The Crab Nebula was first detected with a 37 pixel
camera on the Whipple Observatory 10 m optical reflector in 1989. This
early and somewhat crude instrument yielded a 9
detection with some 60
hours of integration on the source
(Weekes et al. 1989).
The detection relied on discrimination of the
-ray images
from the much more numerous hadron background images. The possibility of
a systematic effect with a new, and not yet proven, technique could not
be completely discounted (although numerous tests were made for
consistency). Nonetheless, it required the independent confirmation of
the detection by other groups using different versions of the technique
to really convince skeptics. The Whipple group subsequently detected the
source at the 20
level using an upgraded
camera (109 pixels)
(Vacanti et al. 1991)
and now routinely detects the source at the 5-6
level in an hour of
observation. The detected photon rate (about 2 per minute) is more than
that registered by EGRET at its optimum energy (100 MeV).
Since then, the Crab has been detected by eight independent groups using different versions of the atmospheric Cerenkov imaging technique (including one group in the Southern Hemisphere). Some of these detections are listed in Table 2. The energy spectrum is now well determined at energies between 300 GeV and 50 TeV (Hillas et al. 1998; Tanimori et al. 1998b). To date there have been no positive detections reported by air shower array experiments using particle detectors (which operate at somewhat higher energies) (Ong 1998).
| Group | VHE Spectrum (10-11 photons cm-2 s-1) |
Eth (TeV) |
| Whipple (1991) a... | [25(E / 0.4 TeV)]-2.4±0.3 | 0.4 |
| Whipple (1998) b... | (3.2 ± 0.7)(E / TeV)-2.49 ± 0.06stat ± 0.05syst | 0.3 |
| HEGRA (1999) c... | (2.7 ± 0.2 ± 0.8)(E / TeV)-2.61 ± 0.06stat ± 0.10syst | 0.5 |
| CAT (1998) d... | (2.7 ± 0.17 ± 0.40)(E / TeV)-2.57 ± 0.14stat ± 0.08syst | 0.25 |
| a Vacanti et al. 1991.
b Hillas et al. 1998. c A. Konopelko 1999, private communication. d M. Punch 1999, private communication. | ||
The simple Compton-synchrotron model (Gould 1965) has been updated to take account of a better understanding of the nebula (de Jager & Harding 1992; Hillas et al. 1998); the measured flux is in good agreement with the predicted flux for a value of magnetic field [(1.6 ± 0.1) × 10-4 G] that is slightly lower than the equipartition value (Fig. 2). Although this model is certainly simplistic given the structure now seen in optical images of the nebula, it shows that there is a viable mechanism that must work at some level.
 |
Figure 2. The VHE spectral energy distribution of the Crab Nebula compared with the predictions of a synchrotron self-Compton emission model (Hillas et al. 1998). |
As in many other bands of the electromagnetic spectrum, the Crab Nebula
has become the standard candle for TeV
-ray
astronomy. Most importantly perhaps, it is available as a steady source
to test and calibrate the ACIT and can be seen from both
hemispheres. Improvements in analysis techniques developed on
Crab Nebula data have led directly to the detections
of the AGNs discussed below.