共查询到20条相似文献，搜索用时 31 毫秒
1.
We offer two possible explanations to account for the characteristics of integrated pulse profiles, in particular their degree
of complexity, their variation from pulsar to pulsar, their stability, and the tendency of complex profiles to be associated
with older pulsars.
It is proposed that the pulse structure could be a reflection of surface irregularities at the polar caps, and it is shown
how the surface relief can affect the number of positrons released into the magnetosphere which are subsequently responsible
for the observed radio radiation. The electrons produced in the vacuum break-down in the gap carry enough energy to allow
creating such a surface relief in ∼ 106 years, and one way in which this could be achieved is discussed.
Alternatively, the presence of multipole components in the magnetic fields of older pulsars could lead to significant variations
in the curvature of the field lines across the gap, and hence to structure in the integrated pulse profiles. An assessment
of the two hypotheses from observed pulse profiles seems to favour the polar cap relief picture.
An erratum to this article is available at . 相似文献
2.
D. C. Backer 《Journal of Astrophysics and Astronomy》1995,16(2):165-171
Two investigations of millisecond pulsar radiation are discussed: average total intensity pulse morphology and individual
pulse to pulse fluctuations. The average emission profiles of millisecond pulsars are compared with those of slower pulsars
in the context of polar cap models. In general the full widths of pulsar emission regions continue to widen inversely with
periodP as P-(0.30-0.5) as expected for dipole polar cap models. Many pulse components are very narrow. The period scaling of pulsar profiles -separations
and widths -can tell us about the angular distribution of radiating currents. An investigation of individual pulses from two
millisecond pulsars at 430 MHz shows erratic pulse to pulse variations similar to that seen in slow pulsars. PSR B1937+21
displays occasional strong pulses that are located in the trailing edge of the average profile with relative flux densities
in the range of 100 to 400. These are similar to the giant pulses seen in the Crab pulsar. 相似文献
3.
By using rather conservative estimates based on the simplest polar cap model, we search the ATNF Pulsar Catalogue for strongly
magnetized stars that could accelerate relativistic protons up to the curvature pion production threshold. The best candidate
turns out to be the 16 ms pulsar J0537-6910, but the corresponding characteristic parameter χ=a/m
p
is yet too small to give origin to observable signals. We show that, for pulsars with period P≈1 ms, a surface polar magnetic field B≈1012 G is required in order to induce detectable curvature pion radiation from accelerated protons in the magnetosphere. Some
other emission processes are also considered. 相似文献
4.
Recent studies suggest that pulsars could be strong sources of TeV muon neutrinos provided positive ions are accelerated by pulsar polar caps to PeV energies. In such a situation, muon neutrinos are produced through the Δ-resonance in interactions of pulsar-accelerated ions with its thermal radiation field. High-energy gamma-rays should also be produced simultaneously in pulsar environment as both charged and neutral pions are generated in the interactions of energetic hadrons with the ambient photon fields. Here, we estimate TeV gamma-ray flux at the Earth from a few nearby young pulsars. When compared with the observations, we find that proper consideration of the effect of polar cap geometry in flux calculation is important. Incorporating such an effect, we obtain the (revised) event rates at the Earth due to a few potential nearby pulsars. The results suggest that pulsars are unlikely to be detected by the upcoming neutrino telescopes. We also estimate TeV gamma-ray and neutrino fluxes from pulsar nebulae for the adopted model of particle acceleration. 相似文献
5.
We have investigated the formation of polar gaps in pulsars as envisaged by Ruderman and Sutherland (1975) in the presence of photon conversion into positronia (Shabad and Usov, 1985; Heroldet al., 1985) and dissociation of positronia by strong electric fields. For the emission of curvature radiation near the pulsar's surface, we have used formulae which are valid in the extreme relativistic regime. We find that the gap formation is not affected, only the gap height is generally slightly increased. In the sample of pulsars that we have studied, in most cases there is likely to be enough plasma in the gap to initiate processes producing radio emission in accordance with the theory. In some cases we found the possibility of emission of high-energy photons outside the pulsar magnetosphere, as was suggested earlier by us (Bhatiaet al., 1987) in conformity with some of the observations. 相似文献
6.
The results of flux pulsar radioemission measurements at meter wavelengths, made at Pushchino Radio Astronomical Observatory
of the Lebedev Physical Institute, are presented. Flux densities at 102, 85, 61 and 39 MHz have been measured for 85, 29,
37 and 23 pulsars correspondingly. Some of them were performed at all frequencies simultaneously.
