MEASUREMENT OF TEV GAMMA-RAY SPECTRA WITH THE CHERENKOV IMAGING TECHNIQUE

In this paper, we seek to establish reliable methods for extracting en ergy spectra for TeV gamma-ray sources observed using the atmospheric Cherenkov Imaging Technique, Careful attention has been paid to the ca lculation of the telescope gain, and we obtain good agreement between direct measurements, with a statistical error of about 10%, and an abs olute calibration from the background cosmic-ray trigger rate that has an overall error of 18%. Two independent analyses that are based on d ifferent Monte Carlo shower simulations, employ different selection cr iteria in order to retain a large fraction of gamma-ray events, and us e different approaches to spectral estimation are presented here. The first is a fairly traditional method that builds on established image selection techniques and calculates the detector collection area and a n energy estimation function. The error in measuring the enrgy of a si ngle event is estimated at 36%, and we try to compensate for this poor energy resolution. The second analysis uses more elegant gamma-ray se lection criteria and implicitly incorporates the properties of the det ector into the simulations that are then compared with the data in ord er to obtain source spectra. The two simulations are compared to each other and to the data, with the aim of establishing that each method i s robust and insensitive to simulation details. Finally, we consider t he main sources of systematic errors, the largest of which is in the t elescope gain calibration, arising from an incomplete knowledge of the relevant factors, and is estimated to be 16%. The effect of possible errors in the simulations is also considered. Both methods have been a pplied to a part of the Whipple observatory database on the Crab Nebul a for the 1988/89 observing season, while the first method has also be en applied to data taken in 1995/96. The statistical error in the flux constant is about 8% and that in the spectral index is about 5%, whil e the corresponding systematic errors are estimated to be 18% and 2%, respectively. The results presented here show good agreement between t he two methods as well as between the two seasons. However, a comprehe nsive consideration of the implications of the derived spectra and a c omparison to other work is addressed in another paper. (C) 1998 Elsevi er Science B.V.