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Astron. Astrophys. 337, 25-30 (1998)

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2. The CANGAROO imaging atmospheric erenkov telescope

The CANGAROO 3.8 m imaging telescope is located near Woomera, South Australia (longitude [FORMULA]E, latitude [FORMULA]S, 160 m a.s.l). The reflector is a single 3.8 m diameter parabolic dish with f/1.0 optics. The imaging camera consists of a square-packed array of 10mm [FORMULA] 10mm Hamamatsu R2248 photomultiplier tubes. The camera originally contained 224 photomultiplier tubes, and was increased to 256 pixels (16 [FORMULA] 16 square array) in May 1995. The tube centers are separated by [FORMULA], giving a total field of view (side-side) of [FORMULA]. The photo-cathode of each tube subtends [FORMULA], giving a photo-cathode coverage of about 40% of the field of view. For a more detailed description of the 3.8 m telescope see Hara et al. 1993.

In the current configuration an event trigger is generated when a sufficiently large number of tubes (3[FORMULA]5) exceed their discriminator threshold. Individual tube discriminator levels are believed to be around 4 photo-electrons. Under these triggering conditions the current gamma-ray energy threshold of the 3.8 m telescope is estimated to be [FORMULA]1.5 TeV. Prior to mirror re-coating in November 1996 (which includes all data presented in this paper) the energy threshold was somewhat higher. Using Monte Carlo simulations we estimate that this energy threshold (as defined by the peak of the differential energy spectrum) was 2.5 TeV. When calculating integral fluxes and flux limits we define our threshold as 2 TeV. This figure is obtained by re-binning the lower energy gamma-rays to produce an integral spectrum defined by a single power law with a sharp cutoff at the threshold energy.

Since starting observations in 1991 the CANGAROO 3.8 m telescope has been used to observe a number of galactic and extragalactic candidate TeV gamma-ray sources (Roberts et al. 1997). From these observations we have evidence for gamma-ray emission from three galactic sources - PSR1706-44 (Kifune et al. 1995), the Crab nebula (Tanimori et al. 1994) and the Vela SNR (Yoshikoshi et al. 1997).

As an indication of the sensitivity of the 3.8 m telescope to extragalactic sources we can use Monte Carlo simulations to estimate the response of the telescope to fluxes observed from Mkn 421 and Mkn 501. Assuming that the gamma-ray emission from Mkn 421 and Mkn 501 extends up to 10 TeV the integral fluxes above 2 TeV are as follows.

For Mkn 421:
(adopting the average 1996 Whipple flux with an assumed integral spectral index of -1.56, McEnery et al. 1997.)

For Mkn 501:
(adopting the March 1997 HEGRA flux assuming an integral photon spectral index of -1.49, Aharonian et al. 1997.)

If any of the sources examined in this paper were capable of providing a steady flux at the level of Mkn 421 or Mkn 501 it would be detectable by the CANGAROO 3.8 m telescope, albeit at low significance.

Observations by the Whipple group of Mkn 421 have shown that it is capable of extremely energetic flares on timescales of hours to days. A flare similar to that seen on 7 May 1996 by the Whipple telescope (McEnery et al. 1997) with a flux of F([FORMULA]250GeV) [FORMULA] photons [FORMULA] (F([FORMULA]2TeV) [FORMULA] photons [FORMULA] for a -1.56 spectral index and maximum photon energy of 10 TeV) lasting two hours would be easily detectable by CANGAROO at a significance of around [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998