3.2. Supernova Remnants: Plerions
The Crab Nebula is a somewhat unique object, and hence one could not confidently predict what other supernova remnants might be detectable. The Crab is a member of that subclass of supernova remnants known as plerions, in which a bubble of relativistic particles is powered by a central pulsar. No other plerions have been seen by telescopes in the Northern Hemisphere (Reynolds et al. 1993) (Table 3). However, two have been detected in the Southern Hemisphere by the CANGAROO group. They first reported a detection of PSR 1706-44 at TeV energies in 1993 (Kifune et al. 1995) based on 60 hours of observation in the summer of 1992. PSR 1706-44 is identified with a pulsar (of period 102 ms) and appears to be associated with a supernova remnant, possibly a plerion. At GeV energies it has a very flat spectrum. The energy spectrum is hard with a flux above 1 TeV of about 0.15 × 10-11 cm-2 s-1. There is no evidence that the signal is periodic. The detection has been confirmed by the University of Durham group working in Narrabri, Australia (Chadwick et al. 1997).
(10-11 cm-2 s-1)
|Crab Nebula...||400||7.0||Whipple, ASGAT, HEGRA, TA, Crimea, Gamma*, CANGAROO, CAT|
|PSR 1706-44...||1000||0.8||CANGAROO, Durham|
|SS 433...||550||< 1.8||Whipple|
|3C 58...||550||< 1.1||Whipple|
|PSR 0656+14...||1000||< 3.4||Whipple|
The CANGAROO group have also reported the detection of a 6 signal from the vicinity of the Vela pulsar (Yoshikoshi et al. 1997). The integral -ray flux above 2.5 TeV is 2.5 × 10-12 photons cm-2 s-1. Again, there is no evidence for periodicity, and the flux limit is about a factor of 10 less than the steady flux. The signal is offset (by 0°.14) from the pulsar position, which makes it more likely that the source is a synchrotron nebula. Since this offset position is coincident with the birthplace of the pulsar, it is suggested that the progenitor electrons are relics of the initial supernova explosion and that they have survived because the magnetic field was weak.