Scargle, Jeffrey D. (1968) Activity in the Crab Nebula. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05022008-113526
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Changes in the structure shown by the continuous emission from the Crab Nebula are investigated observationally and theoretically. A beautiful series of 200-inch plates from 1955 to the present provides the data on the morphology of the activity. Near the center of the nebula is a striking series of strongly polarized, elongated features called "wisps". The wisps in this series appear to move at velocities a significant fraction of the speed of light. The first and most prominent wisp is probably a permanent feature which moves about a characteristic position. Nearby is a fainter but similar region which sometimes shows one or more wisps. Centrally placed with respect to these two regions is a very thin wisp which shows more restricted notions. Since the two active regions are symmetrically placed about it, this thin wisp is in a sense the center of the activity. The nearby star is probably not associated with the nebula. Motion can be detected as well at the edges of the nebula. At the northwest edge in particular are rapidly moving objects which may be wisps which have propagated outward from the central region. The area of the nebula intermediate between the central wisps and these features near the edge is studied with a special technique, and there are indications of motions here too. In a general way all of the motions can be understood as disturbances in the plasma composed of relativistic gas, thermal gas, and magnetic field. The main wisp may act as a piston, exciting the other wisps in the form of propagating compressional waves. The velocity of propagation is set by the density of the thermal gas, which must be on the order of [...] to produce the observed velocities. Damping of these waves by resonance phenomena may be a source of energy for the relativistic particles sufficient to restore the synchrotron losses. Photoelectric observations show that the intrinsic spectrum is probably a power law with a slight excess of ultraviolet radiation. The power law, which decreases with increasing frequency, becomes steeper away from the center of the activity. This may be caused by the evolution of the electron energy distribution or by the injection of energy where the activity is strongest.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Physics, Mathematics and Astronomy|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||8 February 1968|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||16 May 2008|
|Last Modified:||26 Dec 2012 02:39|
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