We study the rotational distortions of the vacuum dipole magnetic field in the context of geometric models of the radio emission from pulsars. We find that at low altitudes the rotation deflects the ...local direction of the magnetic field by at most an angle of the order of r super(2) sub(n), where r sub(n) = r/R sub(lc), r is the radial distance, and R sub(lc) is the light cylinder radius. To the lowest (i.e., second) order in r sub(n), this distortion is symmetric with respect to the plane containing the dipole axis and the rotation axis (, k) plane. The lowest order distortion that is asymmetric with respect to the (, k) plane is third-order in r sub(n). These results confirm the common assumption that the rotational sweepback has negligible effect on the position angle (P.A.) curve. We show, however, that the influence of the sweepback on the outer boundary of the open field line region (open volume) is a much larger effect, of the order of r super(1) sub(n) super(/2). The open volume is shifted backward with respect to the rotation direction by an angle d sub(ov) 6 0.2 sin ar super(1) sub(n) super(/2), where a is the dipole inclination with respect to the rotation axis. The associated phase shift of the pulse profile phi sub(ov) 6 0.2r super(1) sub(n) super(/2) can easily exceed the shift caused by combined effects of aberration and propagation time delays (-2r sub(n)). This strongly affects the misalignment of the center of the P.A. curve and the center of the pulse profile, thereby modifying the delay-radius relation. Contrary to intuition, the effect of sweepback dominates over other effects when emission occurs at low altitudes. For r sub(n) 3 x 10 super(-3) the shift becomes negative; i.e., the center of the P.A. curve precedes the profile center. With the sweepback effect included, the modified delay-radius relation predicts larger emission radii and is in much better agreement with the other methods of determining r sub(n).
Aims. Detection of a γ-ray source above 300 GeV is reported, confirming the unidentified source MGRO J1908+06, discovered by the Milagro collaboration at a median energy of 20 TeV. Methods. The ...source was observed during 27 h as part of the extension of the HESS Galactic plane survey to longitudes >30°. Results. HESS J1908+063 is detected at a significance level of 10.9σ with an integral flux above 1 TeV of (3.76 ± $0.29_{\rm~stat}$± $0.75_{\rm sys}$)$\times$10-12 ph cm-2 s-1, and a spectral photon index Γ = 2.10±$0.07_{\rm~stat}$± 0.2$_{\rm sys}$. The positions and fluxes of HESS J1908+063 and MGRO J1908+06 are in good agreement. Possible counterparts at other wavelengths and the origin of the γ-ray emission are discussed. The nearby unidentified GeV source, GRO J1908+0556 (GeV) which also remains unidentified and the new Fermi pulsar 0FGL J1907.5+0617, may be connected to the TeV source.
Context. Observations of very high-energy γ-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead to a ...better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) γ-rays of the well-known high-frequency-peaked BL Lac object PKS 2155–304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 ± 0.09stat ± 0.86syst) × 10-11 cm-2 s-1 above 200 GeV, or approximately $15\%$ of the Crab Nebula, and a power-law photon index of Γ = 3.53 ± 0.06stat ± 0.10syst. During the flares of July 2006, doubling timescales of ~2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE γ-ray emitting blazars. The variability amplitude characterized by the fractional rms Fvar is strongly energy-dependent and is $\propto E^{0.19\pm0.01}$. The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of ~2. Conclusions. This unique data set shows evidence of a low-level γ-ray emission state from PKS 2155–304 that possibly has a different origin than the outbursts. The discovery of the light curve lognormal behavior might be an indicator of the origin of aperiodic variability in blazars.
Aims. HESS J1745-303 is an extended, unidentified VHE (very high energy) gamma-ray source discovered using HESS in the Galactic Plane Survey. Since no obvious counterpart has previously been found in ...longer-wavelength data, the processes that power the VHE emission are not well understood. Methods. Combining the latest VHE data with recent XMM-Newton observations and a variety of source catalogs and lower-energy survey data, we attempt to match (from an energetic and positional standpoint) the various parts of the emission of HESS J1745-303 with possible candidates. Results. Though no single counterpart is found to fully explain the VHE emission, we postulate that at least a fraction of the VHE source may be explained by a supernova-remnant/molecular-cloud association and/or a high-spin-down-flux pulsar.
