U.S.Department of Energy, NASA, and Eolas (Ireland). REFERENCES. Cawley, M.F. et al., 1991, Experimental Astronomy i, 185. Clear, J. et al., 1987, Astron.
OBSERVATIONS
OF THE CRAB NEBULA AT TEV ENERGIES
T.C.Weekes I, C.W.Akerlof 2, M.Chantell I, E.Colombo I, M.F.Cawley 3, V.Connaughton I'4, D.J.Fegan 4, S.Fennell 4, J.Gaidos s, A.M.Hillas 6, A.D.Kerrick 7, P.Kwok 7, D.A.Lewis z, R.C.Lamb z, D.I.Meyer 2, G.Mohanty z, J.Rose 6, A.Rovero I, G.Sembroski 5, M.S.Schubnell 2, M.Punch 4, M.West 6, T.Whitaker I, c.Wilson 5. I 2 3 4 5 6 7
Whipple Observatory, S.A.O., U.S.A. University of Michigan, U.S.A. St. Patrick's College, Maynooth, Ireland University College, Dublin, Ireland Purdue University, U.S.A. University of Leeds, U.K. Iowa State University, U.S.A. ABSTRACT
The results of four years of observation of the Crab Nebula at TeV energies by the Whipple group are discussed and the status of the Crab Nebula as a standard candle for TeV astronomy is reviewed. INTRODUCTION Observations with the High Resolution Camera on the I0 m reflector of the Whipple Observatory commenced in April, 1988. The camera and the mode of operation and analysis have been described elsewhere (Cawley et al. 1990; Vacanti et al. 1991). The results of a survey of more than 35 sources in the period 1987 to 1993 have been reported (Reynolds et al. 1993; Kerrick et al. 1993). A significant detection was found for only two sources, the Crab Nebula (Vacanti et al. 1991) and Markarian 421 (Punch et al. 1992). The former source will be discussed here; the latter is the subject of a companion paper at this symposium (Schubnell et al. 1993). CREDIBILITY unlike most TeV source detections the credibility of the Crab detection has increased steadily with time. The initial Whipple HRC detection was statistically significant at the 20a level using pre-determined selection criteria which were based on both the shape and orientation of the shower images (Vacanti et al. 1991). A subsequent analysis (Reynolds et ai.1993) refined the selection method and increased its sensitivity by a factor of 1.75; this selection was then applied to three subsequent years of observation (1989-92) leading to an "a priori" detection of 31.5~ (Punch, 1993). The actual numbers involved are shown in Table i. It should be noted that: I) the signal rate is high (about 1 per minute) 2) all but 0.37% of the background has been rejected 3) the selected "signal" events are approximately 60% gamma rays. 270
9 1994 American Institute of Physics
T. C. Weekes et aL Table Observation
1988-89 #ON #OFF Difference Excess (o) 1,808 minutes 1989-92 #ON #OFF Difference Excess (a) 2,872 minutes
271
1.
Raw Shape 489,426 14,218 493,434 11,216 +4,992 +3,002 +5.0 +18.8 R a t e (raw) 273/min
Summary Orientation Combined 44,099 4,452 40,413 1,766 +3,686 +2,686 +12.7 +34.1 Rate (gamma r a y s ) 1.49/min
559,019 15,213 554,755 12,659 +4,264 +2,554 +4.0 +15.3 Rate (raw) 193/min
47,217 4,470 43,696 1,945 +3,521 +2,525 +11.7 +31.5 Rate (gamma rays) 0.88/min
M o r e c o n v i n c i n g t h a n the s t a t i s t i c s of a single e x p e r i m e n t using a new technique is the c o n f i r m a t i o n by i n d e p e n d e n t experiments. This has come from four experiments using s o m e w h a t d i f f e r e n t v e r s i o n s of the C h e r e n k o v t e c h n i q u e and all r e p o r t i n g s i g n a l s at m o r e than the 5a level (Table 2). It is also n o t e w o r t h y t h a t over the range from 0.2 to i0 T e V t h e r e is no e x p e r i m e n t w i t h the r e q u i r e d s e n s i t i v i t y t h a t has failed to see the Crab Nebula. It w o u l d thus a p p e a r t h a t the Crab N e b u l a is a g o o d c a n d i d a t e for b e c o m i n g the s t a n d a r d c a n d l e of T e V g a m m a - r a y a s t r o n o m y as it is for m a n y other astronomies. To be thus r e g a r d e d it m u s t be clear that (a) the a b s o l u t e e n e r g y and flux levels are w e l l - e s t a b l i s h e d and (b) t h a t it is truly a steady source; these considerations will be i n v e s t i g a t e d in the next two sections.
Ground-based
Energy (TeV) 0.2
Flux (xl0 -12 cm-2s-1) 170
0.4 0.6
70 27
2
15 4.4
3
T a b l e 2. Observations of
Level (a) 5.8 45.5 5.7
5 8
the
Crab
Group
Nebula
Technique
atmospheric cherenkov Gamma* timing/imaging Whipple imaging ASGAT timing HEGRA imaging Themistocle wavefront timing air shower array
i0 30 40 75 190
