Jets and Shrapnel

Cas A has long been known to have a "jet" of fast moving knots, with an extent of more than twice the outer shock radius. This knot was discovered optically; spectroscopy suggests that the knots composing it are rich in ejecta. Proper motion studies plus spectroscopy indicate that the motion of the jet is largely perpendicular to our line-of-sight. The optical spectra of the jet knots are dominated by [S II] emission, indicating they are ejecta. The jet is not prominent in the radio band. Early high quality X-ray images suggested low surface brightness emission coincident with the jet, but lacked the resolution or surface brightness contrast to allow detailed correlation study. The initial deep Chandra image reveals low surface brightness X-ray emission from the entirety of the jet with a striking resemblance to the optical image, especially in Si, S, Ar, and Ca [65].

The presence of a counterjet on the west side of the remnant was indicated by the presence of a large number of fast moving optical knots [33]. The initial deep Chandra image showed Si streamers in that direction, but lacked the sensitivity to unequivocally relate them to a counterjet. As shown in Fig. 17.8, the narrow band Si

Fig. 17.8 The jet and counterjet extending to the east and west of Cas A, traced by the Si line emission in the million-second Chandra image [62]. The jet is revealed here via a ratio image between Si He« (1.78-2.0 keV) and 1.3-1.6 keV (Mg He«, Fe L)

map extracted using the recent million second Chandra exposure reveals the clear presence of the counterjet [62].

There is no clear explanation for the orientation of the jet. Models of jet-induced explosions generally predict a jet directed along the rotation axis of the progenitor star and a kick velocity imparted to the neutron star along that same axis (e.g., [80]). In Cas A, however, the jet axis and proper motion direction of the compact source are nearly perpendicular [157].

Fast moving ejecta knots are not found exclusively in young SNRs. One of the most spectacular results from the ROSAT all-sky survey was the discovery of half a dozen bright features well exterior to the shell of the nearby (250 kpc), evolved (<104yrs) large angular diameter (<8.3°) Vela supernova remnant (Fig. 17.9) [6]. Each feature possesses morphology either like a truncated cone (fragments A and E) or an arc opening toward the remnant center (fragments B, C, D, and F). Simple geometrical arguments indicate a common origin, near the location of the Vela pulsar. These features were interpreted as dense fragments of the exploded star, or shrapnel, that have been decelerated less than the forward shock, and are now producing bow shock nebulae and Mach cones as they interact with ambient medium beyond the remnant boundary.

Fig. 17.9 The ROSAT All Sky Survey image of the Vela SNR showing the location of the shrapnel [6]

Fig. 17.9 The ROSAT All Sky Survey image of the Vela SNR showing the location of the shrapnel [6]

Detailed followup studies have largely confirmed this interpretation. Radio observations of fragment A show nonthermal emission along its leading edge, indicating particle acceleration at the shock front. Detailed X-ray observations of fragment A have been performed using ASCA and Chandra [104,160]; XMM-Newton observations were performed on fragment D [77]. Fragment A was found to have a Si/O abundance ratio of 10 or more times the solar ratio, a mass on the order of 0.01 M©, supporting an origin in the progenitor star. It has a velocity of about 500 km s^1, requiring substantial deceleration from its original velocity of —104 km s^1. In fragment D, a complex morphology is found, suggesting the onset of a Rayleigh-Taylor instability between the fragment and an interstellar cloud with which it is colliding. The X-ray spectrum suggests that the fragment is rich in O, Ne, and Mg, but not in Fe. The different composition suggests that fragment D originated in a different layer of the progenitor from fragment A.

Similar features have been found in at least one other SNR. A Chandra image of the evolved, core-collapse remnant N63A in the LMC reveals crescent-shaped plumes exterior to the remnant proper, similar to those associated with Vela [173]. Spectral analysis indicates that the plumes are not dominated by ejecta material, suggesting that if they are of similar origin as the Vela fragments, then they have undergone substantial mixing with ambient material.

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