Gamma-ray observations of blazars and the intergalactic magnetic field spectrum

Abstract

9 pages, 5 figuresInternational audienceVery-high energy observations of blazars can be used to constrain the strength of the intergalactic magnetic field. A simplifying assumption which is often made is that of a magnetic field of constant strength composed by randomly oriented and identical cells. In this paper, we demonstrate that a more realistic description of the structure of the intergalactic magnetic field is indeed needed. If such a description is adopted, the observational bounds on the field strength are significantly affected in the limit of short field correlation lengths: in particular, they acquire a dependence on the magnetic field power spectrum. In the case of intergalactic magnetic fields which are generated causally, for which the magnetic field large scale spectral index is $n_B\geq 2$ and even, the observational lower bound becomes more constraining by about a factor 3. If instead $-3<n_B<-2$, the lower bound is significantly relaxed. Such magnetic fields with very red spectra can in principle be produced during inflation, but remain up to now speculative

Magnetogenesis during inflation and preheating in the Starobinsky model

“…Another logical possibility is that the observed magnetic fields are of a purely astrophysical origin. The recent multifrequency blazar observation of large-scale magnetic fields in voids [11][12][13][14] with strength not less than 10 −16 G [12] coherent on Mpc scale supports the case for primordial magnetic fields.…”

“…(14). For the rescaled field F (k, τ ) = a 1/2 (τ )A(k, τ ) [here τ is a shifted time, defined before Eq.…”

“…[2][3][4][5][6][7][8]. The upper and lower bounds on the strength of the present large-scale magnetic fields B 0 are given as 10 −17 G B 0 10 −9 G by the observations of the cosmic microwave background (CMB) [9,10] and the gamma rays from blazars [11][12][13][14], respectively. The origin and evolution of these magnetic fields is a subject of intense studies.…”

Simulations of extragalactic magnetic fields and of their observables

“…of detected IC cascade component at ∼ GeV energies is probing the magnetic fields in the intervening cosmic volume (mostly dominated by voids) and/or the filling factor of magnetic fields along the line of sight. From this effect, lower limits of ≥ 10 −16 G on ∼ Mpc scales have been derived Neronov and Vovk, 2010;Dolag et al, 2011;Arlen et al, 2014;Caprini and Gabici, 2015;Chen et al, 2015) The Cherenkov Telescope Array (CTA) will have the potential to further limit large-scale magnetic fields through this effect (Sol et al, 2013;Meyer et al, 2016). Finally, the presence of significant large-scale magnetic fields has been suggested as a possible explanation for the puzzling lack of infrared absorption in the observed spectra of distant blazars (e.g.…”

Sensitivity of the Cherenkov Telescope Array to the Detection of Intergalactic Magnetic Fields

“…The maximum allowed delay time of cascade photons is degenerate with the IGMF strength, and small values of δt only yield conservative limits if small values of the IGMF strength are to be constrained. It should be noted that the cell-like morphology of the IGMF adopted in the ELMAG code neglects the dependence of the limits on the actual IGMF power spectrum (Caprini & Gabici 2015). Especially for red power spectra, the cell-like assumption breaks down and lower limits on the IGMF have to be relaxed.…”