WFPC2 Studies of the Crab Nebula. I. HST and ROSAT Imaging of the Synchrotron Nebula

Abstract

We present images of the Crab synchrotron nebula obtained with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. These data are compared with ROSAT HRI images, and with 0':5 resolution Canada-France-Hawaii Telescope (CFHT) images previously published by van den Bergh & Pritchet (1989). These data strengthen the emerging picture of the Crab as a cylindrically symmetrical object with an axis running southeast to northwest, and inclined by ~20°-30° with respect to the plane of the sky. Identification of structure very near to the pulsar which shares this symmetry helps to better establish the link between the symmetry axis of the nebula and the spin axis of the pulsar. We report the discovery of a bright knot of visible emission located 0':65 to the southeast of the pulsar, along the axis of the system. This knot and a second knot 3':8 from the pulsar appear to be present but not well resolved in the 1988 CFHT image, indicating that they are persistent structures. The inner knot is interpreted as a shock in the pulsar wind ~ 1500 AU above the pole of the pulsar. No corresponding knots are seen to the northwest of the pulsar, which may indicate that the characteristics of the wind from the two poles are not symmetrical. The closest of the "wisps" to the northwest of the pulsar appear to close into a ringlike "halo" encircling the axis of the nebula. The wisps are resolved, with widths of ~ 0':2. This allows• calculation of their volumes and volume emissivities, and in turn their equipartition fields and pressures. Equipartition pressures calculated for the knots and wisps are typically 10 to as much as 80 times the equipartition pressure calculated for the nebula as a whole. The wisps show significant substructure which changed considerably between 1988 and 1994. Previous reports of relativistic motions of the wisps were probably due to changes in the unresolved substructure of these features. Comparison of the CFHT and WFPC2 images show remarkable changes in the inner nebula, but inferences about physical conditions based on this comparison are limited by the resolution of the CFHT data and the long 5 year baseline between the images. The structure of the nebula in 1994 may be inconsistent with the recent model by Gallant & Arons (1994). Very fine fibrous texture visible in the WFPC2 image follows the structure of the X-ray torus. A puzzling anticorrelation is seen between the X-ray and visible surface brightness through part of the torus. Long contiguous low contrast features with widths of ~ 1"-2" are seen to run throughout the volume of the nebula. These features are seen to move outward through the nebula at velocities in excess of homologous expansion. These features trace the magnetic structure of the nebula; they are probably due to differences in emissivity accompanying varying degrees of departure from equipartition at roughly constant total pressure. Visible fibers "drapt< over" and appear to expand away from an X-ray counterjet to the northwest of the pulsar, supporting th...

Properties of Nonthermal Emission in Plerions

“…The line a', parallel to a, indicates the case of no spectral bending between optical frequencies and X rays (a:opt == a:ox). Pixels with oox < 1 outline rather well the region of the X-ray "torus" (see, e.g., Hester et al 1995); in the plot, they are located on the lower left side, in agreement with our expectation that this is the main location in which to find freshly injected particles.…”

“…Figure 2-R gives a map of aox for all of the points confined to the two lobes (brighter pixels indicate higher values of aox); a softening of the spectrum in the outer zones is apparent here. In the lobes, the zones with harder spectra seem to lie on the prolongation of the X-ray "jets" (see Hester et al 1995). This is very clear for the S-E lobe and may be an indication that these particles are directly provided by the jets.…”

“…As already pointed out by Hester et al (1995), the outer and faint X-ray emission extends almost to the boundary of the optical nebula, so we have performed a quantitative analysis over a large area, by using 5" x 5" pixels. Figure 1 gives a plot of the optical-to-X-ray averaged spectral index (a:ox) versus the optical spectral index (a:opt), where each dot refers to a single pixel; unfortunately, no high-resolution spectral map in X-rays is available yet.…”

Constraints on particle acceleration sites in the Crab nebula from relativistic magnetohydrodynamic simulations

“…In the last decade, thanks to the unprecedented imaging power of Chandra, high resolution observations of the inner region of the CN have become available also in the X-ray band, revealing a striking jet-torus structure, knots and rings (Weisskopf et al 2000), previously just barely inferred (Brinkmann et al 1985;Hester 1995). These observations prompted a theoretical effort at modelling PWNe, and the CN in particular, within the framework of time-dependent multi-D MHD.…”

Pulsar Winds

“…The "Faint Wisp" to the Southeast was first discovered in van den Bergh & Pritchett's picture. So far, nothing has been reported on its variability, although it does appear in more recent HST images obtained by Hester et al (1995). Since these variable features fall right in the region which might be the particle acceleration zone, understanding what they are telling us might be a promising path to unraveling the physics of how this compact object energizes its surroundings.…”