共查询到20条相似文献，搜索用时 31 毫秒
1.
A comparative analysis of various parameters of pulsars with short (P < 0.1 s) and long (P > 0.1 s) periods is carried out. There is no correlation between the radio and gamma-ray luminosities of the pulsars and their surfacemagnetic fields, but there is a correlation between the X-ray luminosity and the surfacemagnetic field. A dependence of the X-ray and gamma-ray luminosities on the magnetic field at the light cylinder is also found. This result provides evidence for the formation of hard, non-thermal emission at the periphery of the magnetosphere. An appreciable positive correlation between the luminosity and the rate of rotational energy loss by the neutron star is observed, supporting the idea that all radio pulsars have the same basic source of energy. The efficiency of the transformation of rotational energy into radiation is significantly higher in long-period pulsars. The dependence of the pulse width on the pulsar period is steeper for pulsars with short periods than for those with long periods. The results obtained support earlier assertions that there are differences in the processes generating the emission in pulsars with P < 0.1 s and those with P > 0.1 s. 相似文献
2.
We consider the influence of a non-dipolar magnetic field on the gamma-ray emission from the polar regions of a radio pulsar. The pulsar is treated in a Goldreich-Julian model with a free flow of charge from the surface of the neutron star. When finding the intensity of the gamma-ray radiation of the pulsar tube, both curvature gamma-ray radiation from the primary electrons and non-resonance inverse Compton scattering of thermal photons from the polar cap on primary electrons are taken into account. When finding the height of the upper plate of the pulsar diode, we included only positrons created by the curvature radiation of primary electrons. We assumed that the polar cap is heated by the return positron current. The influence on the gamma-ray emission of variations in both the radius of curvature of the magnetic force lines and in the electric field due to the non-dipolarity of the magnetic field were taken into account. The presence of even weak non-dipolarity of the magnetic field leads to a sharp decrease in the intensity of the gamma-ray emission from the pulsar tube at energies 1–100 MeV, while the intensity of the inverse Compton radiation (at energies 1–100 GeV) varies only relatively weakly. 相似文献
3.
A. V. Tutukov 《Astronomy Reports》2003,47(8):637-647
The rate of gamma-ray bursts (GRBs) in the Galaxy is estimated assuming that these events result from the formation of rapidly rotating Kerr black holes during the core collapse of massive, helium, Wolf-Rayet secondary components in very close binary systems. This process brings about rapid rotation of the cores of such Wolf-Rayet stars, inevitably resulting in the formation of Kerr black holes during type Ib,c supernovae. The current rate of formation of Kerr black holes (GRBs) in the Galaxy is about 3×10?5/year. Collimation of the gamma-ray radiation into a small solid angle (about 0.1–0.01 sr) brings this rate into consistency with the observed rate of GRBs, estimated to be 10?6–10?7/year. Possible immediate progenitors of GRBs are massive X-ray binaries with X-ray luminosities of 1038–1040 erg/s. Due to the short lifetimes of the progenitors and the very high brightnesses of GRBs, the GRB rate can provide information about the history of star formation in the Universe on the Hubble time scale. A model in which the star-formation rate is determined by the conditions for ionization of the interstellar gas, whose density and volume are determined by supernovae, yields a Galactic star-formation history that can be viewed as representing the history of star formation in the Universe. The theoretical history of star formation is in satisfactory agreement with the history reconstructed from observations. The theoretical model for the history of star formation in the Galaxy can also be used to assess the influence of dust on optical observations of supernovae and GRBs in galaxies of various ages. 相似文献
4.
It is shown that cyclotron radiation by electrons near the surface of a neutron star with a magnetic field of ～1012 G can easily provide the observed quiescent radiation of magnetars (Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters). Pulsed emission is generated by the synchrotron mechanism at the periphery of the magnetosphere. Short-time-scale cataclysms on the neutron star could lead to flares of gamma-ray radiation with powers exceeding the power of the X-ray emission by a factor of 2γ2, where γ is the Lorentz factor of the radiating particles. It is shown that an electron cyclotron line with an energy of roughly 1 MeV should be generated in the magnetar model. The detection of this line would serve as confirmation of the correctness of this model. 相似文献
5.
