Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pasau..11...82j&link_type=abstract
Astronomical Society of Australia, Proceedings, vol. 11, no. 1, p. 82
Astronomy and Astrophysics
Astrophysics
Cosmic Rays, Gamma Ray Spectra, Gamma Rays, Radiation Sources, Supernova Remnants, Astrophysics, Gamma Ray Astronomy, Spectrum Analysis
Scientific paper
The bright gamma-ray source Geminga has puzzled astronomers for twenty years, because of its lack of emission outside the gamma-ray regime. Recently, the nature of this object has finally been established with the discovery of a 237 ms pulsar in soft X-ray data (Halpern and Holt, 1992, Nature, 357, 222). The measurement of pulsations between 1975 and 1992 yields a steady spin-down consistent with that of an isolated pulsar, with a characteristic age of tau = 3.7 x 105 years (Bignami and Caraveo, 1992, Nature, 357, 287). From the observed gamma-ray flux, the absolute upper limit on the distance is put at R less than 380 pc, but a Vela-like gamma-ray efficiency reduces the estimate to more like R approximately 40 pc. This suggests that Geminga is one of the closest pulsars known and the existence of this neutron star implies that a supernova exploded a few 105 years ago. This explains the current soft X-ray background (Cox and Anderson, 1982, Astrophys. J., 253, 268), and it has also recently been suggested that this nearby cataclysm may be responsible for the local bubble in the Interstellar Medium (Gehrels and Chen, 1993, Nature, 361, 706; Hajivassiliou, 1992, Nature, 355, 232). Young supernova remnants are thought to be capable of efficiently accelerating cosmic rays up to 105 GeV (Lagage and Cesarsky, 1983, Astron. Astrophys., 125, 249), and if this is the case, the contribution to the ambient cosmic ray intensity from the Geminga remnant must be very important, given its proximity to Earth. Here, we assume acceleration and subsequent diffusion from the remnant, and find that both the expected cosmic ray flux and its anisotropy are close to those observed just below the 'knee' in the cosmic ray spectrum. The conventional view, of course, is that cosmic rays permeate the whole of the Galaxy, but there is evidence to suggest that the cosmic rays we obseve are of relatively local origin. This has been used (Streitmatter et al., 1985, Astron. Astrophys., 143, 249) to suggest that cosmic rays are trapped by a local magnetic supershell, and no evidence from composition or anisotropy measurements was found which precluded the production and trapping of cosmic rays in such a region.
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