共查询到20条相似文献，搜索用时 31 毫秒
1.
Xiao-Peng You Jin-lin Han National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2006,6(2):237-246
We update the systematic studies of circular polarization in integrated pulse profiles by Han et al. Data of circular polarization profiles are compiled. Sense reversals can occur in core or cone components, or near the intersection between components. The correlation between the sense of circular polarization and the sense of position angle variation for conal-double pulsars is confirmed with a much large database. Circular polarization of some pulsars has clear changes with frequency. Circular polarization of millisecond pulsars is marginally different from that of normal pulsars. 相似文献
2.
Mateusz Ruszkowski 《Astrophysics and Space Science》2003,288(1-2):133-142
I discuss the transfer of polarized synchrotron radiation in relativistic jets. I argue that the main mechanism responsible for the circular polarization properties of compact synchrotron sources is likely to be Faraday conversion and that, contrary to common expectation, a significant rate of Faraday rotation does not necessarily imply strong depolarization. The long-term persistence of the sign of circular polarization, observed in some sources, is most likely due to a small net magnetic flux generated in the central engine, carried along the jet axis and superimposed on a highly turbulent magnetic field. I show that the mean levels of circular and linear polarizations depend on the number of field reversals along the line of sight and that the gradient in Faraday rotation across turbulent regions can lead to`correlation depolarization'. The model is potentially applicable to a wide range of synchrotron sources. In particular, I demonstrate how the model can naturally explain the excess of circular over linear polarization in the Galactic Center (SgrA*) and the low-luminosity AGN M81*. 相似文献
3.
We report the detection and analysis of circular polarization in solar type III radio storms at hectometric-to-kilometric
wavelengths. We find that a small (usually less than 5%), but statistically significant, degree of circular polarization is
present in all interplanetary type III radio storms below 1 MHz. The sense of the polarization, which is right-hand circular
for some storms and left-hand circular for others, is maintained for the entire duration of the type III storm (usually many
days). For a given storm, the degree of circular polarization peaks near central meridian crossing of the associated active
region. At a given time, the degree of circular polarization is found to generally vary as the logarithm of the observing
frequency. The radiation characteristics, including the polarization, for one interplanetary type III storm exhibits an unusual
1.6 hour oscillation. Based on the standard plasma emission theory of type III radiation, we discuss the implications of these
observations for the magnitude and radial dependence of the solar magnetic field above active regions on the Sun. 相似文献
4.
D. B. Melrose 《Astrophysics and Space Science》1971,12(1):172-192
Formulas describing synchrotron radiation are extended to include the effect of the presence of an ambient medium and the effect of reabsorption and Faraday rotation on the degree of circular polarization. Results are: (1) The onset of Razin-Tsytovich suppression has no significant effect on the degree of circular polarization. (2) The onset of self-absorption in a source subject to weak Faraday rotation (negligible Faraday rotation in an absorption length) causes the degree of circular polarization to reverse its sign and to decrease (by between one half and one quarter for typical parameters) in magnitude. (3) As in (2) but for strong Faraday rotation (many rotations of the plane of polarization in an absorption length) the degree of circular polarization also reverses its sign and becomes slightly smaller in magnitude than for weak Faraday rotation.The transfer equation including the effects of the polarization is discussed in detail. 相似文献
5.
Guo-Jun Qiao Xu-Dong Wang Hong-Guang Wang Ren-Xin XuDepartment of Astronomy CAS-PKU Joint Beijing Astrophysics Center Peking University Beijing 《中国天文和天体物理学报》2002,2(4)
Pulsar radio emission beams have been studied observationally for a long time, and the suggestion is that they consist of the so-called core and conal components. To reproduce these components is a challenge for any emission model, and that the pulse profile of pulsars changes with frequency presents even a greater challenge. Assuming a local surface magnetic structure (to produce the core or central beam) and a global dipole magnetic field (to produce the conal beams), Gil & Krawczyk (1997) applied curvature radiation to the pulse profile simulation of PSR J0437-4715 (hereafter the GK model). Here we present an alternative multi-frequency simulation of the same profiles within the framework of the Inverse Compton Scattering (ICS) model. It is obtained from our simulation (1) that besides the core, the inner cone and the outer cone, there is an outer-outer cone; (2) that the emission components of the core and cones evolve strongly with frequency. Some important differences between the ICS model and the 相似文献
6.
