A Synchrotron Self-Compton Scenario for the Very High Energy Gamma-ray Emission of the Intermediate BL Lacertae object W Comae

TL;DR

This paper models the multi-wavelength emission of W Comae, a BL Lac object, using a synchrotron self-Compton scenario, explaining its variability and predicting detectable GeV emission during flares.

Contribution

It demonstrates that a single-zone SSC model can fit the broadband SEDs during different flaring states of W Comae, providing insights into jet properties and emission regions.

Findings

Both SEDs during TeV flare and optical/X-ray outburst fit well with a single-zone SSC model.

The TeV flare region is larger and has a weaker magnetic field than the optical/X-ray outburst region.

Predicted GeV emission is above Fermi/LAT sensitivity, testable by future observations.

Abstract

W Comae has significant variability in multi-wavelengthes, from the radio to the gamma-ray bands. A bright outburst in the optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae (IBL) source. I find that both the broadband spectral energy distributions (SEDs) quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron + synchrotron-self-Compton (SSC) model. The satisfactory fitting requires a large beaming factor, i.e., $\delta\sim 25$ and $\delta\sim 20$ for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is $\sim 14$ times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The IC component of the TeV flare peaks at around 1.3 GeV, but it is around 20 MeV for the X-ray/optical outburst, lower than that for the TeV flare with 2 orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical to characterize the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of \emph{Fermi}/LAT, and it could be verified with the observations by \emph{Fermi}/LAT in near future.