Spectrum of Very High Energy Gamma-Rays from the blazar 1ES1959+650 during Flaring Activity in 2002

Abstract

The blazar 1ES 1959+650 was observed in a flaring state with the Whipple 10 m Imaging Atmospheric Cherenkov Telescope during May of 2002. A spectral analysis has been carried out on the data from that time period and the resulting very high energy gamma-ray spectrum ($E \geq 316$ GeV) can be well fit by a power-law of differential spectral index \alpha = 2.78 +/- 0.12_{stat.} +/- 0.21_{sys.}. On June 4th 2002, the source flared dramatically in the gamma-ray range without any coincident increase in the X-ray emission, providing the first unambiguous example of an `orphan' gamma-ray flare from a blazar. The gamma-ray spectrum for these data can also be described by a simple power-law fit with \alpha = 2.82 +/- 0.15_{stat.} +/- 0.30_{sys.}. There is no compelling evidence for spectral variability, or for any cut-off to the spectrum.