W Comae Light Curve
Lower panel: The light curve of the integral photon flux above 200 GeV is shown. A spectral shape of dN/dE ~ E^-Gamma with Gamma = 3.8 is assumed. Each flux point corresponds to one observation period (defined by ~3 weeks of operation between two full-moon phases), with the exception of the flare around MJD 54538 (1) for which a night-by-night binning is used (see inlay for details). Upper panel: The X-ray flux as measured by Swift for the same time period. The vertical lines are shown for easier comparison.

 Discovery of the Blazar W Comae


VERITAS detected very high-energy (VHE) gamma-ray emission from the intermediate-frequency-peaked BL Lacertae object W Comae (z=0.102). The source was observed between January and April 2008.· A strong outburst of gamma-ray emission was measured in the middle of March, lasting for only four days (see ATel#1422). The energy spectrum measured during the two highest flare nights is fit by a power-law and is found to be very steep, with a differential photon spectral index of Γ = 3.81 ± 0.35stat ± 0.34syst. The integral photon flux above 200 GeV during those two nights corresponds to roughly 9% of the flux from the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energies were triggered by the VERITAS observations. The spectral energy distribution of the flare data can be described by synchrotron-self-Compton (SSC) or external-Compton (EC) leptonic jet models, with the latter offering a more natural set of parameters to fit the data.

Beginning of June 2008, a second gamma-ray flare was observed by VERITAS from W Comae with fluxes reaching a level twice as high as the first flare (see ATel#1565). Those observations were taken under moon light conditions, work in progress.

W Comae is the first blazar of the IBL class to be detected at energies above 100 GeV. The extension of the VHE catalog to the FRSQ, LBL and IBL classes will play a major role in our understanding of blazar populations and blazar dynamics. The known VHE-emitting blazar 1ES 1218+304 is detected roughly 2deg North of W Comae. For the first time in VHE gamma-ray astronomy two extragalactic VHE gamma-ray sources are detected in the same field of view.

For more information on the science of this object, please see V. A. Acciari et. al., The Astrophysical Journal Letters, 684:L73–L77 (ArXiv:0808.0889). You may also contact Matthias Beilicke