Identification of Shock Structures

In the young remnants, even the highest quality images prior to Chandra lacked the resolution or sensitivity to isolate shock structures. Many of these structures had been elusive in other bands as well.

The most prominent example is Cas A. The radio profile consists of a bright ring, thought to be associated with the reverse shock, and an outer plateau. The plateau emission shows no evidence of limb brightening, thus leaving the location of the forward shock unknown. The inner ring is so complicated that it is not possible to determine the location of the reverse shock. The highly filamentary optical emission was not useful for identifying shock structures. The initial deep Chandra image provided the first clear location of Cas A's forward and reverse shocks [41]. The 4-6 keV continuum image revealed a series of thin wisps at radii between 140 and 165arcsec from the nominal center of the remnant. The correspondence of the outer edge of these X-ray filaments with a large jump in radio polarization indicates that they mark the previously unidentified forward shock. The reverse shock is marked by a correspondingly sharp rise of the emissivity of the X-ray line emission at 1.85 keV and the radio surface brightness, at an approximate radius of 1.8arcmin. The approximate 3:2 diameter ratio of the forward and reverse shocks provides a means of assessing the evolutionary state of Cas A. Comparison with models (e.g., [159]) indicates that mass swept-up by the shock is comparable with that of the ejecta, and that Cas A is just entering the adiabatic phase.

Chandra and XMM-Newton images have also revealed quite dramatically the structure of the forward shocks in the other young SNRs. In Tycho, portions of the forward shock had been identified optically as an arc of nonradiative Ha filaments. The Chandra image reveals similarly thin X-ray filaments [61,171]. As with Cas A, these appear most prominently in the 4-6 keV continuum band. Unlike those in Cas A, the emission is nearly continuous around the remnant, although its surface brightness does vary azimuthally by factors of several over scales of degrees. The radio rim corresponds closely with the X-ray rim, but the surface brightness of the two does not vary in any correlated way. As in Tycho, the X-ray emission from the bright NE and SW rims in SN 1006 corresponds closely with the radio and has a dramatically well-defined outer edge [96].

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