Kinematics of Pulsar Beams

Kapoor, R. C.; Shukre, C. S.

doi:10.1086/305804

Cited by 16 publications

(12 citation statements)

References 24 publications

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“…The outer gap width across magnetic field lines is determined by computing the location of the pair formation surface. From Kapoor & Shukre (1998), the polar angle θ c corresponding to the magnetic field line tangent to the light cylinder is:…”

Section: Gap Widthmentioning

confidence: 99%

Constrainingγ-ray pulsar gap models with a simulated pulsar population

Pierbattista

Grenier

Harding

et al. 2012

A&A

111

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With the large sample of young γ-ray pulsars discovered by the Fermi Large Area Telescope (LAT), population synthesis has become a powerful tool for comparing their collective properties with model predictions. We synthesised a pulsar population based on a radio emission model and four γ-ray gap models (Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic). Applying γ-ray and radio visibility criteria, we normalise the simulation to the number of detected radio pulsars by a select group of ten radio surveys. The luminosity and the wide beams from the outer gaps can easily account for the number of Fermi detections in 2 years of observations. The wide slot-gap beam requires an increase by a factor of ∼10 of the predicted luminosity to produce a reasonable number of γ-ray pulsars. Such large increases in the luminosity may be accommodated by implementing offset polar caps. The narrow polar-cap beams contribute at most only a handful of LAT pulsars. Using standard distributions in birth location and pulsar spin-down power (Ė), we skew the initial magnetic field and period distributions in a an attempt to account for the highĖ Fermi pulsars. While we compromise the agreement between simulated and detected distributions of radio pulsars, the simulations fail to reproduce the LAT findings: all models under-predict the number of LAT pulsars with highĖ, and they cannot explain the high probability of detecting both the radio and γ-ray beams at highĖ. The beaming factor remains close to 1.0 over 4 decades inĖ evolution for the slot gap whereas it significantly decreases with increasing age for the outer gaps. The evolution of the enhanced slot-gap luminosity withĖ is compatible with the large dispersion of γ-ray luminosity seen in the LAT data. The stronger evolution predicted for the outer gap, which is linked to the polar cap heating by the return current, is apparently not supported by the LAT data. The LAT sample of γ-ray pulsars therefore provides a fresh perspective on the early evolution of the luminosity and beam width of the γ-ray emission from young pulsars, calling for thin and more luminous gaps.

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Section: Gap Widthmentioning

confidence: 99%

Constrainingγ-ray pulsar gap models with a simulated pulsar population

Pierbattista

Grenier

Harding

et al. 2012

A&A

111

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“…For an oblique magnetic dipole rotator, the polar angle c for which the last open field line is tangent to the light cylinder is given by (Kapoor & Shukre 1998) …”

Section: Magnetospheric Geometrymentioning

confidence: 99%

Gamma‐Ray Luminosity and Death Lines of Pulsars with Outer Gaps

Zhang¹,

Cheng²,

Jiang³

et al. 2004

ApJ

147

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We reexamine the outer-gap size by taking the geometry of the dipole magnetic field into account. Furthermore, we also consider that instead of taking the gap size at half of the light cylinder radius to represent the entire outer gap, it is more appropriate to average the entire outer-gap size over the distance. When these two factors are considered, the derived outer-gap size f ðP; B; hriðÞÞ is a function not only of the period P and magnetic field B of the neutron star but also of the average radial distance to the neutron star, hri, which depends on the magnetic inclination angle . We use this new outer-gap model to study the -ray luminosity of pulsars, which is given by L ¼ f 3 ðP; B; hriðÞÞL sd , where L sd is the pulsar spin-down power, and to study the death lines of -ray emission of the pulsars. Our model can predict the -ray luminosity of an individual pulsar if its P, B, and are known. Since different pulsars have different , this explains why some -ray pulsars have very similar P and B but very different -ray luminosities. In determining the death line of -ray pulsars, we have used a new criterion based on a concrete physical property, i.e., that the fractional size of the outer gap at the null-charge surface for a given pulsar cannot be larger than unity. In an estimate of the fractional size of the outer gap, two possible X-ray fields are considered: (1) X-rays produced by neutron star cooling and polar-cap heating, and (2) X-rays produced by the bombardment of relativistic particles from the outer gap onto the stellar surface (the outer gap is called a ''self-sustained outer gap''). Since it is very difficult to measure in general, we use a Monte Carlo method to simulate the properties of -ray pulsars in our Galaxy. We find that this new outer-gap model predicts many more weak -ray pulsars, which have a typical age between 0.3 and 3 Myr, than does the old model. For all simulated -ray pulsars with self-sustained outer gaps, the -ray luminosity L satisfies L / L sd , where the value of depends on the sensitivity of the -ray detector. For EGRET, $ 0:38, whereas $ 0:46 for GLAST. For -ray pulsars with L sd P L crit sd , $ 1, and L crit sd ¼ 1:5 Â 10 34 P 1=3 ergs s À1 is determined by f ðhri $ r L Þ ¼ 1. These results are roughly consistent with the observed luminosity of -ray pulsars. These predictions are very different from those of the previous outer-gap model, which predicts a very flat relation between L and L sd .

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“…Recently Kapoor & Shukre (2002) considered the aberration effect and the rotational sweepback to investigate the relative locations of core and cone components in the pulsar magnetosphere. Although included in the model, the sweepback is again estimated with the help of equation (1).…”

Section: Introductionmentioning

confidence: 99%

Rotational Sweepback of Magnetic Field Lines in Geometric Models of Pulsar Radio Emission

Dyks

Harding

2004

ApJ

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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 2 n , where r n ¼ r=R lc , r is the radial distance, and R lc is the light cylinder radius. To the lowest (i.e., second) order in r n , this distortion is symmetric with respect to the plane containing the dipole axis and the rotation axis [ 6; m ð Þplane]. The lowest order distortion that is asymmetric with respect to the (6; m) plane is third-order in r 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 1=2n . The open volume is shifted backward with respect to the rotation direction by an angle ov $ 0:2 sin r 1=2 n , where is the dipole inclination with respect to the rotation axis. The associated phase shift of the pulse profile Á ov $ 0:2r 1=2 n can easily exceed the shift caused by combined effects of aberration and propagation time delays (%2r 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 n P 3 ; 10 À3 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 n .

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Kinematics of Pulsar Beams

Cited by 16 publications

References 24 publications

Constrainingγ-ray pulsar gap models with a simulated pulsar population

Constrainingγ-ray pulsar gap models with a simulated pulsar population

Gamma‐Ray Luminosity and Death Lines of Pulsars with Outer Gaps

Rotational Sweepback of Magnetic Field Lines in Geometric Models of Pulsar Radio Emission

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