CANGAROO‐III Observations of the 2006 Outburst of PKS 2155−304

Abstract

We have used the CANGAROO-III imaging atmospheric Cerenkov telescopes to observe the high-frequency-peaked BL Lacertae (HBL) object PKS 2155-304 between 2006 July 28 (MJD 53,944) and August 2, triggered by the H.E.S.S. report that the source was in a high state of TeV gamma-ray emission. A signal was detected at the 4.8 ? level in an effective live time of 25.1 hr during the outburst period. The flux of very high energy gamma rays from the CANGAROO-III observations shows variability on the timescale of less than a few hours. The averaged integral flux above 660 GeV is (1.6 ? 0.3stat ? 0.5syst) ? 10-11 cm-2 s -1, which corresponds to ?45% of the flux observed from the Crab Nebula. Follow-up observations between August 17 (MJD 53,964) and 25 indicate that the source activity had decreased. ? 2008. The American Astronomical Society. All rights reserved.