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

TL;DR

VERITAS detected very high-energy gamma-ray emission from the BL Lac object W Comae, including a short, intense flare, and modeled its spectral energy distribution using leptonic jet models.

Contribution

First detection of >200GeV gamma-ray emission from W Comae, with detailed spectral analysis and modeling of its emission mechanisms.

Findings

Detected a gamma-ray flare lasting four days.

Measured a steep energy spectrum with index ~3.81.

Modeled the spectral energy distribution with SSC and EC leptonic jet models.

Abstract

We report the detection of very high-energy gamma-ray emission from the intermediate-frequency-peaked BL Lacertae object W Comae (z=0.102) by VERITAS. 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. 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 Gamma = 3.81 +- 0.35_stat +- 0.34_syst. The integral photon flux above 200GeV 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.