Characterization of the Inner Knot of the Crab: The Site of the Gamma-ray Flares?

Abstract

One of the most intriguing results from the gamma-ray instruments in orbit has been the detection of powerful flares from the Crab Nebula. These flares challenge our understanding of pulsar wind nebulae and models for particle acceleration. We report on the portion of a multiwavelength campaign using Keck, HST, and Chandra concentrating on a small emitting region, the Crab's inner knot, located a fraction of an arcsecond from the pulsar. We find that the knot's radial size, tangential size, peak flux, and the ratio of the flux to that of the pulsar are correlated with the projected distance of the knot from the pulsar. A new approach, using singular value decomposition for analyzing time series of images, was introduced yielding results consistent with the more traditional methods while some uncertainties were substantially reduced. We exploit the characterization of the knot to discuss constraints on standard shock-model parameters that may be inferred from our observations assuming the inner knot lies near to the shocked surface. These include inferences as to wind magnetization, shock shape parameters such as incident angle and poloidal radius of curvature, as well as the IR/optical emitting particle enthalpy fraction. We find that while the standard shock model gives good agreement with observation in many respects, there remain two puzzles: (a) The observed angular size of the knot relative to the pulsar--knot separation is much smaller than expected; (b) The variable, yet high degree of polarization reported is difficult to reconcile with a highly relativistic downstream flow.

46 pages, 14 figures, submitted to the Astrophysical Journal

References in corpus (6)

• The Hubble Ultra Deep Field
• Discovery of Powerful Gamma-Ray Flares from the Crab Nebula
• Fitting Pulsar Wind Tori. II. Error Analysis and Applications
• Three-dimensional Analytical Description of Magnetised Winds from Oblique Pulsars
• Optical and infrared observations of the Crab Pulsar and its nearby knot
• A statistical model for the $\gamma$-ray variability of the Crab nebula

Cited by in corpus (21)

• Magnetoluminescence
• Multi-D magnetohydrodynamic modelling of pulsar wind nebulae: recent progress and open questions
• Kinetic study of radiation-reaction-limited particle acceleration during the relaxation of unstable force-free equilibria
• X-Ray Polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR)
• Interpreting Crab Nebula synchrotron spectrum: two acceleration mechanisms
• Particle acceleration in explosive relativistic reconnection events and Crab Nebula gamma-ray flares
• Particle acceleration in explosive relativistic reconnection events and Crab Nebula gamma-ray flares
• Crab flares due to turbulent dissipation of the pulsar striped wind
• A recent change in the optical and γ-ray polarization of the Crab nebula and pulsar
• Reconnection and particle acceleration in interacting flux ropes I. Magnetohydrodynamics and test particles in 2.5D
• The imprint of pulsar parameters on the morphology of Pulsar Wind Nebulae
• Modelling Jets, Tori and Flares in Pulsar Wind Nebulae
• The inner knot of the Crab nebula
• Detection of Small Flares from the Crab Nebula with Fermi-LAT
• Correcting Imaging Atmospheric Cherenkov Telescope data with atmospheric profiles obtained with an elastic light detecting and ranging system
• On the Implications of Recent Observations of the Inner Knot in the Crab Nebula
• Phenomenological modelling of the Crab Nebula's broad band energy spectrum and its apparent extension
• The optical and near-infrared spectrum of the Crab pulsar with X-shooter
• A systematic study of $\gamma$-ray flares from the Crab Nebula with Fermi-LAT: I. flare detection
• Fermi Large Area Telescope Observations of the Fast-dimming Crab Nebula in 60-600 MeV
• Multi-wavelength Polarimetry of Isolated Neutron Stars