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

TL;DR

This study analyzes the Crab Nebula's inner knot using multiwavelength data and introduces a new time series analysis method to better understand the site of gamma-ray flares and constrain shock-model parameters.

Contribution

It provides a detailed characterization of the Crab's inner knot and introduces a novel singular value decomposition approach for analyzing time series in astrophysical imaging.

Findings

Knot size and flux are correlated with distance from the pulsar.

Standard shock models fit many observations but face puzzles regarding knot size and polarization.

New analysis reduces uncertainties in knot characterization.

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.