Rotational asymmetry of pulsar profiles Dyks, J.; Wright, G. A. E.; Demorest, P.
Monthly notices of the Royal Astronomical Society,
06/2010, Letnik:
405, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We analyse the influence of rotation on shapes of pulse profiles of fast-rotating (millisecond) pulsars. Corotation has two opposing effects: (1) the caustic enhancement of the trailing side (TS) by ...aberration and retardation (AR), which squeezes the emission into a narrower phase interval; (2) the weakening of the TS caused by the asymmetry of curvature radiation about the dipole axis. Analysis of the radii of curvature of electron trajectories in the inertial observer's frame (IOF) enables these two effects to be considered together. We demonstrate that for dipolar magnetic field lines on the TS there exists a 'caustic phase' beyond which no emission can be observed. This phase corresponds to the zero (or minimum) curvature of the IOF trajectories and maximum bunching of the emission. The maximum gradient of polarization angle (PA) in the S-shaped PA curve is also associated with the curvature minimum and occurs at exactly the same phase. The asymmetry of trajectory curvature with respect to the dipole axis affects the curvature emissivity and the efficiency of pair production, suggesting a minimum at the caustic phase. Emission over a fixed range of altitudes, as expected in millisecond pulsars, leads to broad leading profiles and sharp peaks with a cut-off phase on the TS. We apply our results to the main pulse of the 5 ms pulsar J1012+5307.
Aims. The previously unidentified very high-energy (VHE; E > 100 GeV) γ-ray source HESS J1303−631, discovered in 2004, is re-examined including new data from the H.E.S.S. Cherenkov telescope array in ...order to identify this object. Archival data from the XMM-Newton X-ray satellite and from the PMN radio survey are also examined. Methods. Detailed morphological and spectral studies of VHE γ-ray emission as well as of the XMM-Newton X-ray data are performed. Radio data from the PMN survey are used as well to construct a leptonic model of the source. The γ-ray and X-ray spectra and radio upper limit are used to construct a one zone leptonic model of the spectral energy distribution (SED). Results. Significant energy-dependent morphology of the γ-ray source is detected with high-energy emission (E > 10 TeV) positionally coincident with the pulsar PSR J1301−6305 and lower energy emission (E < 2 TeV) extending ~0.4° to the southeast of the pulsar. The spectrum of the VHE source can be described with a power-law with an exponential cut-off N0 = (5.6 ± 0.5) × 10-12 TeV-1 cm-2 s-1, Γ = 1.5 ± 0.2) and Ecut = (7.7 ± 2.2) TeV. The pulsar wind nebula (PWN) is also detected in X-rays, extending ~2−3′ from the pulsar position towards the center of the γ-ray emission region. A potential radio counterpart from the PMN survey is also discussed, showing a hint for a counterpart at the edge of the X-ray PWN trail and is taken as an upper limit in the SED. The extended X-ray PWN has an unabsorbed flux of \hbox{$F_{\rm 2{-}10~keV} \sim 1.6^{+0.2}_{-0.4}\times 10^{-13}\textrm{~erg\,cm}^{-2}\textrm{\,s}^{-1}$}F2−10 keV~1.6-0.4+0.2×10-13 erg cm-2 s-1 and is detected at a significance of 6.5σ. The SED is well described by a one zone leptonic scenario which, with its associated caveats, predicts a very low average magnetic field for this source. Conclusions. Significant energy-dependent morphology of this source, as well as the identification of an associated X-ray PWN from XMM-Newton observations enable identification of the VHE source as an evolved PWN associated to the pulsar PSR J1301−6305. This identification is supported by the one zone leptonic model, which suggests that the energetics of the γ-ray and X-ray radiation are such that they may have a similar origin in the pulsar nebula. However, the large discrepancy in emission region sizes and the low level of synchrotron radiation suggest a multi-population leptonic nature. The low implied magnetic field suggests that the PWN has undergone significant expansion. This would explain the low level of synchrotron radiation and the difficulty in detecting counterparts at lower energies, the reason this source was originally classified as a “dark” VHE γ-ray source.
