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Published on November 13, 2007

Author: Alien

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Particle acceleration in pulsar magnetospheres:  Particle acceleration in pulsar magnetospheres Kouichi HIROTANI MPI für Kernphysik, Heidelberg (To move to TIARA, Taiwan, from July) Torun, June 23, 2005 The seven highest-confidence g-ray pulsars:  The seven highest-confidence g-ray pulsars §2 Accelerator models (cont’d):  §2 Accelerator models (cont’d) The modulation of the GeV light curves §2 Accelerator models (cont’d):  §2 Accelerator models (cont’d) The modulation of the GeV light curves testifies to the γ-ray production position: (1) polar cap (Harding et al. 1978, ApJ 225, 226; Daugherty & Harding 1982 ApJ 252, 337; 1996 ApJ 458, 278; Sterner et al. 1995, ApJ 445, 736), (2) outer magnetosphere (Cheng, Ho, Ruderman 1986, ApJ 300, 500; 300, 522 other refs…later.), or (3) in the wind region (close to the light cylinder) (Petri & Kirk 2005, ApJL in press) §3 Accelerator models: Outer-gap model:  Various properties of high-energy emissions such as double-peak light curves with strong bridges, phase-resolved spectra of the Crab pulsar (Cheng et al. 2000, ApJ 537, 964), or the evolution of GeV photon-production efficiency (Romani 1996, ApJ 470, 469) have been explained with outer-gap models. §3 Accelerator models: Outer-gap model §3 Outer-gap Models: Problems:  §3 Outer-gap Models: Problems However… the hypothesized gap geometry does not satisfy the Maxwell equation (Hirotani, Harding, Shibata 2003). Back to solution §3 Outer-gap Models: Problems:  §3 Outer-gap Models: Problems However… the hypothesized gap geometry does not satisfy the Maxwell equation (Hirotani, Harding, Shibata 2003). Moreover, traditional outer gap models cannot explain the off-pulse emission from the Crab pulsar (Dyks & Rudak 2003). proposed the two-pole caustic model. Gap extends from star to light cylinder. Double peak arise from a crossing of two caustics, each of which is associated with a different magnetic pole. §3 Two-pole caustic model:  §3 Two-pole caustic model Crab-like pulsars: Peaks at all wavelengths are in phase! Double-peaks profiles (both poles) with Df < 1800 Off-pulse emission Observer Angle z Phase Phase Dyks, Harding, Rudak 2004 §3 Two-pole caustic model: physical basis:  §3 Two-pole caustic model: physical basis Thus, next issue is to give a physical basis for the two-pole caustic model. Does a gap extend from the star to the outer magnetosphere? By extending traditional polar-gap model into outer magnetosphere, Muslimov & Harding (2003, 2004) proposed a polar-slot gap model. §3 Polar-slot gap model:  §3 Polar-slot gap model Muslimov & Harding (2003, 2004). Outer part has non-vanishing E|| but is geometrically very thin and hence pair-production-free. §3 Polar-slot gap model:  However, polar-slot gap model (and all traditional polar-cap model) predicts a negative E|| when , which induces an opposite gap current from the global current flow patterns. §3 Polar-slot gap model §3 Polar-slot gap model:  However, polar-slot gap model (and all traditional polar-cap model) predicts a negative E|| when , which induces an opposite gap current from the global current flow patterns. Moreover, the assumption of geometrically thin gap in the outer magnetosphere has not been shown so far. If outer part of gap becomes thick, a non-negligible g-g pair production takes place. This leads to a positive E|| throughout the gap because of the strong g-B pair production in the inner-most region. another extended gap solution may be found. §3 Polar-slot gap model §3 New accelerator model:  §3 New accelerator model On these grounds, I examined the gap trans-field thickness by assuming that the gap thickness is determined so that the created current in the gap may be maximized, assuming the gap upper boundary coincides with a specific field line. To this aim, I solve the set of Maxwell & Boltzmann equations in pulsar magnetospheres on 2-D poloidal plane. (Beskin, Istomin, Par’ev 1992, Sov. Astron. 