D. J. Saikia G. F. Holmes A. R. Kulkarni C. J. Salter & S. T. Garrington 《Monthly notices of the Royal Astronomical Society》1998,298(3):877-887
We present total-intensity and linear-polarization observations at a single epoch for a sample of 11 quasars and one BL Lac object. The data were taken with the VLA A array at λλ 20, 18, 6 and 2 cm. We examine the variation of the degree of polarization, p , and polarization position angle, PA, with wavelength, and attempt to determine the rotation measure, RM, of the cores in these sources. The degree of polarization does not exhibit any systematic variation with wavelength, the median values ranging from 2.3 to 3.5 per cent at the different wavelengths. The variation of PA with λ2 is not linear over the entire wavelength range. However, for most sources the λλ 20-, 18- and 6-cm PAs do follow such a linear relationship, yielding a median |RM| of about 15 rad m−2 . In contrast, the λλ 6- and 2-cm observations give a median |RM| of about 129 rad m−2 . The long-wavelength emission is likely to originate from a spatially different part of the milliarcsec-scale jet from the λ 2-cm emission, which could turn over at a higher frequency and is likely to be more compact and located closer to the quasar nucleus. We have attempted to obtain linear fits over the entire wavelength range allowing for n π ambiguities in the PAs, but the fits are not statistically significant. The low values of RM for these core-dominated sources suggest that either the radio emission from the jet intercepts few of the emission-line clouds and their confining medium, or the clouds have a small filling factor and are possibly magnetically confined. 相似文献
7.
Dipanjan Mitra Joanna M. Rankin 《Monthly notices of the Royal Astronomical Society》2008,385(2):606-613
New Giant Metre-Wave Radio Telescope (GMRT) observations of the five-component pulsar B1857−26 provide detailed insight into its pulse-sequence modulation phenomena for the first time. The outer conal components exhibit a 7.4-rotation period, longitude-stationary modulation. Several lines of evidence indicate a carousel circulation time of about 147 stellar rotations, characteristic of a pattern with 20 beamlets. The pulsar nulls some 20 per cent of the time, usually for only a single pulse, and these nulls show no discernible order or periodicity. Finally, the pulsar's polarization-angle traverse raises interesting issues: if most of its emission comprises a single polarization mode, the full traverse exceeds 180°; or if both polarization modes are present, then the leading and the trailing halves of the profiles exhibit two different modes. In either case, the rotating-vector model fails to fit the polarization-angle traverse of the core component. 相似文献
8.
Simon Johnston Joel M. Weisberg 《Monthly notices of the Royal Astronomical Society》2006,368(4):1856-1870
We present polarization profiles at 1.4 and 3.1 GHz for 14 young pulsars with characteristic ages less than 75 kyr. Careful calibration ensures that the absolute position angle of the linearly polarized radiation at the pulsar is obtained. In combination with previously published data, we draw three main conclusions about the pulse profiles of young pulsars. (i) Pulse profiles are simple and consist of either one or two prominent components. (ii) The linearly polarized fraction is nearly always in excess of 70 per cent. (iii) In profiles with two components, the trailing component nearly always dominates, only the trailing component shows circular polarization and the position angle swing is generally flat across the leading component and steep across the trailing component.
Based on these results, we can make the following generalizations about the emission beams of young pulsars. (i) There is a single, relatively wide cone of emission from near the last open field lines. (ii) Core emission is absent or rather weak. (iii) The height of the emission is between 1 and 10 per cent of the light cylinder radius. 相似文献
Based on these results, we can make the following generalizations about the emission beams of young pulsars. (i) There is a single, relatively wide cone of emission from near the last open field lines. (ii) Core emission is absent or rather weak. (iii) The height of the emission is between 1 and 10 per cent of the light cylinder radius. 相似文献
9.
