RADIO-TO-TeV PHASE-RESOLVED EMISSION FROM THE CRAB PULSAR: THE ANNULAR GAP MODEL

Abstract

The Crab pulsar is a quite young famous pulsar which radiates multi-wavelength pulsed photons. The latest detection of GeV and TeV pulsed emission with unprecedented signal-to-noise ratio, supplied by the powerful telescopes: Fermi, MAGIC and VERITAS, challenges the current popular pulsar models, which can be a valuable discriminator to justify the pulsar high-energy-emission models.Our work is divided into two steps. First of all, taking reasonable parameters (the magnetic inclination angle α = 45 • and the view angle ζ = 63 • ), we use the latest high-energy data to calculate radio, X-ray, γ-ray and TeV light curves from a geometric view to obtain some crucial information on emission locations. Secondly, we calculate the phase-averaged spectrum and phase-resolved spectra for the Crab pulsar and take a theoretical justification from a physical view for the emission properies as found in the first step. It is found that a Gaussian emissivity distribution with the peak emission near the null charge surface in the so-called annular gap region gives the best modeled light curves. The pulsed emission of radio, X-ray, γ-ray and TeV are mainly generated from the emission of primary particles or secondary particles with different emission mechanisms in the nearly similar region of the annular gap located in the only one magnetic pole, which leads to the nearly "phasealigned" multi-wavelength light curves. The emission of peak 1 (P1) and peak 2 (P2) is originated from the annular gap region near the null charge surface, while the emission of bridge is mainly originated from the core gap region.The charged particles cannot corotate with the pulsar and escape from the magnetosphere, which determines the original flowing primary particles. The acceleration electric field and potential in the annular gap and core gap are huge enough in the several tens of neutron star radii. Thus the primary particles are accelerated to ultrarelativistic energies, and produce numerous secondary particles (pairs) in the inner region of the annular gap and core gap. We emphasize that there are mainly two types of pairs, i.e., one is curvature-radiation induced (CR-induced), and the other is inverse-Compton-scattering induced (ICS-induced). The phase-averaged spectrum and phase-resolved spectra from soft X-ray to TeV band are produced by four components: synchrotron radiation from CR-induced and ICS-induced pairs dominates the X-ray band to soft γ-ray band (100 eV to 10 MeV); curvature radiation and synchrotron radiation from the primary particles mainly contribute to γ-ray band (10 MeV to ∼ 20 GeV); ICS from the pairs significantly contributes to the TeV γ-ray band (∼ 20 GeV to 400 GeV).The multi-wavelength pulsed emission from the Crab pulsar can be well modeled with the annular gap and core gap model. To distinguish our single magnetic pole model from two-pole models, the convincing values of the magnetic inclination angle and the viewing angle will play a key role.

Phase-resolved energy spectra of the Crab pulsar in the range of 50–400 GeV measured with the MAGIC telescopes

“…Other possible Ansätze to explain the VHE emission include the production of inverse Compton radiation in the unshocked pulsar wind outside the light cylinder by pulsed photons (Aharonian et al 2012;Aharonian & Bogovalov 2003), a striped pulsar wind (Pétri 2011), or the annular gap model presented in Du et al (2012). The two crucial spectral features to establish to test these models are the expected spectral upward-kink in the transition region between the curvature and the hard component, and the detection or exclusion of a terminal cutoff at a few hundred GeV.…”

Constraining Lorentz Invariance Violation Using the Crab Pulsar Emission Observed up to TeV Energies by MAGIC

“…Generally, the origin of VHE pulsar emission up to TeV energies is still debated in the literature (Aharonian et al 2012;Du et al 2012;Hirotani 2013), albeit it is out of the question that inverse Compton scattering must be at play in some way or another (Hirotani 2001;Aleksić et al 2011;Lyutikov et al 2012;Bogovalov 2014). Energy-dependent time drifts of the pulse can then follow those of the illuminating electron population, or the illuminated seed UV and X-ray photons, although this is less plausibly because of the involved Klein-Nishina scattering regime.…”

Gamma-ray pulsars: A gold mine

“…This subexponential cut-off can be conceptually explained by the blending in space of curvature radiation produced with different spectra throughout the caustic region [52], or by the blending in time of curvature radiation produced in outer gaps threaded by variable currents [50]. In the Crab case, the highest-energy photons correspond to an additional radiation component [53], produced for instance by secondary and tertiary generations of pairs near the light cylinder, up-scattering the IR-UV synchrotron photons of the cascades [51].Paradoxically, the young pulsars become more efficient at producing γ rays as they slow down and lose less rotational power (see the orange and red data in Figure 5). γ-ray luminosities, Lγ, have been derived from the observed energy flux (Sγ) when a distance estimate (D) is available.…”