequence positions as well as the evolutionary behavior of Population III stars up to an evolution age of 2×10¹⁰ yr, taking this time as the age of the Universe, have been investigated in the mass range 0.2 and 0.8M . While Population III stars with masses greater than 0.3M develop a radiative core during the approach to the Main Sequence, stars with masses smaller than 0.3M reach the Main Sequence as a wholly convective stars. Population III stars with masses greater than 0.5M show a brightening of at most 2.2 in bolometric magnitude when the evolution is terminated as compared to the value which corresponds to zero-age Main Sequence. The positions of stars with masses smaller than 0.5M remain almost the same in the H-R diagram.If Population III stars have formed over a range of redshifts, 6相似文献   

The saturation of the cosmic diffuse γ-ray background by the Seyfert galaxy component

The nature of the diffuse cosmic background in the low -ray energy range has been widely discussed but not definitively solved. Unresolved active galaxies have become good candidates for its origin ever since they have been identified as -ray emitting objects. In particular, the summed contribution of Seyfert galaxies is expected to be dominant to such an extent that a straight forward calculation, based on their volume emissivity, predicts a considerable excess above the observed level of the isotropic background flux. In this letter we explore different ways to reconcile the observational data on these active galactic nuclei with the measured intensity of the cosmic diffuse -radiation.     

Passage of the solar current disk and major geomagnetic storms

It is shown that major geomagnetic storms (¦Dst¦ > 100) tend to develop at about the time of the passage of the solar current sheet or disk at the location of the Earth, provided this passage is associated with (1) a large impulsive increase of the IMF magnitude B, (2) a negative value of the IMF angle (Theta), and (3) an increasing solar wind speed. The passage occurs in association with the 27-day rotation of the warped current disk or a temporal up-down movement of the latter. The period in which ¦Dst¦/t< 0 during major storms coincides approximately with the period when the solar windmagnetosphere energy coupling function becomes 10¹⁹ erg s^–1. These conclusions do not depend on the phase of the sunspot cycle.These results may be interpreted as follows: A high speed solar wind flow, originating either from flare regions or coronal holes, tends to push the solar current disk to move upward or downward for either a brief period (1 3 days) or an extended period (2 weeks). A relatively thin region of a large IMF B > 10 is often present near the moving current disk. Waves are also generated on the moving current disk, and some of them cause large changes of . A high value of is found in the region of a large IMF B near the wavy solar current disk, where has a large negative value.     

Spectrophotometric study of P/Halley during post perihelion passage

Spectrophotometric observations of P/Halley are presented for four nights in the optical region (3200–7000 ) during post-perihelion period. Emission features due to CN, CH, C₃ and C₂ molecules are identified and the total apparent fluxes in each emission band are measured. It is found that the comet display large variations in the emission bands as well as continuum.     

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.     

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.     

A search for steep low-frequency radio spectra among quasars and clusters of galaxies

Using published flux densitiesS at low frequenciesv, radio spectra were constructed for 3C, 4C, and 4CT radio sources in Abell clusters of galaxies, radio galaxies outside Abell clusters, and quasars with known redshifts. About half the sources in rich Abell clusters (richness classesR>-2) have steep spectra between 38 and 178 MHz with spectral indices ₃₈ ¹⁷⁸ > whereSv ^–. However, radio galaxies outside clusters have values of ₃₈ ¹⁷⁸ 1.2, and no steep spectra were found among 170 quasars. The radio sources in rich clusters are probably confined by intergalactic gas, and the steep spectra develop over a period of 10⁹ yr as relativistic electrons lose energy. The absence of steep spectra among quasars does not necessarily mean that quasars never occur in rich clusters of galaxies, since quasars are probably being observed only in their early high-luminosity phases. The possibility that some quasar events occur in the nuclei of the dominant cD galaxies in clusters is discussed, but quasar events may occur in more than one type of galaxy.     

A correlation study between ozone and volcanic activity

A cross-correlation study for time-lags of ±5 yrs between eleven ground based ozone stations (1957–1985) for = 40°N–75° N and = 30° E-114° W and five volcanic emissivity indices has shown their close connection: significant correlations well above 90% were obtained. Intepretation of these positive/negative correlations () was based on the global wind circulation (aided also by a 2-D, 3-D representation between, , ), and the types of volcanic aerosols leading to heterogeneous chemical reactions with ozone.     

The contribution of normal galaxies to the low energy gamma-ray cosmic diffuse background

Theories to explain the origin of the cosmic diffuse -ray background generally fall into one of two broad categories: those which attribute the emission to particle interctions in intergalactic space and those which attribute it to the summation of numerous, unresolved discrete sources, including normal field galaxies, active galactic nuclei and clusters of galaxies. Strong support for the latter interpretation is given by recent measurements of -ray emission from external galaxies, mainly Seyfert galaxies. Their summed contribution has been evaluated elsewhere; here instead, we use recent observational data on the -ray emission from our own galaxy to estimate the contribution of normal galaxies to the cosmic diffuse -radiation. Our result indicates that this contribution is limited to less than 0.1% and can therefore be neglected.     

The Radio-Silent Start of an Intense Solar Gamma-Ray Flare

Radio-silent -ray flares are solar flares that lack any significant emission in the (non-thermal) radio wave band during their impulsive hard X-ray and -ray emission phases. Flares with extremely suppressed long-wavelength spectra have previously been reported by White et al. (1992) and have been discussed in different context by Hudson and Ryan (1995). A striking example of a radio-silent flare was observed by SMM during the onset of the 6 March 1989 energetic -ray flare. We argue that the absence of radio emission at wavelengths longer than microwave wavelengths is an indication of the compactness of the flare rather than that the flare did not exhibit non-thermal properties. Probably the flare site was restricted to altitudes above the photosphere in a newly emerging loop configuration lower than the equivalent altitude corresponding to an emission frequency of 1.4 GHz. This implies the presence of a dense and highly magnetized closed field configuration confining the electron component which causes the impulsive -ray continuum. Reconnection in such a configuration did not lead to open magnetic fields and streamer formation. Acceleration of particles in the and hard X-ray bursts was restricted to closed field lines. Thermal expansion of the loop system may subsequently lead to the generation of radially propagating blast waves in the solar corona which are accompanied by type II solar radio bursts and decimetre emissions. The emission during the onset of the flare was dominated by a continuum originating from electron bremsstrahlung at X-ray and -ray energies with only little evidence for the presence of energetic ions. It is, therefore, concluded that energetic electrons have been primary and not secondary products of the particle acceleration process.     

A radiation model for the vela pulsar

We model the Vela emission in radio, optical and gamma wavelengths. We assume that radio emission occurs near the polar axis deep in the magnetosphere. Optical and gamma radiation arises through ordinary synchrotron mechanisms in a wide hollow cone, near the light cylinder. Fitting of observed frequencies and luminosities in the gamma and optical give reasonable plasma parameters, and indicate that field-plasma pressure balance breaks down before the light cylinder, as required by the picture. Some optical coherence is necessary if we attribute both radio and optical emission to the same species of particles, with 100, but it is not required otherwise. Gamma-ray pulses arise from 10⁵ electrons. We suggest a single-pole orthogonal rotor picture which might account for the Vela pulse phases.     

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.