Discovery of Very High Energy $gamma$-Ray Emission from the Low-Frequency-peaked BL Lacertae Object BL Lacertae
Journal
The Astrophysical Journal Letters 666, L17, 2007
ticlealbert2007discovery, title=Discovery of Very High Energy $gamma$-Ray Emission from the Low-Frequency-peaked BL Lacertae Object BL Lacertae, author=Albert, J and Aliu, E and Anderhub, H and Antoranz, P and Armada, A and Baixeras, C and Barrio, JA and
Abstract:
The MAGIC collaboration observed BL Lacertae for 22.2 hr during 2005 August to December and for 26 hr during 2006 July to September. The source is the historical prototype and eponym of a class of low-frequency peaked BL Lacertae (LBL) objects. A very high energy (VHE) gamma-ray signal was discovered with a 5.1 sigma excess in the 2005 data. Above 200 GeV, an integral flux of 0.6+-0.2 x 10^(-11) cm^(-2) s^(-1) was measured, corresponding to approximately 3% of the Crab flux. The differential spectrum between 150 and 900 GeV is rather steep with a photon index of -3.6+-0.5. The light curve shows no significant variability during the observations in 2005. For the first time a clear detection of VHE gamma-ray emission from an LBL object was obtained with a signal below previous upper limits. The 2006 data show no significant excess. This drop in flux follows the observed trend in optical activity.