Abstract
Average radio pulse profile of a pulsar B in a double pulsar system PSR J0737−3039A/B exhibits an interesting behaviour. During the observation period between 2003 and 2009, the profile ...evolves from a single-peaked to a double-peaked form, following disappearance in 2008, indicating that the geodetic precession of the pulsar is a possible origin of such behaviour. The known pulsar beam models can be used to determine the geometry of PSR J0737−3039B in the context of the precession. We study how the fan-beam geometry performs in explaining the observed variations of the radio profile morphology. It is shown that the fan beam can successfully reproduce the observed evolution of the pulse width, and should be considered as a serious alternative for the conal-like models.
The known population of pulsars contains objects with four- and five-component profiles, for which the peak-to-peak separations between the inner and outer components can be measured. These Q- and ...M-type profiles can be interpreted as a result of sightline cut through a nested-cone beam, or through a set of azimuthal fan beams. We show that the ratio R... of the components' separations provides a useful measure of the beam shape, which is mostly independent of parameters that determine the beam scale and complicate interpretation of simpler profiles. In particular, the method does not depend on the emission altitude and the dipole tilt distribution. The different structures of the radio beam imply manifestly different statistical distributions of R..., with the conal model being several orders of magnitude less consistent with data than the fan-beam model. To bring the conal model into consistency with data, strong effects of observational selection need to be called for, with 80 per cent of Q and M profiles assumed to be undetected because of intrinsic blending effects. It is concluded that the statistical properties of Q and M profiles are more consistent with the fan-shaped beams, than with the traditional nested-cone geometry. (ProQuest: ... denotes formulae/symbols omitted.)
We present the influence of the special relativistic effects of aberration and light-travel time delay on pulsar high-energy light curves and polarization characteristics predicted by three models: ...the two-pole caustic model, the outer gap model, and the polar cap model. Position angle curves and degree of polarization are calculated for the models and compared with the optical data on the Crab pulsar. The relative positions of peaks in gamma-ray and radio light curves are discussed in detail for the models. We find that the two-pole caustic model can qualitatively reproduce the optical polarization characteristics of the Crab pulsar: fast swings of the position angle and minima in polarization degree, associated with both peaks. The anticorrelation between the observed flux and the polarization degree (observed in the optical band also for B0656+14) naturally results from the caustic nature of the peaks, which are produced in the model because of the superposition of radiation from many different altitudes, i.e. polarized at different angles. The two-pole caustic model also provides an acceptable interpretation of the main features in the Crab's radio profile. Neither the outer gap model nor the polar cap model is able to reproduce the optical polarization data on the Crab. Although the outer gap model is very successful in reproducing the relative positions of gamma-ray and radio peaks in pulse profiles, it can reproduce the high-energy light curves only when photon emission from regions very close to the light cylinder is included.
The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has – together with the PAMELA detection of a rise in the positron fraction up to ...≈100 GeV – motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H.E.S.S. starting at 340 GeV. While the overall electron flux measured by H.E.S.S. is consistent with the ATIC data within statistical and systematic errors, the H.E.S.S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H.E.S.S. data follow a power-law spectrum with spectral index of 3.0±0.1(stat.)± 0.3(syst.), which steepens at about 1 TeV.
High-quality integrated radio profiles of some pulsars contain bifurcated, highly symmetric emission components (BECs). They are observed when our line of sight traverses through a split-fan shaped ...emission beam. It is shown that for oblique cuts through such a beam, the features appear asymmetric at nearly all frequencies, except for a single 'frequency of symmetry'νsym, at which both peaks in the BEC have the same height. Around νsym, the ratio of flux in the two peaks of a BEC evolves in a way resembling the multifrequency behaviour of J1012+5307. Because of the inherent asymmetry resulting from the oblique traverse of the sightline, each minimum in double notches can be modelled independently. Such a composed model reproduces the double notches of B1929+10 if the fitted function is the microscopic beam of curvature radiation in the orthogonal polarization mode. These results confirm our view that some of the double components in radio pulsar profiles directly reveal the microscopic nature of the emitted radiation beam as the microbeam of the curvature radiation polarized orthogonally to the trajectory of electrons.
The bifurcated emission component (BEC) in the radio profile of the millisecond pulsar J1012+5307 can be interpreted as the signature of the curvature radiation beam polarized orthogonally to the ...plane of electron trajectory. Since the beam is intrinsically narrow (∼1°), the associated emission region must be small for the observed BEC to avoid smearing out by spatial convolution. We estimate whether the energy available in the stream is sufficient to produce such a bright feature in the averaged profile. The energy considerations become complicated by the angular constraints imposed by the width of the microbeam, and by the specific spectrum of the BEC which is found to have the spectral index ξBEC −0.9 in comparison to the index of ξ −2 for the total profile. For typical parameters, the luminosity of the BEC is determined to be 4 × 1025 erg s−1, whereas the maximum-possible beam-size-limited power of the stream is L
ΔΦ 2 × 1029 erg s−1. This implies the minimum energy-conversion efficiency of ηΔΦ 2 × 10−4. The BEC's luminosity does not exceed any absolute limits of energetics, in particular, it is smaller than the power of primary electron and/or secondary plasma stream. However, the implied efficiency of energy transfer into the radio band is extreme if the coherently emitting charge-separated plasma density is limited to the Goldreich-Julian value. This suggests that the bifurcated shape of the BEC has macroscopic origin; however, several uncertainties (e.g. the dipole inclination and spectral shape) make this conclusion not firm.
