High-state SED of W Com for MJD 54624 to 54626 including VERITAS, Swift XRT/UVOT, optical, and radio data (filled circular markers). See Figure 5 (below) for more details.

Multiwavelength Observations of a Bright TeV Flare from W Comae

Reference:  V. A. Acciari et al. (The VERITAS Collaboration with AGILE, Swift, and XMM collaborators), The Astrophysical Journal, 707:612-620, 2009

Full text version

ArXiv version: ArXiV:0910.3750

Contact person: Gernot Maier

W Comae is an intermediate-frequency peaked BL Lacertae object (z=0.102) discovered by VERITAS at energies above 200 GeV in March 2008. A second flare, with a flux of F(E>200 GeV) = (5.7+-0.6)x10^-11 cm^-2 s^-1 about three times brighter was detected by VERITAS during observation in moonlight conditions in June 2008. This triggered an intensive multiwavelength campaign including ground-based observatories (radio, UV and optical wavelengths), the space-based gamma-ray telescope AGILE, and the XMM-Newton and Swift X-ray telescopes.

Contemporaneous multiwavelength data like this help to increase our understanding of particle acceleration and high-energy emission in the highly relativistic jets of blazars. The extensive modeling of the spectral energy distribution (SED) of W Comae considers leptonic emission in form of synchrotron, synchrotron self Compton (SSC) and external Compton (EC) emission. The wide separation of the peaks in the SED disfavors a simple SSC model. A SSC+EC model, assuming thermal emission from the accretion disk as source of low-energy photons provides a good description of the broadband SED.  The strong variability of W Comae at X-ray and gamma-ray energies on timescales of days or less shows than only truly contemporaneous measurements can provide serious constraints on emission models.

Figures from paper (click to get full size image):

Figure 1: Multiwavelength light curve of W Com for MJD 54622 to 54636. Panel (a): VHE gamma-ray light curve (E > 200 GeV) as measured by VERITAS. The flux in VHE gamma rays corresponds to approximately 25% of the flux of the Crab Nebula above 200 GeV. Panel (b): gamma-ray light curve (E > 100 MeV) as measured by AGILE. Panel (c): X-ray (Swift XRT: 2-10 keV, circles; XMM-Newton EPIC: squares). Panel (d): Swift UVOT (UVW1: squares; UVM2: downward-pointing triangles; UVW2: upward-pointing triangles). Panel (e): light curves of negative optical magnitudes (R-band; filled circles: Tuorla; filled squares: Abastumani; filled triangles: San Pedro Martir; diamonds: Sapienza University; open circles: KVA; open squares: Crimean; open stars: NOT; open triangles: Torino; open crosses: Talmassons). Panel (f): radio light curve (circles: UMRAO 14.5 GHz; triangles: Metsähovi 37 GHz). Downward pointing arrows indicate upper flux limits (99% confidence level; Helene 1983).

Figure 2: Sky map of significances of gamma-ray emission from the region around W Com. The background is estimated using the reflected region model (10 background regions, oversampling radius 0.12 degrees). The position of W Com derived from radio data (Fey et al. 2004) is indicated by a white cross. The dashed circles indicate positions of bright stars and their B-band magnitudes in the field of view; regions around these stars are excluded from the background estimation. Two sources listed in the Fermi bright gamma-ray source list (Abdo et al. 2009a), and firmly associated with the blazars W Com and B2 1215 (Abdo et al. 2009b), are shown with their 95% confidence area as circles with

Figure 3: Differential VHE photon spectrum for W Com for MJD 54624.16 to 54625.24 (2008 June 7-8). The markers indicate measured data points and the continuous line a fit assuming a power-law distribution. Error bars show statistical errors only. For comparison, the photon spectrum of W Com derived from VERITAS measurements in 2008 March (Acciari et al. 2008b) is indicated by a dashed line.

Figure 4: SED for five different X-ray measurements with Swift XRT and XMM-Newton EPIC for 2008 June 7-18 and the Swift XRT measurements in 2008 March (MJD 54539.4).

Figure 5 (shown above): High-state SED of W Com for MJD 54624 to 54626 including VERITAS, Swift XRT/UVOT, optical, and radio data (filled circular markers). The averages of the optical, NIR, and radio fluxes calculated over the time range from MJD 54610 to 54645 are shown here. Downward-pointing arrows indicate upper flux limits (99% confidence level; Helene 1983). For comparison, the VERITAS, AGILE, and Swift XRT/UVOT data for MJD 54626 to 54626.9 are shown as gray open squares and gray downward pointing arrows. Results from synchrotron-self-Compton (SSC) and external-Compton (SSC+EC) models are shown as continuous and dashed lines, respectively (see the text for details). The different components (disk, sync=synchrotron, ec, ssc) of the SSC+EC models are indicated by dotted lines.