On the basis of these data and high frequencies data obtained by other authors, spectra of 52 pulsars were plotted. In practically
all investigated pulsars we have detected a turn-over frequency at which the flux density of pulsar radioemission attained
its maximum. Its mean value isv
m
=130±80 MHz. Averaged on many pulsars, the spectral index is negative in the 39–61 MHz frequency range
and passes through zero at frequencies of about 100 MHz, becoming positive in the 100–400 MHz frequency range. It was noticed
that the spectral index in the 100–400 MHz interval depends upon such pulsar periods as α100−=0.7logp+0.9. Using the spectra, more precise radio luminosities of pulsars have been computed. 相似文献
7.
We develop a numerical code for simulating the magnetospheres of millisecond pulsars, which are expected to have unscreened
electric potentials due to the lack of magnetic pair production. We incorporate General Relativistic (GR) expressions for
the electric field and charge density and include curvature radiation (CR) due to primary electrons accelerated above the
stellar surface, whereas inverse Compton scattering (ICS) of thermal X-ray photons by these electrons are neglected as a second-order
effect. We apply the model to PSR J0437-4715, a prime candidate for testing the GR-Electrodynamic theory, and find that the
curvature radiation spectrum cuts off at energies below 15 GeV, which are well below the threshold of the H.E.S.S. telescope,
whereas Classical Electrodynamics predict a much higher cutoff near 100 GeV, which should be visible for H.E.S.S., if standard
assumed Classical Electrodynamics apply. GR theory also predicts a relatively narrow pulse (2φ
L
∼ 0.2 phase width) centered on the magnetic axis, which sets the beaming solid angle to ∼0.5 sr per polar cap (PC) for a magnetic
inclination angle of 35∘ relative to the spin axis, given an observer which sweeps close to the magnetic axis. We also find that EGRET observations
above 100 MeV of this pulsar constrain the polar magnetic field strength to B
pc < 4× 108 G for a pulsar radius of 10 km and moment of inertia of 1045 g cm2. The field strength constraint becomes even tighter for a larger radius and moment of inertia. Furthermore, a reanalysis
of the full EGRET data set of this pulsar, assuming the predicted pulse shape and position, should lead to even tighter constraints
on neutron star and GR parameters, up to the point where the GR-derived potential and polar cap current may be questioned. 相似文献
8.
Estelle Asseo David Khechinashvili 《Monthly notices of the Royal Astronomical Society》2002,334(4):743-759
We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, 'neutral points' and 'neutral lines' exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these 'favourable' subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943+10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars. 相似文献
9.
Vyacheslav E. Zavlin 《Astrophysics and Space Science》2007,308(1-4):297-307
Millisecond pulsars represent an evolutionarily distinct group among rotation-powered pulsars. Outside the radio band, the soft X-ray range (～0.1–10 keV) is most suitable for studying radiative mechanisms operating in these fascinating objects. X-ray observations revealed diverse properties of emission from millisecond pulsars. For the most of them, the bulk of radiation is of a thermal origin, emitted from small spots (polar caps) on the neutron star surface heated by relativistic particles produced in pulsar acceleration zones. On the other hand, a few other very fast rotating pulsars exhibit almost pure nonthermal emission generated, most probably, in pulsar magnetospheres. There are also examples of nonthermal emission detected from X-ray nebulae powered by millisecond pulsars, as well as from pulsar winds shocked in binary systems with millisecond pulsars as companions. These and other most important results obtained from X-ray observations of millisecond pulsars are reviewed in this paper, as well as results from the search for millisecond pulsations in X-ray flux of the radio-quite neutron star RX J1856.5-3754. 相似文献
10.
Marco Morini 《Astrophysics and Space Science》1981,79(1):203-211
The physical conditions near pulsar surface that might give rise to gamma ray emission from Crab and Vela pulsars are not yet well understood. Here I suggest that, in the context of the vacuum discharge mechanism proposed by Ruderman and Sutherland (1975), gamma rays are produced by inverse Compton scattering of secondary electrons with the thermal radiation of the star surface as well as for curvature and synchrotron radiation. It is found that inverse Compton scattering is relevant if the neutron star surface temperature is greater than 106K or if the polar cap temperature is of the order of 5×106K. Inverse Compton scattering in anisotropic photon fields and Klein-Nishina regime is here carefully considered. 相似文献
11.
W. Sutantyo 《Astrophysics and Space Science》1978,54(2):479-488
We investigate the effect of asymmetric supernova explosions on the orbital parameters of binary systems with a compact component. We relate such explosions to the origin of binary pulsars. The degree of asymmetry of the explosion is represented by the kick velocity gained by the exploding star due to the asymmetric mass ejection. The required kick velocity to produce the observed parameters of the binary pulsar PSR 1913+16 should be larger than 80 km s–1 if the mass of the exploding star is larger than 4M
. We examine the mean survival probability of the binary system (<f>) for various degrees of asymmetry in the explosion. The rare occurrence of a binary pulsar does not necessarily imply that such a probability is low since not all pulsars have originated in a binary system. Assuming the birth rate of pulsars derived by Taylor and Manchester (1977), we derive that <f> would be as high as 0.25. Such values of <f> can be obtained if the mass of the exploding stars is, in general, not large (10M
). 相似文献
12.
