Abstract
"The Duck" is a complicated nonthermal radio system, consisting of the energetic radio pulsar B1757-24, its surrounding pulsar wind nebula G5.27-0.90, and the adjacent supernova remnant (SNR) G5.4-1.2. PSR B1757-24 was originally claimed to be a young (≈15,000 yr) and extreme-velocity (≳1500 km s-1) pulsar, which had penetrated and emerged from the shell of the associated SNR G5.4-1.2; but recent upper limits on the pulsar's motion have raised serious difficulties with this interpretation. We here present 8.5 GHz interferometric observations of the nebula G5.27-0.90 over a 12 yr baseline, doubling the time span of previous measurements. These data correspondingly allow us to halve the previous upper limit on the nebula's westward motion to 14 mas yr-1 (5 σ), allowing a substantive reevaluation of this puzzling object. We rule out the possibility that the pulsar and SNR were formed from a common supernova explosion ≈15,000 yr ago, as implied by the pulsar's characteristic age, but conclude that an old (≳70,000 yr) pulsar/SNR association, or a situation in which the pulsar and SNR are physically unrelated, are both still viable explanations.
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