MODELING OF GAMMA-RAY PULSAR LIGHT CURVES USING THE FORCE-FREE MAGNETIC FIELD

By Xue-Ning Bai, A. Spitkovsky. 2009

187 citations recorded by the source corpus.

Gamma-ray emission from pulsars has long been modeled using a vacuum dipole field. This approximation ignores changes in the field structure caused by the magnetospheric plasma and strong plasma currents. We present the first results of gamma-ray pulsar light-curve modeling using the more realistic field taken from three-dimensional force-free (FF) magnetospheric simulations. Having the geometry of the field, we apply several prescriptions for the location of the emission zone, comparing the light curves to observations. We find that when the emission region is chosen according to the conventional slot-gap (or two-pole caustic) prescription, the model fails to produce double-peak pulse profiles, mainly because the size of the polar cap in the FF magnetosphere is larger than the vacuum field polar cap. This suppresses caustic formation in the inner magnetosphere. The conventional outer-gap model is capable of producing only one peak under general conditions because a large fraction of open field lines does not cross the null charge surface. We propose a novel “separatrix layer” model, where the high-energy emission originates from a thin layer on the open field lines just inside of the separatrix that bounds the open flux tube. The emission from this layer generates two strong caustics on the sky map due to the effect we term “Sky Map Stagnation” (SMS). It is related to the fact that the FF field asymptotically approaches the field of a rotating split monopole, and the photons emitted on such field lines in the outer magnetosphere arrive to the observer in phase. The double-peak light curve is a natural consequence of SMS. We show that most features of the currently available gamma-ray pulsar light curves can be reasonably well reproduced and explained with the separatrix layer model using the FF field. Association of the emission region with the current sheet will guide more detailed future studies of the magnetospheric acceleration physics.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/bce2679983a3d3186f11377f25b4ebde12dbb3d8

How researchers see this paper

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In Astronomy and Astrophysics

1. A. Spitkovsky
2. B. Metzger
3. E. Quataert
4. J. Arons
5. A. Philippov
6. A. N. Timokhin
7. T. Thompson
8. B. Cerutti

In Nuclear and High Energy Physics

1. A. Spitkovsky
2. A. Philippov
3. B. Metzger
4. B. Cerutti
5. E. Quataert
6. D. Giannios
7. J. Arons
8. T. Thompson

Citations in the corpus

• TOWARD A REALISTIC PULSAR MAGNETOSPHERE (2011, methodology): “There are also kinematic models of pulsar emission derived by injecting photons along the magnetic field lines of either the FFE magnetosphere (Bai & Spitkovsky 2010b) or those of the vacuum field geometry (Bai & Spitkovsky 2010a), or along the electric current near the equatorial current sheet…”
• Time-dependent 3D magnetohydrodynamic pulsar magnetospheres: oblique rotators (2012, background): “†E-mail: atchekho@princeton.edu for powering the observed γ -ray emission (e.g. Bai & Spitkovsky 2010b, 2010a; Pétri 2012b; Uzdensky & Spitkovsky 2012; Arka & Dubus 2013).”
• Particle acceleration in axisymmetric pulsar current sheets (2014, result): “Our results suggest that the current sheet, and the separatrix layers, should be intense sources of high-energy radiation in pulsars (Lyubarskii 1996; Kirk et al. 2002; Bai & Spitkovsky 2010; Pétri 2012a; Arka & Dubus 2013; Uzdensky & Spitkovsky 2014).”
• RESISTIVE SOLUTIONS FOR PULSAR MAGNETOSPHERES (2011, methodology): “4 in Bai & Spitkovsky 2010).”
• The Proto-Magnetar Model for Gamma-Ray Bursts (2010, background): “…opening angle of the polar cap, corrected by a factor (1 + sin2 χ)1/2 to account for the larger cap size of an oblique rotator (e.g. Cheng et al. 2000; Bai & Spitkovsky 2010); Rns is the NS radius; RY is the ‘Y’ point radius where the close zone ends in the magnetic equatorial plane; and the…”
• Gamma-ray binaries and related systems (2013, background): “Numerical simulations of pulsar magnetospheres support the idea that part of the gamma-ray emission arises outside the light cylinder (Bai & Spitkovsky 2010).”
• Time-dependent pair cascades in magnetospheres of neutron stars - I. Dynamics of the polar cap cascade with no particle supply from the neutron star surface (2010, methodology): “Recently the force-free pulsar magnetosphere model has been studied in great detail (e.g. Contopoulos, Kazanas & Fendt 1999; Gruzinov 2005; Timokhin 2006, 2007a; Spitkovsky 2006; Kalapotharakos & Contopoulos 2009; Bai & Spitkovsky 2010).”
• Modelling high-energy pulsar light curves from first principles (2015, background): “Working directly with the force-free fields gives new insight into the formation of gamma-ray lightcurves by tracing potential emitting field lines (Bai & Spitkovsky 2010a), or using resistive force-free fields and test particles (Kalapotharakos et al. 2012, 2014).”
• Current flow and pair creation at low altitude in rotation-powered pulsars’ force-free magnetospheres: space charge limited flow (2012, background): “Moreover, modelling of pulsar gamma-ray light curves suggests that the gamma-ray emission originates from the regions close to the boundary between the open and closed magnetic filed lines (e.g. Bai & Spitkovsky 2010).”
• Pulsar Wind Nebulae as Cosmic Pevatrons: A Current Sheet’s Tale (2012, background): “• The model specifies the polar flux tube size and shape—for i = 0, it is noncircular with a polar cap center displaced from the magnetic axis, even when the magnetic field is the simplest, that of a star centered, point dipole (Bai and Spitkovsky 2010).”