The production of pairs of magnetic monopoles-antimonopoles should be expected in the interactions of the high energy particles accelerated by pulsars. In the frame of the Sturrock model, the interactions of the very high energy protons emitted from the polar caps with the secondary electrons can be a source of magnetic monopoles. It may be the dominating process in very young pulsars such as the Crab pulsar. In the polar gap model of Ruderman and Sutherland, magnetic monopoles can be created by the electrons accelerated across the cap and interacting with the neutron star crust or by the negatons and positrons interacting head-on inside the sparks.Half of these monopoles are accelerated towards the interstellar medium by the pulsar magnetic field and the others are likely to be trapped inside the neutron star crust. This leads to a decrease in the pulsar magnetic field which would imply that the characteristic age
may not give the true age of the pulsar This can be related to the discrepancy between and the real age of the Crab pulsar and the kinematical ages obtained from the measurement of the proper motion of some pulsars. Furthermore, the trapping of magnetic monopoles close under the surface of the neutron star perturbates the pulsar electrodynamics. To have such observable effects, it is shown that the cross-sections for the magnetic monopoles production can be several orders of magnitude smaller than the upper limits so far derived from cosmic rays or accelerator data.The possibility that the magnetic monopoles, accelerated outwards, are responsible for the highest energy extensive air showers, is considered.The production of an avalanche of secondary monopoles, due to acceleration by the magnetic field in the neutron star crust, is possible and the consequences of this process are considered. 相似文献
13.
The curvature -quanta emitted in the pulsar magnetospheres tangentially to the curved lines of force of the magnetic field are shown to be later canalized along the magnetic field—if it is strong enough,B0.1B
cr
=4×1012G—by gradually converting into a mutually bound electron-positron pair, i.e. a positronium atom. This happens before the photon reaches the threshold of free-pair creation. The positronium thus arising is stable against the ionizing action of the electric field near the pulsar unless it reaches a critical value about 4×107 CGSE forB1013G. This prevents the screening of the electric field up to the distances from the pulsar, where the magnetic field is already below the value of 0.1B
cr
and the free pair creation may become essential. This effect provides, at least within the Arons model, a higher theoretical estimate for the total luminosity of pulsars whose field at the surfaceB
s
exceeds 0.1B
cr
as compared with the conventional one. 相似文献
14.
The influence of the Landau-Pomeranchuk effect on the development of a shower generated by ultrarelativistic particles bombarding the surface of a pulsar is discussed. Because of this effect, the path length of the shower increases while low-energy photon generation is strongly suppressed. In view of this, the mechanism of pair production suggested by Cheng, Ruderman, and Jones for the pulsar magnetosphere, may be essential only for pulsars whose magnetic field intensity at the surface lies in a relatively narrow range of aroundB 1012 G. 相似文献
15.
Qinghuan Luo Shinpei Shibata Donald B. Melrose 《Monthly notices of the Royal Astronomical Society》2000,318(3):943-951
The viability of polar cap models for high-energy emission from millisecond pulsars is discussed. It is shown that in millisecond pulsars, polar gap acceleration along the last open field lines is radiation-reaction limited, that is, the maximum energy to which particles can be accelerated is determined by balancing the energy-loss rate (due to curvature radiation) with the gap-acceleration rate. The maximum Lorentz factor is limited by curvature radiation and is not sensitive to the specific acceleration model. However, the distance (from the polar cap) at which the Lorentz factor achieves the limit is model dependent, and can be between one-hundredth (for the vacuum gap) and above one-tenth (for the space-charge limited gap) of a stellar radius distant from the polar cap for a pulsar period P =2 ms and a surface magnetic field B =7.5104 T. Because of the radiation reaction constraint and the relatively weak magnetic field, both the expected multiplicity (number of pairs per primary particle) and the Lorentz factor of the outflowing one-dimensional magnetospheric e± plasma from the polar gap are considerably lower than those for normal pulsars. Assuming space-charge limited flow, the location of the pair production front (PPF) is estimated to occur at about one stellar radius above the polar cap, which is significantly higher than that for normal pulsars. If the observed X-ray emission originates in the region near or above the PPF, the wide hollow-cone can reproduce the observed wide double-peaked feature of the light curves without using the aligned rotator assumption. 相似文献
16.