36(6), 642) (Hirotani & Shibata 1999 MNRAS 308, 54) (Hirotani, Harding, Shibata 2003 ApJ 591, 334) §4 New accelerator model:  §4 New accelerator model Let us first describe the physical processes that take part in a stationary pair-production in a gap. §3 New accelerator model:  §3 New accelerator model Three free parameters: magnetic inclination (assumed to be 45o), azimuthal gap width [rad], Df fraction of soft-photon field illuminating gap, fr Other quantities such gap gap geometry (inner, outer BDs), acceleration electric field distribution, particle density and energy spectrum, g-ray flux and energy spectrum, pair creation rate outside of the gap, are all solved by these three parameters. §3 Application to the Crab Pulsar:  §3 Application to the Crab Pulsar I applied the theory to the Crab pulsar. It is found that the gap extends from the vicinity of the N.S. to the outer magnetosphere. Cf. conventional models: §3 Crab Pulsar: acceleration field:  §3 Crab Pulsar: acceleration field Because of B expansion, E|| increases outwards. 25%, 75% 50% §3 Crab Pulsar: Predicted Spectrum:  §3 Crab Pulsar: Predicted Spectrum EGRET observations are reproduced (below 3 GeV) if fr ~ 0. That is, the gap is not illuminated by the magnetospheric soft photons. Thus, pulsation not detectable above 30 GeV. Flat spectrum below GeV by secondary and higher-generation synch. rad. §3 Crab Pulsar: Magnetization Parameter:  §3 Crab Pulsar: Magnetization Parameter Pulsar wind is heavily concentrated in the equatorial region (at the light cylinder). Because… Pairs are mainly produced in the inner-most region due to magnetic pair production. e+’s attain large Lorentz factor, while e-’s not. §3 Crab Pulsar: Magnetization Parameter:  §3 Crab Pulsar: Magnetization Parameter In-going g-rays (emitted by e-’s) materialize as pairs due to mag. pair production and cascade into higher-generation pairs. Such pairs quickly lose energy by synchrotron process to become non-relativistic, and migrate outwards by resonant scattering of surface photons, forming a pulsar wind above the gap. However, the energy of such wind above the gap is much small compared with that originates from the gap. §3 Crab Pulsar: Magnetization Parameter:  §3 Crab Pulsar: Magnetization Parameter Creation rate, dN/dt = 2.0 1038 pairs s-1. Less than the constraints that arise from magnetic dissipation in wind zone (Kirk & Skjaeraasen 2003). Pulsar wind is heavily concentrated in the equatorial region (at the light cylinder). 2D MHD wind model without reconnection Meridional distr. of s @ LC magnetization parameter 104 106 105 Summary:  Summary A stationary pair-production cascade in pulsar magnetospheres is self-consistently solved from the set of Maxwell & Boltzmann eqs. on 2-D poloidal plane. The gap extends from the vicinity of the star to outer magnetosphere above the last-open field line. If the magnetic flux tube threading the gap is constant, outer part becomes active. Trans-field thickness attains 12.5 % of open-field line flux. Crab’s pulsed spectrum 30 MeV- 10 TeV is reproduced, expect for EGRET fluxes at 8 and 20 GeV. Initial pulsar wind is strongly concentrated at the lower latitudes. lower latitudes: s ~3000, G >10 7 higher latitudes: dN/dt =2*1038 particles s-1 §1 Introduction:  §1 Introduction Pulsar Wind Pulsed (beamed) Radio Emission Pulsed (beamed) High Energy Emission Pulsar Activities Chandra HST Komissarov & Lyubarksy 2D MHD wind model:  2D MHD wind model Komissarov & Lyubarksy 2004 Formation of torus in equatorial flow initial s0~10-100 is acceptable widely accepted value s0~104 gives too thick equatorial totus (100 times larger) EGRET on board CGRO:  EGRET on board CGRO Energy range: 30 MeV to 30 GeV Technique: high-voltage gas-filled spark chambers Targets: diffuse g-ray emission, g-ray bursts, cosmic rays, pulsars, and blazars. Multi-wavelength spectra of 7 g-ray pulsars (pulsed emission):  Multi-wavelength spectra of 7 g-ray pulsars (pulsed emission) Maximum of emission in hard X-ray and g-ray energies. High-energy spectral cutoff. Cutoff energy vs. surface B field:  Cutoff energy vs. surface B field P-Pdot diagram:  P-Pdot diagram Lg vs. Lspin:  Lg vs. Lspin Light curves: total vs. hard g-rays:  Light curves: total vs. hard g-rays g-ray pulsar observability:  g-ray pulsar observability Seven highest-confidence g-ray pulsars:  Seven highest-confidence g-ray pulsars Multi-wavelength detections of high-energy pulsars:  Multi-wavelength detections of high-energy pulsars High-energy Lightcurves:  High-energy Lightcurves

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