Spatial scans with a resolution of 3.4 arc sec of the broad-band circular polarization of several sunspots have been made in five filter bands over the wavelength range 0.4–1.7µ with a sensitivity of 1 × 10–6 fractional polarization. The scans, across a spot through the penumbra and umbra center, revealed two important features: (1) The broad-band circularly polarized fluxV reverses in sign, or diminishes to near zero, at the center of the umbral region relative to the outer penumbra. This effect was wavelength dependent and was most clearly detected as a definite reversal in a band at 1.2µ, although a reversal was also detected in a very broad band extending from 0.8 to 1.6µ. (2) There is a marked asymmetry: in all cases the limbward penumbral region exhibited strongerV values than did the disk-center (inward) side of the spot, at all observed wavelengths. Such previously unreported structure in the magnetic circular polarization of sunspots provides new clues for understanding the anomalous large broad-band polarization at short wavelengths and at the same time imposes new constraints on sunspot models. For example, the polarization reversal in the umbra relative to the penumbra can be naively explained by return-flux sunspot models; but this is not the only interpretation. Alternatively, it can relate to reversals in mass-flow velocities and/or vertical velocity gradients, as between the umbra and penumbra. 相似文献
10.
High resolution studies of circular polarization allow us see where it arises in a jet, study its local fractional level and spectrum, and compare these results to local measures of linear polarization and Faraday rotation. Here we not only review past results from Very Long Baseline Array (VLBA) circular polarization studies, but we also present preliminary new results on two quasars. In the core of PKS 0607–157, we find strong circular polarization at 8 GHz and much weaker levels at 15 GHz. Combined with the linear polarization data, we favor a simple model where the circular is produced by Faraday conversion driven by a small amount of Faradayrotation. In the core of 3C 345, we find strong circular polarization at 15 GHz in a component with distinct linear polarization. This core component is optically thick at 8 GHz, where we detect no circular polarization. With opposite trends in frequency for PKS 0607–157 and 3C 345, it seems clear that local conditions in a jet can have a strong effect on circular polarization and need to be taken into account when studying inhomogeneous objects with multi-frequency observations. 相似文献
11.
K. Krzeszowski D. Mitra Y. Gupta J. Kijak J. Gil A. Acharyya 《Monthly notices of the Royal Astronomical Society》2009,393(4):1617-1624
The magnetospheric locations of pulsar radio emission region are not well known. The actual form of the so-called radius-to-frequency mapping should be reflected in the aberration–retardation (A/R) effects that shift and/or delay the photons depending on the emission height in the magnetosphere. Recent studies suggest that in a handful of pulsars the A/R effect can be discerned with respect to the peak of the central core emission region. To verify these effects in an ensemble of pulsars, we launched a project analysing multifrequency total intensity pulsar profiles obtained from the new observations from the Giant Meterwave Radio Telescope (GMRT), Arecibo Observatory (AO) and archival European Pulsar Network (EPN) data. For all these profiles, we measure the shift of the outer cone components with respect to the core component, which is necessary for establishing the A/R effect. Within our sample of 23 pulsars, seven show the A/R effects, 12 of them (doubtful cases) show a tendency towards this effect, while the remaining four are obvious counterexamples. The counterexamples and doubtful cases may arise from uncertainties in the determination of the location of the meridional plane and/or the core emission component. Hence, it appears that the A/R effects are likely to operate in most pulsars from our sample. We conclude that in cases where those effects are present the core emission has to originate below the conal emission region. 相似文献
12.