Aims. Galaxy clusters are key targets in the search for ultra high energy particle accelerators. The Coma cluster represents one of the best candidates for such a search owing to its high mass, ...proximity, and the established non-thermal radio emission centred on the cluster core. Methods. The HESS (High Energy Stereoscopic System) telescopes observed Coma for ~8 h in a search for γ-ray emission at energies >1 TeV. The large 3.5° FWHM field of view of HESS is ideal for viewing a range of targets at various sizes including the Coma cluster core, the radio-relic (1253+275) and merger/infall (NGC 4839) regions to the southwest, and features greater than $1^\circ$ away. Results. No evidence for point-like nor extended TeV γ-ray emission was found and upper limits to the TeV flux $F(E)$ for $E>1$, >5, and >10 TeV were set for the Coma core and other regions. Converting these limits to an energy flux $E^2F(E)$ the lowest or most constraining is the $E>5$ TeV upper limit for the Coma core (0.2° radius) at ~8% Crab flux units or ${\sim}10^{-13}$ ph cm-2 s-1. Conclusions. The upper limits for the Coma core were compared with a prediction for the γ-ray emission from proton-proton interactions, the level of which ultimately scales with the mass of the Coma cluster. A direct constraint using our most stringent limit for $E>5$ TeV, on the total energy content in non-thermal protons with injection energy spectrum ${\propto} E^{-2.1}$ and spatial distribution following the thermal gas in the cluster, is found to be ~0.2 times the thermal energy, or ${\sim} 10^{62}$ erg. The $E>5$ TeV γ-ray threshold in this case corresponds to cosmic-ray proton energies $\ga$50 TeV. Our upper limits rule out the most optimistic theoretical models for gamma ray emission from clusters and complement radio observations which constrain the cosmic ray content in clusters at significantly lower proton energies, subject to assumptions on the magnetic field strength.
The pulse profile of pulsar B1822-09 exhibits a very peculiar kind of mode changing: a "precursor" appearing just in front of the main pulse (MP) exhibits periods of nulling, during which an ...interpulse (IP) becomes detectable at a rotation phase separated by roughly 180 degree from the precursor. We propose that this bizarre phenomenon, which requires an information transfer between the two components, occurs by means of a reversal of the direction of coherent radio emission generated in the same emission region. This interpretation naturally explains the lack of weak radio emission in the off-pulse regions, as well as the problem of information transfer between emission regions associated with the MP precursor and the IP. The reversals also imply nulling. The model has profound physical implications: (1) the mechanism of coherent radio emission must allow radiation into two, opposite, intermittently changing directions; and (2) the radio waves must be able to propagate through inner regions of the neutron star magnetosphere with strong magnetic field. Most importantly, the model implies inward radio emission in the pulsar magnetosphere.
Aims. Previous observations with the HESS telescope array revealed the existence of extended very-high-energy (VHE; E > 100 GeV) γ-ray emission, HESS J1023–575, coincident with the young stellar ...cluster Westerlund 2. At the time of discovery, the origin of the observed emission was not unambiguously identified, and follow-up observations have been performed to further investigate the nature of this γ-ray source. Methods. The Carina region towards the open cluster Westerlund 2 has been re-observed, increasing the total exposure to 45.9 h. The combined dataset includes 33 h of new data and now permits a search for energy-dependent morphology and detailed spectroscopy. Results. A new, hard spectrum VHE γ-ray source, HESS J1026–582, was discovered with a statistical significance of 7σ. It is positionally coincident with the Fermi LAT pulsar PSR J1028–5819. The positional coincidence and radio/γ-ray characteristics of the LAT pulsar favors a scenario where the TeV emission originates from a pulsar wind nebula. The nature of HESS J1023–575 is discussed in light of the deep HESS observations and recent multi-wavelength discoveries, including the Fermi LAT pulsar PSR J1022–5746 and giant molecular clouds in the region. Despite the improved VHE dataset, a clear identification of the object responsible for the VHE emission from HESS J1023–575 is not yet possible, and contribution from the nearby high-energy pulsar and/or the open cluster remains a possibility.
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