Narrow Results

The BL Lac object 3C 66A was the target of an intensive multiwavelength monitoring campaign organized in 2003–2004. During the campaign, its spectral energy distribution (SED) was measured and flux measurements from radio to X-ray frequencies as well as upper limits in the very high energy (VHE) γ-ray regime were obtained. Here, we reproduce the SED and optical spectral variability pattern observed during our multiwavelength campaign using a time-dependent leptonic jet model. Our model could successfully simulate the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~4 GeV implying the effect of the optical depth due to the intergalactic infrared background radiation (IIBR) to be negligible. Also, the contribution of external Comptonization (EIC), due to the presence of a broad-line region (BLR), in the emission of γ-ray photons could be significant early-on when the emission region is very close to the central engine but as it travels farther out, the production mechanism of hard X-ray and γ-ray photons becomes dominated by synchrotron self-Compton mechanism (SSC).

In this work, spectral energy distribution has been calculated for a sample of 597 Fermi blazars. Based on the calculation, we proposed a classification for subclasses of blazars as lower synchrotron peak sources (LSPs) if $log{\nu }_{\mathrm{\text{p}}}(\mathrm{\text{H}}z)<14$, intermediate synchrotron peak sources (ISPs) if $14\le log{\nu }_{\mathrm{\text{p}}}(\mathrm{\text{H}}z)<16$, higher synchrotron peak sources (HSPs) if $16log{\nu }_{\mathrm{\text{p}}}(\mathrm{\text{H}}z)<19.5$, ultra higher synchrotron peak sources (UHSPs) if $log{\nu }_{\mathrm{\text{p}}}(\mathrm{\text{H}}z)\ge 19.5$. $\gamma$-ray luminosity is found correlated with lower energetic wavebands (radio, optical and X-rays). When redshift effect is removed, correlations are still strong for FSRQs; As for BL lacertaes, correlation between $\gamma$-ray and radio or between $\gamma$-ray and optical band is strong, but that between $\gamma$-ray and X-ray is only marginal.

On the basis of the beaming model, we have derived a relation between the observed polarization (P^ob) and the observed magnitude (m^ob), log P^ob = -0.4(k-1)m^ob+C, which indicates that the observed polarization is anti-correlated with the observed magnitude. Making use of the optical data of polarization and magnitude observed simultaneously for 47 blazars, we found that there is a significant anti-correlation between the observed polarization and the observed magnitude in the V-band, the linear regression equation of which is , with the correlation coefficient of γ = -0.4537 and the chance probability of p = 1.360×10^-3. Obviously, the statistical results are consistent with the theoretical results derived by the beaming model. Our results of both theory and statistical analysis of the data observed simultaneously are consistent with the observations of 3C345, Mrk421.

In addition, using these data, we have also analyzed the relation between the maximum optical polarization ( in the V-band) and the maximum infrared polarization ( in the H-band). For 20 objects with both the maximum optical and infrared polarization, a linear regression analysis gives a strong correlation of , with a correlation coefficient of γ = 0.9071 and a chance probability of p < 1.0 ×10^-4. The results suggest that both the infrared and optical emissions of blazars originate from the synchrotron radiation.

We consider some implications of the rapid X-ray and TeV variability observed in M87 and the TeV blazars. We outline a model for jet focusing and demonstrate that modest radiative cooling can lead to recollimation of a relativistic jet in a nozzle having a very small cross-sectional radius. Such a configuration can produce rapid variability at large distances from the central engine and may explain recent observations of the HST-1 knot in M87. Possible applications of this model to TeV blazars are discussed. We also discuss a scenario for the very rapid TeV flares observed with H.E.S.S. and MAGIC in some blazars, that accommodates the relatively small Doppler factors inferred from radio observations.

The current very high energy (VHE; E > 100 GeV) experiments have tremendously increased the number of detected extragalactic sources. We present a synchrotron self-Compton modeling tour of the active galactic nuclei currently established as VHE emitters so far, and investigate possible correlations among the intrinsic and derived parameters.

The total set of the 14 active galactic nuclei detected by MAGIC so far includes well-studied bright blazars like Mkn 501, the giant radio galaxy M 87, but also the distant flat-spectrum radio quasar 3C 279, and an intriguing gamma-ray source in the 3C 66A/B region, whose energy spectrum is not compatible with the expectations from 3C 66A. Besides scheduled observations, so far MAGIC succeeded in discovering TeV gamma rays from three blazars following triggers from high optical states. I report selected highlights from recent MAGIC observations of extragalactic TeV gamma-ray sources, emphasizing and discussing the new physics insights the MAGIC observations were able to contribute.

Blazars are effective emitters of γ-rays with spectra extending to GeV and TeV energies. In the case of distant TeV blazars, γ-rays undergo severe absorption due to interactions with diffuse intergalactic radiation fields resulting in significant deformations of the initial (source) spectra. Even for the lowest possible level of the Extragalactic Background Light (EBL), the absorption-corrected γ-ray spectra from some TeV blazars appear unusually hard, which cannot be explained within the standard particle acceleration models of blazars. The simplest and most natural solution of this problem seems to us the realization of internal absorption of γ-rays in narrow (e.g. Planckian type) radiation fields in the immediate vicinity of the γ-ray production region.

The BL Lac object 3C 66A was the target of an intensive multiwavelength monitoring campaign organized in 2003–2004. During the campaign, its spectral energy distribution (SED) was measured and flux measurements from radio to X-ray frequencies as well as upper limits in the very high energy (VHE) γ-ray regime were obtained. Here, we reproduce the SED and optical spectral variability pattern observed during our multiwavelength campaign using a time-dependent leptonic jet model. Our model could successfully simulate the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~ 4 GeV.

We report about recent progress in the first numerical implementations of spectral energy distribution (SED) fits to estimate synchrotron-self Compton (SSC) model parameters. Using two well observed objects, Markarian 421 and Markarian 501, we will highlight the strength of the method, as well as plans for future improvements.