VLBI total intensity and linear polarization images of the BL Lacertae object OJ 287 have been obtained at using a global ground array and the HALCA orbiting antenna, and at two weeks earlier using the VLBA. In the ground-based 6-cm images, the source is dominated by a core–jet double structure the components of which are essentially unresolved. The baselines to the orbiting antenna resolve both of these compact components. In the VSOP images, the ground-based 'core' breaks up into several distinct components, demonstrating that this region is dominated by the contribution of bright, optically thin knots of jet emission. A very similar structure is observed in the 1.3-cm image. The magnetic field in the core is transverse, becomes longitudinal in the inner jet, then makes a sharp transition to a region of transverse field further from the core. This suggests that the field in the outer jet has become highly ordered in the transverse direction owing to the action of a shock; the physical nature of the extended region of longitudinal field closer to the core is not clear. The availability of nearly simultaneous observations with comparable resolution at widely spaced frequencies enabled detection of a ≃90° rotation in polarization position angle for the core, owing to the transition from the optically thick (6 cm) to the optically thin (1.3 cm) regime. 相似文献
13.
Polarimetric observations of 300 pulsars have been conducted with the 76-m Lovell telescope at Jodrell Bank at radio frequencies centred around 230, 400, 600, 920, 1400 and 1600 MHz. More than half of the pulsars have no previously published polarization profiles and this compilation represents about three times the sum of all previously published pulsar polarization data. A selection of integrated polarization profiles is provided. Tables of pulse widths and the degree of both linear and circular polarization are given for all pulsars, and these act as an index for all the data, which are available by anonymous ftp in numerical and graphical form. 相似文献
14.
Integrating 26 624 pairs of video frames, the authors have mapped the circular polarization in an active-region filament against
the solar disk by using a traditional magnetograph working at the Hβ line. This filament, offset the disk center, appeared
at the boundary of three decayed active regions. It was quiet and away from any strong enhanced network. The mapped circular
polarization in the filament has an average polarization degree of 1.1×10−3 with a measurement precision of 4×10−4. The mapping of circular polarization in a filament may provide a supplementary diagnosis of the filament magnetic field,
in addition to the mapping of linear polarization via the Hanle effect. However, the interpretation of the circular polarization
requires treatment of the full quantum problem of Zeeman and non-Zeeman effects of Stokes line profiles. 相似文献
15.
Comet Hale-Bopp was observed with the 2.6-m and 1.25-m telescopes of the Crimean Astrophysical Observatory on March 9 and
11, 1997. We determined the linear and circular polarization in the coma. For the dust coma, the linear polarization varied
from 7.8% to 12.4%. The degree of circular polarization was always negative and did not exceed 0.3% with an accuracy of ±0.04%
on average. The passage of a bright star through the cometary coma was monitored with polarimetry and photometry. The wavelength
dependence and spatial variations of optical thickness of dust are obtained from the stellar occultation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
We present the results of studying the impact linear polarization of 32 solar flares of X-ray classes C, M, and X (two flares) observed with the Large Solar Vacuum Telescope. It has turned out that there is evidence for impact polarization only in 13 of them. The newly obtained data have confirmed that the linear Stokes parameters are predominantly 2–7%, while the spatial sizes of flaring points with nonzero Stokes parameters are small (1″-2″). Two features of the manifestation of impact polarization in flares revealed by these studies are of greatest interest: (1) at the two foot points of a single flare loop or an arcade of loops, both the Hα intensity profiles and the Stokes profiles differ in behavior; (2) based on the Hα line, we have found for the first time that the sign of the Stokes parameters changes not only across the flare ribbon but also with depth of the chromosphere. 相似文献
17.
