The Polar Regions of Cassiopeia A: The Aftermath of a Gamma-Ray Burst?

摘要

Are the polar regions of Cas A the aftermath of a gamma-ray burst? Probably not, but it is interesting nevertheless to investigate just how close Cas A might have come to generating such an event. Focusing on the northeast jet filaments, we analyze the polar regions of the recently acquired very deep 1 Ms Chandra X-ray observation. We infer that the so-called "jet" regions are indeed due to jets emanating from the explosion center and not due to polar cavities in the circumstellar medium at the time of explosion. We place limits on the equivalent isotropic explosion energy in the polar regions (around 2.3 × 1052 ergs) and the opening angle of the X-ray-emitting ejecta (around 7°), which give a total energy in the northeast jet of the order of 1050 ergs, an order of magnitude or more lower than inferred for "typical" GRBs. While the Cas A progenitor and explosion exhibit many of the features associated with GRB hosts, e.g., extensive presupernova mass loss and rotation and jets associated with the explosion, we speculate that the recoil of the compact central object, with a velocity of 330 km s-1, may have rendered the jet unstable. In such cases the jet rapidly becomes baryon loaded, if not truncated altogether. Although unlikely to have produced a gamma-ray burst, the jets in Cas A suggest that such outflows may be common features of core-collapse SNe.