VERITAS Discovery of >200GeV Gamma-ray Emission from the Intermediate-frequency-peaked BL Lac Object W Comae

摘要

We report the detection of very high-energy gamma-ray emission from theintermediate-frequency-peaked BL Lacertae object W Comae (z=0.102) by VERITAS.The source was observed between January and April 2008. A strong outburst ofgamma-ray emission was measured in the middle of March, lasting for only fourdays. The energy spectrum measured during the two highest flare nights is fitby a power-law and is found to be very steep, with a differential photonspectral index of Gamma = 3.81 +- 0.35_stat +- 0.34_syst. The integral photonflux above 200GeV during those two nights corresponds to roughly 9% of the fluxfrom the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energieswere triggered by the VERITAS observations. The spectral energy distribution ofthe flare data can be described by synchrotron-self-Compton (SSC) orexternal-Compton (EC) leptonic jet models, with the latter offering a morenatural set of parameters to fit the data.