Observation of Gamma Rays Greater than 10 T[CLC]e[/CLC]V from Markarian 421

K. Okumura, A. Asahara, G. V. Bicknell, P. G. Edwards, R. Enomoto, S. Gunji, S. Hara, T. Hara, S. Hayashi, C. Itoh, S. Kabuki, F. Kajino (+33 others)
2002 Astrophysical Journal  
We have observed Markarian 421 in January and March 2001 with the CANGAROO-II imaging Cherenkov telescope during an extraordinarily high state at TeV energies. From 14 hours observations at very large zenith angles, ∼70^∘, a signal of 298 ± 52 gamma-ray--like events (5.7 σ) was detected at E>10 TeV, where a higher sensitivity is achieved than those of usual observations near the zenith, owing to a greatly increased collecting area. Under the assumption of an intrinsic power-law spectrum, we
more » ... ved a differential energy spectrum dN/dE = (3.3 ± 0.9_stat.± 0.3_syst.)×10^-13 (E/10 Te V)^-(4.0 ^+0.9_-0.6_stat.± 0.3_syst.) ph./cm^2/sec/TeV, which is steeper than those previously measured around 1 TeV, and supports the evidence for a cutoff in the spectrum of Markarian 421. However, the 4 σ excess at energies greater than 20 TeV in our data favors a cutoff energy of ∼8 TeV, at the upper end of the range previously reported from measurements at TeV energies.
doi:10.1086/344831 fatcat:zyt72w3dtncargulgtgm2rkege