Multi-wavelength observations of the flaring gamma-ray blazar 3C66A in October 2008 PDF Print

 

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Broadband SED of 3C 66A during the October 2008 multiwavelength campaign. The observation that corresponds to each set of data points is indicated in the legend. See Figure 6 below for more details.

Reference:  A. A. Abdo et al., Astrophysical Journal 726: 43, 2011 (Fermi-LAT, VERITAS, GASP-WEBT & multi-wavelength partners). Erratum: Astrophys. J. 731: 77, 2011.

Full text version here, erratum here

ArXiv version: ArXiV:1011.1053

Contact person: Luis C. Reyes

 

One of the main obstacles in the broadband study of gamma-ray blazars is the lack of simultaneity, or at least contemporaneousness, of the data at the various wavelengths. At high energies the situation is made even more difficult due to the lack of objects that can be detected by MeV/GeV and TeV observatories on comparable time scales. Fortunately,  the current generation of gamma-ray instruments is closing the gap between the two energy regimes due to improved instrument sensitivities.  An example of the successful synergy of space-borne and ground-based observatories is provided by the joint observations of 3C 66A by Fermi and VERITAS during its strong flare of 2008 October. Follow-up observations were obtained at radio, optical and X-ray wavelengths in order to measure the flux and spectral variability of the source across the electromagnetic spectrum and to obtain a quasi-simultaneous SED. This marks the first occasion that a gamma-ray flare is detected by GeV and TeV instruments in comparable time scales. The light curves obtained show strong variability at every observed wavelength and in particular, the flux increase observed by VERITAS and Fermi is coincident with an optical outburst. The clear correlation between the Fermi-LAT and optical light curves permits one to go beyond the source association reported in the 1st Fermi-LAT source Catalog and finally identify the gamma-ray source 1FGL J0222.6+4302 as blazar 3C 66A.


For the modeling of the overall SED, reasonable agreement can be achieved using both a pure SSC model and an SSC + EC model with an external near-infrared radiation field as an additional source for Compton scattering. However, the pure SSC model requires (a) a large emission region, which is inconsistent with the observed intra-night scale variability at optical wavelengths and (b) low magnetic fields, about a factor 10−3 below equipartition. In contrast, an SSC + EC interpretation allows for variability on time scales of a few hours, and for magnetic fields within about an order of magnitude of, though still below, equipartition. Intermediate synchrotron peaked blazars (ISPs) like 3C 66A are well suited for simultaneous observations by Fermi-LAT and ground-based IACTs like VERITAS. Relative to the sensitivities of these instruments, ISPs are bright enough to allow for time-resolved spectral measurements in each band during flaring episodes. These types of observations coupled with extensive multiwavelength coverage at lower energies will continue to provide key tests of blazar emission models.

 

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

 

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Figure 5: Discrete correlation function (DCF) of the F(E > 1 GeV) gamma-ray light curve with respect to the R band light curve. A positive time lag indicates that the gamma-ray band leads the optical band. Different symbols correspond to different bin sizes of time lag as indicated in the legend. The profile of the DCF is independent of bin size and is well described by a Gaussian function of the form DCF(tau) = C_max x Exp (t-to)^2/s^2. The fit to the 3-day bin size distribution is shown in the plot as solid black line and the best-fit parameters are C_max = 1.1 ± 0.3, to = (0.7±0.7) days and s = (3.3±0.7) days.
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Figure 6: Broadband SED of 3C 66A during the October 2008 multiwavelength campaign. The observation that corresponds to each set of data points is indicated in the legend. As an example, the EBL-absorbed EC+SSC model for z = 0.3 is plotted here for reference. A description of the model is provided in the paper.
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Figure 7: SSC models for redshifts z =0.444, 0.3, 0.2, and 0.1 from top to bottom. The Fermi-LAT and VERITAS data points follow the same convention used in Figures 1 and 6 to distinguish between flare (red) and dark run (blue) data points. In each panel the EBL-absorbed model is shown as a solid red line and the de-absorbed model as a red dashed line. De-absorbed VERITAS flare points are shown as open squares. In all cases the optical depth values from Franceschini et al. (2008) are used. The best agreement between the model and the data is achieved when the source is located at z = 0.2 − 0.3. For lower redshifts the model spectrum is systematically too hard, while at z = 0.444, the model spectrum is too soft.
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Figure 8: EC + SSC model for redshifts z =0.444, 0.3, 0.2, and 0.1 from top to bottom. The individual EBL- absorbed EC and SSC components are indicated as a dash-dotted and dotted lines, respectively. The sum is shown as solid red line (dashed when de-absorbed). The best agreement between the model and the data is achieved when the source is located at z ∼ 0.2.
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Table 1: Parameters used for the SSC models displayed in Figure 7. All SSC models require very low magnetic fields, far below the value expected from equipartition (i.e. e_B
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Table 2: Parameters used for the EC+SSC model fits displayed in Figure 8. These model fits require magnetic fields closer to equipartition and allow for the intra-night variability observed in the optical data. The weighted sum of squared residuals to the VERITAS and Fermi-LAT data and the total value for the combined data set are included at the bottom of the table.

 

 

 

Last Updated on Saturday, 09 July 2011 06:43
 

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