Supernovae and gamma-ray bursts (GRBs) are among the brightest events in the universe. Excluding Type Ia supernovae and short GRBs, they are the result of the core collapse of a massive star with material being ejectedwith speeds of several 1000 km/s to nearly the speed of light, and with a neutron star or a black hole left over as the compact remnant of the explosion. Synchrotron radiation in the radio is generated in a shell when the ejecta interact with the surrounding medium and possibly also in the central region near the compact remnant itself. VLBI has allowed resolving some of these sources and monitoring their expansion in detail, thereby revealing characteristics of the dying star, the explosion, the expanding shock front, and the expected compact remnant. We report on updates of some of the most interesting results that have been obtained with VLBI so far. Movies of supernovae are available from our website. They show the evolution from shortly after the explosion to decades thereafter, in one case revealing an emerging compact central source, which may be associated with shock interaction near the explosion center or with the stellar corpse itself, a neutron star or a black hole. 相似文献
6.
Bolometric light curves for the afterglow resulting from the passage of a gamma-ray burst through a molecular cloud are computed. The profile and duration of the afterglow light curve depend strongly on the distribution of matter in the cloud, the degree of collimation of the gamma-ray radiation, and the observing conditions. The peak can be reached as soon as seven days (the gamma-ray burst is located at some distance from the center of a molecular cloud with small-scale density enhancements), or as long as one to three years (the gamma-ray burst is located at the center of a uniform molecular cloud) after the burst. The bolometric luminosity of the re-radiated signal can reach 6.5 × 1042 erg/s. 相似文献
7.
We use a two-phase model for the structure of the circumstellar nebulae of hot stars to analyze the radiative cooling of a dense, compact cloud behind the shock produced by the compression of the cloud by hot gas from the stellar wind, taking into account ionization and heating by radiation from the central star. We can distinguish three stages of the evolution of the cloud during its compression. In the first stage, relevant for the entire cloud before compression and the gas ahead of the shock front, the state of the gas is determined purely by ionization by the stellar radiation. The next stage is characterized by the simultaneous action of two gas excitation mechanisms—photoionization by the stellar radiation and shock heating. In this stage, the gas intensively radiates thermal energy received at the shock front. After radiative cooling, in the final stage, ionization and heating of the gas are again determined mainly by the star. To compute the spectrum of the cloud radiation, we solved for the propagation of a plane-parallel, homogeneous flux through the shock front in the radiation field of the hot star. The computations show that a combination of two excitation mechanisms considerably enriches the theoretical spectrum. The relative intensities of emission lines of a single cloud may resemble either those for an HII region or of a supernova remnant. 相似文献
8.
We present a model in which the nonthermal radio emission of binary systems containing Wolf-Rayet and O components is due to collisions between clouds belonging to dense phases of the wind of each star. The relativistic electrons are generated during the propagation of fast shock waves through the clouds and their subsequent de-excitation. The initial injection of superthermal particles is due to photoionization of the de-excited cold gas by hard radiation from the shock front. Therefore, the injection takes place in cloud regions fairly far from the front. Further, the superthermal electrons are accelerated by the betatron mechanism to relativistic energies during the isobaric compression of the cloud material, when most of the gas radiates its energy. Collisions between the clouds can occur far beyond the contact boundary between the rarefied wind components. Thus, the model avoids the problem of strong low-frequency absorption of the radiation. 相似文献
9.