We make a statistical analysis of the periodsP and period-derivativesP of pulsars using a model independent theory of pulsar flow in theP-P diagram. Using the available sample ofP andP values, we estimate the current of pulsars flowing unidirectionally along theP-axis, which is related to the pulsar birthrate. Because of radio luminosity selection effects, the observed pulsar sample
is biased towards lowP and highP. We allow for this by weighting each pulsar by a suitable scale factor. We obtain the number of pulsars in our galaxy to
be 6.05−2.80
+3.32 × 105 and the birthrate to be 0.048−0.011
+0.014 pulsars yr−1 galaxy−1. The quoted errors refer to 95 per cent confidence limits corresponding to fluctuations arising from sampling, but make no
allowance for other systematic and random errors which could be substantial. The birthrate estimated here is consistent with
the supernova rate. We further conclude that a large majority of pulsars make their first appearance at periods greater than
0.5 s. This ‘injection’, which runs counter to present thinking, is probably connected with the physics of pulsar radio emission.
Using a variant of our theory, where we compute the current as a function of pulsar ‘age’ (1/2P/P), we find support for the
dipole braking model of pulsar evolution upto 6 × 106 yr of age. We estimate the mean pulsar braking index to be 3.7−0.8
+0.8. 相似文献
17.
A model of Cyg X-3, as a binary cocooned star system with two sources of X-rays, one above the polar caps of the neutron star — the usual pulsar radiation — and the other around the equatorial plane of the magneto-bounding surface formed due to the interaction of the infalling plasma and the magnetic field of the neutron star, is made. The X-ray, -ray, and IR radiation light curves are considered from the shadow effect. An upper limit on the mass of the neutron star is estimated from the consideration of periodic derivative purely due to mass loss. A comparison is made with the results of Elsneret al. (1980) and Ghoshet al. (1981), which they derived from the consideration of period derivative purely from apsidal motion. 相似文献
18.
G. S. Sahakian 《Astrophysics》1997,40(4):311-319
A model of compact galactic nuclei in statistical equilibrium was developed in [L. Sh. Grigorian and G. S. Sahakian, Astrofizika (in press)]. It was shown that they should consist predominantly of neutron stars (pulsars) and white dwarfs. The problem of the energy reserves of galactic nuclei is discussed in terms of this concept. The mechanism of conversion of a white dwarf into a neutron star due to the accretion of interstellar matter is considered. This means that a galactic nucleus has an energy reserve of some 5·1060 N8 erg (N is the number of stars in the nucleus). It is shown that galactic nuclei are powerful sources of hard γ radiation [power L » 2·1044µ30N8(Ω/50)17/7 erg/sec, where µ is the magnetic moment and Ω is the angular rotation rate of a neutron star ] due to curvature radiation from relativistic electron fluxes flowing along channels of open magnetic field lines of pulsars. The x-ray and ultraviolet emission are due to synchrotron emission from the same electron fluxes in the magnetic field of the galactic nucleus (L » 1042-1044 erg/sec). The optical (visible and infrared) and radio emission are due to bremsstrahlung from electrons in the interstellar medium [L » 6·1046N 8 2 (5/Rpc)3 erg/sec, where R is the radius of the galactic nucleus]. An equation is obtained for the magnetic moment of a pulsar: µ ≈ 3.4·10-5LγP17/7, where P is the pulsar’s period and L03B3; is the luminosity of the pulsar’s y radiation. 相似文献
19.
The power law spectra of the six known-ray pulsars are seen to cut off at energies < 1 TeV so that no steady pulsed TeV component is observed from any known-ray pulsar. In some cases we show that the cutoff is steeper than exponential, which is consistent with magnetic pair production above the polar cap region. The small upper limits on the ratio of TeV energy flux to optical - GeV energy flux is again consistent with the polar cap model, but appears to be inconsistent with inverse Compton controlled outer gaps. 相似文献
20.
Yu. A. Rylov 《Astrophysics and Space Science》1981,75(2):423-436
The case of an aligned rotator magnetosphere is considered. Provided the ions ejection from the neutron stellar surface is absent, the pulsar magnetosphere consists of two polar electron caps. The upper parts of the caps are unstable. Electrons precipitate from these parts, fall onto the star and are accelerated to Lorentz-factor 106–107. Electrons radiate -quanta in the direction of the star. These -quanta are converted into electron-positron pairs. The region of size, about 10 stellar radii, around the star appears to be filled with electron-positron plasma. The inflow of electron-positron plasma interacts with the electron gas of the polar cap. For this reason longitudinal plasma vibrations arise, and bunched outflows of electron-positron plasma appear. 相似文献