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Register a new userDegree-scale GeV jets from active and dead TeV blazars
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We show that images of TeV blazars in the GeV energy band should contain, along with point-like sources, degree-scale jet-like extensions. These GeV extensions are the result of electromagnetic cascades initiated by TeV gamma-rays interacting with extragalactic background light and the deflection of the cascade electrons/positrons in extragalactic magnetic fields (EGMF). Using Monte-Carlo simulations, we study the spectral and timing properties of the degree-scale extensions in simulated GeV band images of TeV blazars. We show that the brightness profile of such degree-scale extensions can be used to infer the lightcurve of the primary TeV gamma-ray source over the past 1e7 yr, i.e. over a time scale comparable to the life-time of the parent active galactic nucleus. This implies that the degree-scale jet-like GeV emission could be detected not only near known active TeV blazars, but also from TeV blazar remnants, whose central engines were switched off up to ten million years ago. Since the brightness profile of the GeV jets depends on the strength and the structure of the EGMF, their observation provides additionally information about the EGMF.
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Black holes, anywhere in the stellar-mass to supermassive range, are often associated with relativistic jets. Models suggest that jet production may be a universal process common in all black hole systems regardless of their mass. Although in many cases observations support such hypotheses for microquasars and Seyfert galaxies, little is known on whether boosted blazar jets also comply with such universal scaling laws. We use uniquely rich multiwavelength radio light curves from the F-GAMMA program and the most accurate Doppler factors available to date to probe blazar jets in their emission rest frame with unprecedented accuracy. We identify for the first time a strong correlation between the blazar intrinsic broad-band radio luminosity and black hole mass, which extends over $sim$ 9 orders of magnitude down to microquasars scales. Our results reveal the presence of a universal scaling law that bridges the observing and emission rest frames in beamed sources and allows us to effectively constrain jet models. They consequently provide an independent method for estimating the Doppler factor, and for predicting expected radio luminosities of boosted jets operating in systems of intermediate or tens-of-solar mass black holes, immediately applicable to cases as those recently observed by LIGO.
Fermi-LAT spectra at high energies (HE, 0.1-100 GeV) are often extrapolated to very high energies (VHE, >100 GeV) and considered either a good estimate or an upper limit for the blazars intrinsic VHE spectrum. This assumption seems not well justified, neither theoretically nor observationally. Besides being often softer, observations do indicate that spectra at VHE could be also harder than at HE, even when adopting the limit of Gamma=1.5. Results based on such straightforward GeV-TeV extrapolations are in general not reliable, and should be considered with caution.
Several high-frequency peaked BL Lac objects such as Mrk 501 are strong TeV emitters. However, a significant fraction of the TeV gamma rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding electron-positron pairs. Hence, the observed TeV spectrum must be steeper than the intrinsic one. Using the recently derived intrinsic $gamma$-ray spectrum of Mrk 501 during its 1997 high state, we study the inverse-Compton scattering of cosmic microwave photons by the resulting electron-positron pairs, which implies the existence of a hitherto undiscovered GeV emission. The typical duration of the GeV emission is determined by the flaring activity time and the energy-dependent magnetic deflection time. We numerically calculate the scattered photon spectrum for different intergalactic magnetic field (IGMF) strengths, and find a spectral turnover and flare duration at GeV energies which are dependent on the field strength. We also estimate the scattered photon flux in the quiescent state of Mrk 501. The GeV flux levels predicted are consistent with existing EGRET upper limits, and should be detectable above the synchrotron -- self Compton (SSC) component with the {em Gamma-Ray Large Area Space Telescope} ({em GLAST}) for IGMFs $lesssim 10^{-16}$ G, as expected in voids. Such detections would provide constraints on the strength of weak IGMFs.
The extragalactic gamma-ray sky at TeV energies is dominated by blazars, a subclass of accreting super-massive black holes with powerful relativistic outflows directed at us. Only constituting a small fraction of the total power output of black holes, blazars were thought to have a minor impact on the universe at best. As we argue here, the opposite is true and the gamma-ray emission from TeV blazars can be thermalized via beam-plasma instabilities on cosmological scales with order unity efficiency, resulting in a potentially dramatic heating of the low-density intergalactic medium. Here, we review this novel heating mechanism and explore the consequences for the formation of structure in the universe. In particular, we show how it produces an inverted temperature-density relation of the intergalactic medium that is in agreement with observations of the Lyman-alpha forest. This suggests that blazar heating can potentially explain the paucity of dwarf galaxies in galactic halos and voids, and the bimodality of galaxy clusters. This also transforms our understanding of the evolution of blazars, their contribution to the extra-galactic gamma-ray background, and how their individual spectra can be used in constraining intergalactic magnetic fields.
We present Very Long Baseline Array (VLBA) images of 20 TeV blazars (HBLs) not previously well-studied on the parsec scale. Observations were made between August and December 2013, at a frequency of 8.4 GHz. These observations represent the first epoch of a VLBA monitoring campaign on these blazars, and they significantly increase the fraction of TeV HBLs studied with high-resolution imaging. The peak VLBI flux densities of these sources range from about 10 to 100 mJy/beam, and parsec-scale jet structure is detected in all sources. About half of the VLBI cores are resolved, with brightness temperature upper limits of a few times 10^10 K, and we find that a brightness temperature of about 2x10^10 K is consistent with the VLBI data for all but one of the sources. Such brightness temperatures do not require any relativistic beaming to reduce the observed value below commonly invoked intrinsic limits; however, the lack of detection of counter-jets does place a modest limit on the bulk Lorentz factor of greater than about 2. These data are thus consistent with a picture where weak-jet sources like the TeV HBLs develop significant velocity structures on parsec-scales. We also extend consideration to the full sample of TeV HBLs by combining the new VLBI data with VLBI and gamma-ray data from the literature. By comparing measured VLBI and TeV fluxes to samples with intrinsically uncorrelated luminosities generated by Monte Carlo simulations, we find a marginally significant correlation between the VLBI and TeV fluxes for the full TeV HBL sample.
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