The geometry of a radio pulsar beam Dyks, J.
Monthly notices of the Royal Astronomical Society. Letters,
10/2017, Letnik:
471, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract Taxonomy of radio pulsar profiles is mostly based on a system of Ptolemaic artificiality, consisting of separated rings and a core, arbitrarily located at disparate altitudes in the ...magnetosphere. Diversity of observed profile shapes clearly exceeds the interpretive capability of such conal model. Moreover, bifurcated features observed in pulsar profiles imply a system of fan beams radially extending away from the dipole axis. The bifurcations can be understood as the imprint of the elementary radiation pattern of the long-sought radio emission mechanism, thus identifying the latter. Their size, however, is several times larger than implied by the curvature of magnetic dipole lines. Here, I show that the illusion of disconnected rings and the size of bifurcated features can be explained through a natural geometry that combines the properties of both the cone and the fan beam. It is a flaring spiral that makes several revolutions around the dipole axis on its way to leave the magnetosphere. Such geometry is consistent with a stream of outflowing and laterally drifting plasma. The bifurcated components are so wide because the curvature on such a spiral is larger than that of the dipolar magnetic field, and hence they are consistent with the extraordinary mode curvature radiation.
Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV γ-ray band, coupled with those at millimetre radio wavelengths, are motivated by the search for cosmic-ray ...accelerators in our Galaxy. The old-age mixed-morphology SNR W 28 (distance ~2 kpc) is a prime target due to its interaction with molecular clouds along its northeastern boundary and other clouds situated nearby. Methods. We observed the W 28 field (for ~40 h) at very high energy (VHE) γ-ray energies ($E > 0.1$ TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET $E > 100$ MeV data was also undertaken. Results from the NANTEN 4 m telescope Galactic plane survey and other CO observations were used to study molecular clouds. Results. We have discovered VHE γ-ray emission (HESS J1801-233) coincident with the northeastern boundary of W 28 and a complex of sources (HESS J1800-240A, B and C) ~0.5° south of W 28 in the Galactic disc. The EGRET source (GRO J1801-2320) is centred on HESS J1801-233 but may also be related to HESS J1800-240 given the large EGRET point spread function. The VHE differential photon spectra are well fit by pure power laws with indices Γ ~2.3 to 2.7. The spectral indices of HESS J1800-240A, B, and C are consistent within statistical errors. All VHE sources are ~10′ in intrinsic radius except for HESS J1800-240C, which appears pointlike. The NANTEN 12CO($J = 1{-}0$) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15 km s-1 range in local standard of rest velocity. Conclusions. The VHE/molecular cloud association could indicate a hadronic origin for HESS J1801-233 and HESS J1800-240, and several cloud components in projection may contribute to the VHE emission. The clouds have components covering a broad velocity range encompassing the distance estimates for W 28 (~2 kpc) and extending up to ~4 kpc. Assuming hadronic origin and distances of 2 and 4 kpc for cloud components, the required cosmic-ray density enhancement factors (with respect to the solar value) are in the range ~10 to ~30. If situated at 2 kpc distance, such cosmic-ray densities may be supplied by SNRs like W 28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII region W 28A2, and the HII regions M 8 and M 20, along with their associated open clusters. Further sub-mm observations would be recommended to probe in detail the dynamics of the molecular clouds at velocites > 10 km s-1 and their possible connection to W 28.
Aims. Recent theoretical predictions of the lowest very high energy (VHE) luminosity of SN 1006 are only a factor 5 below the previously published HESS upper limit, thus motivating further in-depth ...observations of this source. Methods. Deep observations at VHE energies (above 100 GeV) were carried out with the high energy stereoscopic system (HESS) of Cherenkov Telescopes from 2003 to 2008. More than 100 h of data have been collected and subjected to an improved analysis procedure. Results. Observations resulted in the detection of VHE γ-rays from SN 1006. The measured γ-ray spectrum is compatible with a power-law, the flux is of the order of 1% of that detected from the Crab Nebula, and is thus consistent with the previously established HESS upper limit. The source exhibits a bipolar morphology, which is strongly correlated with non-thermal X-rays. Conclusions. Because the thickness of the VHE-shell is compatible with emission from a thin rim, particle acceleration in shock waves is likely to be the origin of the γ-ray signal. The measured flux level can be accounted for by inverse Compton emission, but a mixed scenario that includes leptonic and hadronic components and takes into account the ambient matter density inferred from observations also leads to a satisfactory description of the multi-wavelength spectrum.
Aims. Results obtained in very-high-energy (VHE; E ≥ 100 GeV) γ-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity ...of the young massive stellar cluster Westerlund 1 (Wd 1). Methods. Imaging of Cherenkov light from γ-ray induced particle cascades in the Earth’s atmosphere is used to search for VHE γ rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. Results. The detection of the degree-scale extended VHE γ-ray source HESS J1646–458 is reported based on 45 h of H.E.S.S. observations performed between 2004 and 2008. The VHE γ-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20σ. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of Γ = 2.19 ± 0.08stat ± 0.20sys and a flux normalisation at 1 TeV of Φ0 = (9.0 ± 1.4stat ± 1.8sys) × 10-12 TeV-1 cm-2 s-1. The integral flux above 0.2 TeV amounts to (5.2 ± 0.9) × 10-11 cm-2 s-1. Conclusions. Four objects coincident with HESS J1646–458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2 − 455216, the X-ray binary 4U 1642–45, the pulsar PSR J1648–4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648–4611 could be viable to explain parts of the VHE γ-ray emission of HESS J1646–458.
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