F. Argüeso J. L. Sanz D. Herranz M. López-Caniego J. González-Nuevo 《Monthly notices of the Royal Astronomical Society》2009,395(2):649-656
Given a set of images, whose pixel values can be considered as the components of a vector, it is interesting to estimate the modulus of such a vector in some localized areas corresponding to a compact signal. For instance, the detection/estimation of a polarized signal in compact sources immersed in a background is relevant in some fields like astrophysics. We develop two different techniques, one based on the Neyman–Pearson lemma, the Neyman–Pearson filter (NPF), and another based on pre-filtering before fusion, the filtered fusion (FF), to deal with the problem of detection of the source and estimation of the polarization given two or three images corresponding to the different components of polarization (two for linear polarization, three including circular polarization). For the case of linear polarization, we have performed numerical simulations on two-dimensional patches to test these filters following two different approaches (a blind and a non-blind detection), considering extragalactic point sources immersed in cosmic microwave background (CMB) and non-stationary noise with the conditions of the 70 GHz Planck channel. The FF outperforms the NPF, especially for low fluxes. We can detect with the FF extragalactic sources in a high noise zone with fluxes Jy for (blind/non-blind) detection and in a low noise zone with fluxes Jy for (blind/non-blind) detection with low errors in the estimated flux and position. 相似文献
18.
Egidio Landi Degl'innocenti 《Solar physics》1982,79(2):291-322
A method is presented to measure the magnetic field vector in prominences by means of the polarimetric observations in the D3 line of He obtained with the High Altitude Observatory Stokes polarimeter. The characteristics of the observed Stokes profiles are discussed. The theory of the Hanle effect is reformulated in the representation of the irreducible tensors of the density matrix, and is generalized to derive the circular polarization profiles across the spectral line in terms of the intensity and direction of the prominence magnetic field. The circular polarization profile so deduced can be employed to obtain useful information which adds to that carried by the linear polarization observations. A non-linear least-squares algorithm is proposed to derive the measurement of the magnetic field from the observations, and a consistency check is suggested to test the adequacy of the theoretical model to describe the physics of the He I atomic excitation in prominences.On leave from: Astrophysical Observatory of Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
19.
We explore the detailed polarization behaviour of pulsar 0823 + 26 using the technique of constructing partial ‘mode-separated’
profiles corresponding to the primary and secondary polarization modes. The characteristics of the two polarization modes
in this pulsar are particularly interesting, both because they are anything but orthogonal and because the secondary mode
exhibits a structure seen neither in the primary mode nor in the total profile. The new leading and trailing features in the
secondary mode, which appear to represent a conal component pair, are interpreted geometrically on the basis of their width
and the associated polarization-angle traverse as an outer cone.
If the secondary-mode features are, indeed, an outer cone, then questions about the significance of the pulsar’s postcursor
component become more pressing. It seems that 0823 + 26 has a very nearly equatorial geometry, in that both magnetic poles
and the sightline all fall close to the rotational equator of the star. We thus associate the postcursor component with emission
along those bundles of field lines which are also equatorial and which continue to have a tangent in the direction of our
sight line for a significant portion of the star’s rotation cycle. It seems that in all pulsars with postcursor components,
this emission follows the core component, and all may thus have equatorial emission geometries. No pulsars with precursors
in this sense — including the Crab pulsar — are known.
The distribution of power between the primary and secondary modes is very similar at both 430 and 1400 MHz. Our analysis shows
that in this pulsar considerable depolarization must be occurring on time scales that are short compared to the time resolution
of our observations, which is here some 0.5–1.0 milliseconds. One of the most interesting features of the modeseparated partial
profiles is a phase offset between the primary and secondary modes. The secondary-mode ‘main pulse’ arrives some 1.5 ± 0.1‡
before the primary-mode one at 430 MHz and some 1.3 +0.1 ‡ at 21 cm. Given that the polar cap has an angular diameter of 3.36‡,
we consider whether this is a geometric effect or an effect of differential propagation of the two modes in the inner magnetosphere
of the pulsar. 相似文献
20.
S. Suzuki 《Solar physics》1978,57(2):415-422
The projected source positions at 43, 80, and 160 MHz and the sense and degree of circular polarization in the range 24 to 220 MHz, as observed with the Culgoora radioheliograph and spectropolarimeter respectively, are used:
- To substantiate the hypothesis that metric U bursts originate in high coronal, magnetic loops.
- To strengthen the hypothesis that U-burst radiation is in the ordinary magneto-ionic mode.