Extreme TeV blazars and the intergalactic magnetic field

Monthly Notices of the Royal Astronomical Society (Impact Factor: 4.9). 07/2011; 414:3566. DOI: 10.1111/j.1365-2966.2011.18657.x
Source: arXiv


We study the four BL Lac objects (RGB J0152+017, 1ES 0229+200, 1ES 0347-121 and PKS 0548-322) detected in the TeV band but not present in the 1FGL catalogue of the Fermi/Large Area Telescope. We analize the 24 months of LAT data deriving gamma-ray fluxes or upper limits that we use to assemble their spectral energy distributions (SED). We model the SEDs with a standard one-zone leptonic model, also including the contribution of the reprocessed radiation in the multi GeV band, emitted by the pairs produced through the conversion of the primary TeV emission by interaction with the cosmic optical-IR background. For simplicity, in the calculation of this component we adopt an analytical approach including some simplifying assumptions, in particular i) the blazar high energy emission is considered on average stable over times of the order of 10^7 years and ii) the observer is exactly on-axis. We compare the physical parameters derived by the emission model with those of other high-energy emitting BL Lacs, confirming that TeV BL Lacs with a rather small GeV flux are characterized by extremely low values of the magnetic field and large values of the electron energies. The comparison between the flux in the GeV band and that expected from the reprocessed TeV emission allows us to confirm and strengthen the lower limit of B >10^{-15} G for the intergalactic magnetic field using a theoretically motivated spectrum for the primary high-energy photons.

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Available from: Giacomo Bonnoli,
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    • "It is generally accepted that magnetic fields are omnipresent in the universe, from stars and planets to the interstellar medium of galaxies [15] [16], to the hot intracluster medium of galaxy clusters [17] [18] [19] and even in the vast cosmic voids [20] [21] [22]. The fundamental question raised by these observations is clearly the origin of the magnetic field. "

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    • "One possible explanation for this observation is a deflection of the electron-positron pairs due to the presence of a diffuse magnetic field (see Neronov & Vovk 2010, Taylor et al. 2011, Tavecchio et al. 2011, Vovk et al. 2012, and Neronov et al. 2013b for details on this explanation and see Broderick et al. 2012 for alternative scenarios). Constraints on the GeV emission provide lower limits on the amplitude of intergalactic fields of the order of 10 −18 –10 −15 G (Tavecchio et al., 2010; Taylor et al., 2011; Dermer et al., 2011; Vovk et al., 2012), if this scenario is correct. "
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    ABSTRACT: We predict and investigate four types of imprint of a stochastic background of primordial magnetic fields (PMFs) on the cosmic microwave background (CMB) anisotropies: the impact of PMFs on the CMB spectra; the effect on CMB polarization induced by Faraday rotation; magnetically-induced non-Gaussianities; and the magnetically-induced breaking of statistical isotropy. Overall, Planck data constrain the amplitude of PMFs to less than a few nanogauss. In particular, individual limits coming from the analysis of the CMB angular power spectra, using the Planck likelihood, are $B_{1\,\mathrm{Mpc}}< 4.4$ nG (where $B_{1\,\mathrm{Mpc}}$ is the comoving field amplitude at a scale of 1 Mpc) at 95% confidence level, assuming zero helicity, and $B_{1\,\mathrm{Mpc}}< 5.6$ nG when we consider a maximally helical field. For nearly scale-invariant PMFs we obtain $B_{1\,\mathrm{Mpc}}<2.1$ nG and $B_{1\,\mathrm{Mpc}}<0.7$ nG if the impact of PMFs on the ionization history of the Universe is included in the analysis. From the analysis of magnetically-induced non-Gaussianity we obtain three different values, corresponding to three applied methods, all below 5 nG. The constraint from the magnetically-induced passive-tensor bispectrum is $B_{1\,\mathrm{Mpc}}< 2.8$ nG. A search for preferred directions in the magnetically-induced passive bispectrum yields $B_{1\,\mathrm{Mpc}}< 4.5$ nG, whereas the the compensated-scalar bispectrum gives $B_{1\,\mathrm{Mpc}}< 3$ nG. The analysis of the Faraday rotation of CMB polarization by PMFs uses the Planck power spectra in $EE$ and $BB$ at 70 GHz and gives $B_{1\,\mathrm{Mpc}}< 1380$ nG. In our final analysis, we consider the harmonic-space correlations produced by Alfv\'en waves, finding no significant evidence for the presence of these waves. Together, these results comprise a comprehensive set of constraints on possible PMFs with Planck data.
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    • "On galactic scales, magnetic fields are observed with a coherence length of a few kiloparsecs and a strength of around 1µG [2] [3] [4] [5], while on galaxy cluster scales similar strength magnetic fields are found with larger coherence lengths, of a few megaparsecs [6] [7] [8]. Recently there have been some exciting observations showing the existence of inter-cluster magnetic fields within voids, with strengths between 10 −17 − 10 −14 G [9] [10] [11] [12] [13]. Despite their importance, surprisingly little is known about the origin of the magnetic fields in our universe. "
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    ABSTRACT: Magnetic fields are present on all scales in the Universe. While we understand the processes which amplify the fields fairly well, we do not have a "natural" mechanism to generate the small initial seed fields. By using fully relativistic cosmological perturbation theory and going beyond the usual confines of linear theory we show analytically how magnetic fields are generated. This is the first analytical calculation of the magnetic field at second order, using gauge-invariant cosmological perturbation theory, and including all the source terms. To this end, we have rederived the full set of governing equations independently. Our results indicate that magnetic fields of the order of $10^{-29}$ G can be generated. This is largely in agreement with previous results that relied upon numerical calculations. These fields are likely too small to act as the primordial seed fields for dynamo mechanisms.
    Journal of Cosmology and Astroparticle Physics 12/2013; 2014(09). DOI:10.1088/1475-7516/2014/09/023 · 5.81 Impact Factor
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