The coalescence of components of a binary star with equal masses (M 1 = M 2 = M ⊙) and moving in circular orbits is considered. The equation of state for degenerate neutrons is used, leading to the equation of state for an ideal gas. The initial model has zero temperature, corresponding to a polytrope with n = 1.5. To reduce the required computational time, the initial close binary is constructed using the self-consistent field method. The computations use Newtonian gas dynamics, but the back reaction of the gravitational radiation is taken into account in a PN2.5 post-Newton approximation, obtained using ADM formalism. This makes it possible to apply previous experienceof constructing high-order Godunov-type difference schemes, which are suitable for end-to-end calculations of discontinuous solutions of the gas-dynamics equations on a fixed Eulerian grid. The Poisson equations were solved using an original spherical-function expansion method. The 3D computations yielded the parameters of the gravitational signal. Near the radiation maximum, the strain amplitude is rh ～ 4 × 104 cm, the power maximum is 4 × 1054 erg/s, and the typical radiation frequency is ?1 kHz. The energy carried away by gravitational waves is ?1052 erg. These parameters are of interest, since they form an inherent part of a rotational mechanism for the supernova explosion. They are also of interest for the planning of gravitational-wave detection experiments. 相似文献
10.
The generation of infrared (IR) radiation and the observed IR-intensity distribution at wavelengths of 8, 24, and 100 µm in the ionized hydrogen region around a young, massive star is investigated. The evolution of the HII region is treated using a self-consistent chemical-dynamical model in which three dust populations are included—large silicate grains, small graphite grains, and polycyclic, aromatic hydrocarbons (PAHs). A radiative transfer model taking into account stochastic heating of small grains and macromolecules is used to model the IR spectral energy distribution. The computational results are compared with Spitzer and Herschel observations of the RCW 120 nebula. The contributions of collisions with gas particles and the radiation field of the star to stochastic heating of small grains are investigated. It is shown that a model with a homogeneous PAH content cannot reproduce the ring-like IR-intensity distribution at 8 µm. A model in which PAHs are destroyed by ultraviolet radiation of the star, generating region HII, provides a means to explain this intensity distribution. This model is in agreement with observations for realistic characteristic destruction times for the PAHs. 相似文献
11.
Interaction of a cosmological gamma-ray burst with a dense molecular cloud and the formation of jets
The interaction of a powerful cosmological gamma-ray burst (GRB) with a dense molecular cloud is modeled. Two-dimensional gas-dynamical flows were computed for various configurations of the cloud. In the spherically symmetrical case, the gas velocity does not exceed $ \sim 2 \times 10^3 \sqrt {E/1.6 \times 10^{53} } km/s$ km/s. If the GRB precursor has an anisotropic wind, a conical cavity can form in the nearby region of the molecular cloud. The propagation of the gamma-ray pulse in this cavity leads to the formation of a rapidly moving hot clump of matter, with the gas velocity reaching 1.8 × 104 km/s for gamma-ray energy of E = 1.6 × 1053 erg. In all the computations, the velocity of the moving material is much lower than the velocity of light, the volume of gas affected by the motion is small, and the influence of the gas motions on the light curve of the optical afterglow is insignificant. 相似文献
12.
We consider the evolution of binary systems formed by a Supermassive Black Hole (SMBH) residing in the center of a galaxy
or a globular cluster and a star in its immediate vicinity. The star is assumed to fill its Roche lobe, and the SMBH accretes
primarily the matter of this star. The evolution of such a system is mainly determined by the same processes as for an ordinary
binary. The main differences are that the donor star is irradiated by hard radiation emitted during accretion onto the SMBH;
in a detached system, nearly all the donor wind is captured by the black hole, which strongly affects the evolution of the
semi-major axis; it is not possible for companions of the most massive SMBHs to fill their Roche lobes, since the corresponding
orbital separations are smaller than the radius of the last stable orbit in the gravitational field of the SMBH. Moreover,
there may not be efficient exchange between the orbital angular momentum and the angular momentum of the overflowing matter
in such systems. Our computations assumed that, if the characteristic timescale for mass transfer is smaller than the thermal
timescale of the star, no momentum exchange occurs. Absorption of incident external radiation in the stellar envelope was
treated using the same formalism that was used when computing the radiative transfer in the stellar atmosphere. Numerical
simulations show that Roche-lobe overflow is possible for a broad range of initial system parameters. The evolution of semi-detached
systems containing a star and a SMBH nearly always ends with the dynamical disruption of the star. Stars with masses close
to the solar mass are destroyed immediately after they fill their Roche lobes. During the accretion of matter of disrupted
stars, the SMBH can achieve quasar luminosities. If the SMBH accretes ambient gas as well as gas stripped from stars, the
star is subject to additional radiation in the detached phase of its evolution, strengthening its stellar wind. This leads
to an increase of the semi-major axis and subsequent decrease of the probability of Roche-lobe overflow during the subsequent
evolution of the system. 相似文献
13.
