Article

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

ABSTRACT

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 first Fermi-LAT (1FGL) catalogue of the Fermi/Large Area Telescope (LAT). We analyse the 24 months of LAT data deriving γ-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–infrared 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 107 yr 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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    • "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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