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Register a new userIntrinsic Correlations for Flaring Blazars Detected by Fermi
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Blazars are an extreme subclass of active galactic nuclei. Their rapid variability, luminous brightness, superluminal motion, and high and variable polarization are probably due to a beaming effect. However, this beaming factor (or Doppler factor) is very difficult to measure. Currently, a good way to estimate it is to use the time scale of their radio flares. In this $Letter$, we use multiwavelength data and Doppler factors reported in the literatures for a sample of 86 flaring blazars detected by Fermi to compute their intrinsic multiwavelength data and intrinsic spectral energy distributions, and investigate the correlations among observed and intrinsic data. Quite interestingly, intrinsic data show a positive correlation between luminosity and peak frequency, in contrast with the behavior of observed data, and a tighter correlation between $gamma$-ray luminosity and the lower energy ones. For flaring blazars detected by Fermi, we conclude that (1) Observed emissions are strongly beamed; (2) The anti-correlation between luminosity and peak frequency from the observed data is an apparent result, the correlation between intrinsic data being positive; and (3) Intrinsic $gamma$-ray luminosity is strongly correlated with other intrinsic luminosities.
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Multiwavelength observations are essential to constrain physical parameters of the blazars observed by Fermi/LAT. Among the 187 AGN significantly detected in public INTEGRAL data above 20 keV by the imager IBIS/ISGRI, 20 blazars were detected. 15 of these sources allowed significant spectral extraction. They show hard X-ray spectra with an average photon index of 2.1+-0.1 and a hard X-ray luminosity of L(20-100 keV) = 1.3e46 erg/s. 15 of the INTEGRAL blazars are also visible in the first 16 months of the Fermi/LAT data, thus allowing to constrain the inverse Compton branch in these cases. Among others, we analyse the LAT data of four blazars which were not included in the Fermi LAT Bright AGN Sample based on the first 3 months of the mission: QSO B0836+710, H 1426+428, RX J1924.8-2914, and PKS 2149-306. Especially for blazars during bright outbursts, as already observed simultaneously by INTEGRAL and Fermi (e.g. 3C 454.3 and Mrk 421), INTEGRAL provides unique spectral coverage up to several hundred keV. We present the spectral analysis of INTEGRAL and Fermi data and demonstrate the potential of INTEGRAL observations of Fermi detected blazars in outburst by analysing the combined data set of the persistent radio galaxy Cen A.
Blazars are known for their energetic multiwavelength flares from radio wavelengths to high-energy $gamma$-rays. In this work, we study radio, optical, and $gamma$-ray light curves of 145 bright blazars spanning up to 8~yr, to probe the flaring activity and interband correlations. Of these, 105 show $>1sigma$ correlations between one or more wavebands, 26 of which have a $>3sigma$ correlation in at least one wavelength pair, as measured by the discrete correlation function. The most common and strongest correlations are found between the optical and $gamma$-ray bands, with fluctuations simultaneous within our $sim 30$~d resolution. The radio response is usually substantially delayed with respect to the other wavelengths with median time lags of $sim 100$--160~d. A systematic flare identification via Bayesian block analysis provides us with a first uniform sample of flares in the three bands, allowing us to characterise the relative rates of multiband and orphan flares. Multiband flares tend to have higher amplitudes than orphan flares.
Beaming effect is important for the observational properties of blazars. In this work, we collect 91 $Fermi$ blazars with available radio Doppler factors. $gamma$-ray Doppler factors are estimated and compared with radio Doppler factors for some sources. The intrinsic (de-beamed) $gamma$-ray flux density ($f^{rm in}_{gamma}$), intrinsic $gamma$-ray luminosity ($L^{rm in}_{gamma}$), and intrinsic synchrotron peak frequency ($ u_{rm p}^{rm in}$) are calculated. Then we study the correlations between $f^{rm in}_{gamma}$ and redshift and find that they follow the theoretical relation: $log f = -2.0 log z + {rm const}$. When the subclasses are considered, we find that stationary jets are perhaps dominant in low synchrotron peaked blazars. 63 $Fermi$ blazars with both available short variability time scales ($Delta T$) and Doppler factors are also collected. We find that the intrinsic relationship between $L ^{rm in}_{gamma}$ and $Delta T^{rm in}$ obeys the Elliot & Shapiro and the Abramowicz & Nobili relations. Strong positive correlation between $f_{gamma}^{rm in}$ and $ u_{rm p}^{rm in}$ is found, suggesting that synchrotron emissions are highly correlated with $gamma$-ray emissions.
We use the complete MOJAVE 1.5 Jy sample of active galactic nuclei (AGN) to examine the gamma-ray detection statistics of the brightest radio-loud blazars in the northern sky. We find that 23% of these AGN were not detected above 0.1 GeV by the Fermi LAT during the 4-year 3FGL catalog period partly because of an instrumental selection effect, and partly due to their lower Doppler boosting factors. Blazars with synchrotron peaks in their spectral energy distributions located below $10^{13.4}$ Hz also tend to have high-energy peaks that lie below the 0.1 GeV threshold of the LAT, and are thus less likely to be detected by Fermi. The non-detected AGN in the 1.5 Jy sample also have significantly lower 15 GHz radio modulation indices and apparent jet speeds, indicating that they have lower than average Doppler factors. Since the effective amount of relativistic Doppler boosting is enhanced in gamma-rays (particularly in the case of external inverse-Compton scattering), this makes them less likely to appear in the 3FGL catalog. Based on their observed properties, we have identified several bright radio-selected blazars that are strong candidates for future detection by Fermi.
The curvature of the $gamma$-ray spectrum in blazars may reflect the intrinsic distribution of the emitting electron distribution, which will further give some information on the possible acceleration and cooling processes in the emitting region. The $gamma$-ray spectra of Fermi blazars are normally fitted either by a single power-law (PL) or a log-normal (call Logarithmic Parabola, LP) form. The possible reason for this differnece is not unclear. We statistically explore this issue based on the different observational properties of 1419 Fermi blazars in the 3LAC Clean sample. We find that the $gamma$-ray flux (100 MeV-100 GeV) and variability index follow bimodal distributions for PL and LP blazars, where $gamma$-ray flux and variability index show {a positive correlation}. However, the distributions of the $gamma$-ray luminosity and redshift follow a unimodal distribution. Our results suggest that the bimodal distribution of $gamma$-ray flux for LP and PL blazars may be not intrinsic and all blazars may have an intrinsic curved $gamma$-ray spectrum and the PL spectrum is just caused by the fitting effect due to the less photons.
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