V. P. Grinin D. N. Shakhovskoi V. I. Shenavrin A. N. Rostopchina L. V. Tambovtseva 《Astronomy Reports》2002,46(8):646-655
We present observations of the UX Ori star RR Tau in the optical (UBVRI)and near infrared (JHKLM)acquired between November 2000 and April 2001. We recorded a uniquely long (about half a year) Algol-like minimum with an amplitude of ΔV≈2.9. The dimming of RR Tau was accompanied by an increase of the linear polarization, typical of UX Ori stars and testifying to the eclipsing nature of the minimum. The J and H infrared fluxes varied synchronously with the optical variations. However, the K and L brightness changes were in the opposite sense: the flux in these two bands increased for the entire duration of the optical minimum. Our analysis suggests that the source of the K and L radiation is the dust cloud itself, moving at a distance of about 1 AU from the star. The flux increase in these bands was not due to an increase in the dust temperature, but instead to an increase in the number of emitting grains in the cloud. This could be associated either with an actual increase in the number of fine grains due to sublimation and the disruption of larger grains or with the distortion and disruption of the cloud due to tidal perturbation, permitting the star’s light to penetrate and heat the densest regions of the cloud. Based on the observed L fluxes, we estimate the mass of the emitting dust in the cloud to be ≈1023 g. Taking into account the presence of cool dust and a gaseous component in the cloud in addition to the dust heated by the star’s radiation, and adopting a ratio for the masses of the dust and gas components similar to that in the interstellar medium (1:100), we estimate the cloud’s total mass to be ≥1025 g. Judging from this value and the duration of the minimum, we observed an extremely rare episode associated with a giant gas and dust cloud with a total mass on the order of 0.1 lunar mass or higher, which passed very near the young star (and may be falling onto it). 相似文献
14.
Modeling of hydrogen emission lines is a powerful tool to study physical processes in the nearest vicinity of young stars because spectral lines carry information on the kinematics and physical conditions of the gas. One of the lines that probe emitting regions closest to the star is the Br$$\gamma $$ line. We consider different types of hybrid models to reproduce both interferometric VLTI-AMBER observations and LBT-LUCIFER spectroscopic observations of the single-peak profile of the Br$$\gamma $$ line of the Herbig AeBe star (HAEBE) VV Ser, a member of the UX Ori type subclass. We computed models of a magneto-centrifugal disk wind, a magnetospheric accretion region (magnetosphere), Cranmer’s polar wind, and scattered light from circumstellar polar dust. Furthermore, we calculated hybrid two-component models consisting of a disk wind and one of the aforementioned models. We computed visibilities and line profiles for all types of models and compared them with the available interferometric observations to constrain model parameters. We conclude that for the inclinations reported for this star (60°–70°), the disk wind alone cannot explain the Br$$\gamma $$ line profile although it may be a dominant contributor to the hydrogen line radiation. However, magneto-centrifugal disk wind in combination with aforementioned emitting regions (magnetosphere, polar wind, or scattered light from polar dust) may be able to reproduce the observations. 相似文献
15.
V. A. Hagen-Thorn V. M. Larionov D. A. Morozova A. A. Arkharov E. I. Hagen-Thorn E. S. Shablovinskaya M. S. Prokop’eva V. A. Yakovleva 《Astronomy Reports》2018,62(2):103-115
The results of optical, radio, and gamma-ray observations of the blazar AO 0235+16 are presented, including photometric (BV RIJHK) and polarimetric (R)monitoring carried out at St. Petersburg State University and the Central (Pulkovo) Astronomical Observatory in 2007–2015, 43 GHz Very Long Baseline Interferometry radio observations processed at Boston University, and a gamma-ray light curve based on observationswith the Fermi space observatory are presented. Two strong outbursts were detected. The relative spectral energy distributions of the variable components responsible for the outbursts are determined; these follow power laws, but with different spectral indices. The degree of polarization was high in both outbursts; only an average relationship between the brightness and polarization can be found. There was no time lag between the variations in the optical and gamma-ray, suggesting that the sources of the radiation in the optical and gamma-ray are located in the same region of the jet. 相似文献
16.
