An Upper Limit to Circularly Polarized Radiation from the Crab Nebula

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RECENTLY a small intense source of low frequency radio emission has been detected in the Crab Nebula1,2. Subsequent investigations have revealed that this source has a flux spectral index of a α 1.2 (refs. 2 and 3), an angular diameter of 0.2 sec arc (refs. 3 and 4), and a brightness temperature of 1014 °K (ref. 4). Neither the thermal nor the synchrotron emission mechanisms can satisfactorily account for these characteristics3–5. Accordingly, a plasma mechanism, possibly similar to that of a solar flare, has been proposed4,5. It seems likely that such a mechanism would be associated with the stellar remnant of the supposed supernova which gave rise to the Crab Nebula, and a recent measurement of the position of the compact source indicates that it does indeed coincide with the position of the stellar remnant6.

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