TeV and Multiwavelength Observations of Mrk421 in 20062008 
Reference: V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal 738: 25, 2011Full text versionArXiv: ArXiV:1106.1210Contacts: Matthias Beilicke
The blazar Mrk 421 is one of the strongest (and best studied) extragalactic TeV gammaray source in the sky. It is located at a redshift of 0.031 and was first discovered at TeV energies by the Whipple 10m telescope in 1992 (Punch et al 1992). Mrk 421 was extensively observed with the VERITAS observatory and the Whipple 10m telescope in 20062008. The excellent sensitivity of VERITAS allowed us to sample the TeV gammaray fluxes and energy spectra with unprecedented accuracy. On May 2nd and 3rd, 2008, bright TeV gammaray flares were detected with fluxes reaching the level of 10 Crab. A spectral hardening with increasing TeV flux level is observed. The TeV gammaray data were complemented with radio, optical, and Xray observations, resulting in one of the best covered multiwavelength light curve of a TeV blazar measured so far. Flux variability is found in all bands except for the radio waveband. The Xray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV gammarays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably well described by a onezone SSC model. Figures from paper (click to get full size image):
Figure 6: Photon index Γ versus flux normalization I0 (at 1TeV) obtained from a powerlaw fit with exponential cutoff (fixed to Ecut = 4TeV, statistical errors only). Each gray data point corresponds to a data run with a duration of 10 to 20min. The black points correspond to the values obtained from the fits to the spectra grouped according to their flux states (defined in Figure 5 and Table 2). Correlation coefficients r and the corresponding chance probabilities for the null hypothesis (no correlation) are given in the legend. Results obtained from earlier Whipple 10m observations (Krennrich et al. 2002) are shown for reference.
Figure 7: Correlations between the spectral shape and the flux level for the Xray data sets taken with RXTE/PCA and Swift/XRT. Correlation coefficients r and the corresponding chance probability for the null hypothesis (no correlation) are given in the legends. Left: The logparabola parameter a vs. the 2 − 10 keV photon flux. Right: The logparabola curvature parameter b vs. the 2 − 10 keV photon flux. Figure 8: Correlation between TeV and optical fluxes. The maximum allowed time difference between the data points is Δt ≤ 0.5 days.
Figure 9: Hard/soft Xray flux correlation between the 15 − 150 keV Swift/BAT flux and the 2 − 10 keV RXTE/ASM flux. All dwellbydwell flux points with a statistical significance above 2σ measured in the first half of 2008 (MJD 54457– 54640) are shown for two cases of a maximum time lag between the observed data points. The legend gives the corresponding correlation coefficient and the corresponding chance probability for the noncorrelation.
Figure 10: Correlation between TeV fluxes (VERITAS and Whipple) and Xray fluxes (RXTE: PCA & ASM, Swift: XRT & BAT). All Xray fluxes are given between 2 − 10 keV, except for the Swift/BAT fluxes which are given between 15 − 150 keV. Only data measured in the first half of 2008 (MJD 54457–54640) are shown. The RXTE/ASM and Swift/BAT fluxes are dwell by dwell with the requirement that the statistical significance is above 2 standard deviations. The maximum time difference between the data points is Δt ≤ 15min. The correlation coefficients including the chance probability for the null hypothesis are shown in the legend. The flux pairs measured during the first (2b(1)) and second night (2b(2)) of phase 2b are marked.
Figure 11: SEDs of Mrk 421 for individual dates showing all available MWL data (statistical errors only) if the corresponding time lag criterion is met (see text for more detail). The legends indicate the time lags ΔT = TMWL−TTeV between the individual MWL data and the VERITAS data. For the Xray observations, the duration of the pointing Δ is given, as well. The number in parentheses refers to the phase (2a or 2b) during which the The SEDs were fit by a onezone SSC model (dashed curves); the corresponding model parameters are summarized in Table 4. The nonsimultaneous MeV/GeV spectrum measured by Fermi (August 2008 to July 2009) is shown for reference (Abdo et al. 2010).
Figure 12: SEDs of Mrk 421, continued from figure 11. The SEDs of the two flare nights are also shown: MJD 54588.30 (no Xray coverage in this night) and MJD 54589.21 with Xray coverage during the onset of the flare (solid TeV points) but not during the high flare state (open TeV data points).


Last Updated on Saturday, 17 September 2011 18:58 