A. N. Rostopchina V. P. Grinin D. N. Shakhovskoi A. A. Lomach N. Kh. Minikulov 《Astronomy Reports》2007,51(1):55-66
We present the results of simultaneous UBVRI photometry and polarimetry of the classical T Tauri star CO Ori carried out at the Crimean Astrophysical Observatory during the 18 years between 1986 and 2004. We show that the variations of linear polarization accompanying the star’s brightness variations follow the law characteristic of UX Ori stars. This suggests that the brightness variations of the star are mainly due to changes of the circumstellar extinction due to non-uniform structure of the circumstellar environment, and to an “optimal” orientation of the circumstellar gas and dust disk relative to the observer, whose line of sight crosses the gas and dust atmosphere of the disk. We determine the star’s intrinsic polarization due to scattering of light in the circumstellar disk. The polarization position angle indicates the orientation of the disk’s symmetry axis in the plane of the sky. Our analysis of an archival light curve for CO Ori confirms the existence of a many-year cycle of photometric activity, suspected by us earlier. The refined period of this cycle is 12.4 years. The existence of such activity cycles of UX Ori stars testifies to considerable deviations of their circumstellar disks from axial symmetry, a reflection of either stellar binarity or the commencement of the process of planetary formation. 相似文献
17.
Several scenarios for the formation of accretion and decretion disks in single and binary Ae and Be stars are proposed. It is shown that, in order for a rapidly rotating main-sequence Be star to lose mass via a disk, the star’s rotation must be quasi-rigid-body. Estimates show that such rotation can be maintained by the star’s magnetic field, which is probably a relict field. The evolution of single Be main-sequence stars is numerically simulated allowing for mass loss via the stellar wind and rotational mass loss assuming rigid-body rotation. The stellar wind is the factor that determines the maximum mass of Be stars, which is close to 30M ⊙. The evolution of Be stars in close binaries is analyzed in the approximation adopted in our scenario. Long gamma-ray bursts can be obtained as a result of the collapse of rapidly rotating oxygen—neon degenerate dwarfs—the accreting companions of Be stars—into neutron stars. 相似文献
18.
19.
A new model is put forward to explain the observed features of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs). It is shown that drift waves can be excited in the magnetosphere of a neutron star with a rotational period of P～0.1 s, surface magnetic field Bs～1012 G, and angle between the rotational axis and magnetic moment β<10°. These waves lead to the formation of radiation pulses with a period of Pdr～10 s. The rate of loss of rotational energy by such a star (～1037 erg/s) is sufficient to produce the observed increase in the period \((\dot P \sim 10^{ - 10} )\), the X-ray luminosities of AXPs and SGRs (～1034–1036 erg/s), and an injection of relativistic particles into the surrounding supernova remnant. A modulation of the constant component of the radiation with a period of P～0.1 s is predicted. In order for SGRs to produce gamma-ray bursts, an additional source of energy must be invoked. Radio pulsars with periods of Pobs>5 s can be described by the proposed model; in this case, their rotational periods are considerably less than Pobs and the observed pulses are due to the drift waves. 相似文献
20.
We consider the evolution of stars that are located in the near vicinity of bright quasars and illuminated by their hard radiation. The absorption of the external radiation flux in the stellar envelope was calculated applying the formalism used to determine the opacity of the stellar material. The simulations show that the external irradiation heats the outer layers of the star, altering their structure and reducing the thickness of the convective envelope in low-mass stars, but leaves the inner layers essentially unaffected. In addition, the irradiation substantially increases the mass loss in the stars. This is important for our understanding of the evolution of quasar masses, since this increased mass loss by nearby stars can supply the quasar with additional accreting gas. The results can also be applied to detached binaries in which a low-mass star is irradiated by a very massive companion. 相似文献