Supernova Remnants
Pulsars and Their Wind Nebulae
Pulsars are the most extreme form of matter short of black holes. Their ultra-high densities, rapid rotations, and strong magnetic fields carry imprints of their formation processes in core-collapse SN. As they age pulsars spin down, depositing vast amounts of energy into their surroundings through relativistic winds and, in some cases, jets. Key elements of their evolutionary history are revealed by these wind nebulae and the properties of the ejecta they sweep up.
Chandra image of 3C 58 with the Constellation-X calorimeter field of view (white) and two 15" × 15" extraction regions indicated.
Simulated spectra of the entire region of 3C 58 (white) and the Ne IX (helium-like) K line complex from the two smaller regions (yellow and cyan, corresponding to the colored extraction regions). The two-component spectrum observed with Chandra is assumed, with a power law accompanied by a thermal spectrum assumed to originate in a shell. The expansion velocity of the shell is assumed to be 900 km s-1, as observed in optical filaments. The Doppler-shifted features from the front and back shells are clearly separated, and the variation in the projected velocity with radius is also evident as a smaller separation between the lines in the western-most (cyan).
Of particular importance are "naked" pulsar wind nebulae (PWNe) like the Crab Nebula and 3C 58, which exhibit little or no evidence of ejecta or material swept up by the SNR blast wave. This lack of observed thermal emission implies that they are expanding into extremely rarefied surroundings. However, ejecta must have formed in the explosion. Interestingly the size of the PWN (9.3 pc × 5.6 pc assuming a distance of 3.2 kpc) requires average expansion velocities of >= 10,000 km s-1, while the highest velocities measured are of order 900 km s-1. Identification of the higher-velocity gas is crucial to understand how this nebula has evolved. Based on Chandra observations, a 100-ks Constellation-X observation of the western region of 3C 58 will yield a surface brightness of 7 × 10-3 cts ks-1 arcsec-2 in the Ne IX triplet. With Constellation-X's goal of 2-eV spectral resolution, velocities as low as ~ 600 km s-1 can be resolved which can easily be measured with roughly 100 counts in the line. This will allow the ejecta expansion to be measured on 10"-15" scales. Using line ratios, the temperature and ionization state of the gas can also be determined and the density estimated. These measurements will establish the expansion profile of the ejecta, and provide constraints on the wind of the progenitor star.
