LOWER BOUNDS ON MAGNETIC FIELDS IN INTERGALACTIC VOIDS FROM LONG-TERM GeV-TeV LIGHT CURVES OF THE BLAZAR MRK 421

Abstract

Lower bounds are derived on the amplitude B of intergalactic magnetic fields (IGMFs) in the region between Galaxy and the blazar Mrk 421, from constraints on the delayed GeV pair-echo flux that are emitted by secondary e − e + produced in γ γ interactions between primary TeV gamma rays and the cosmic infrared background. The distribution of galaxies mapped by the Sloan Digital Sky Survey shows that this region is dominated by a large intergalactic void. We utilize data from long-term, simultaneous GeV-TeV observations by the Fermi Large Area Telescope and the ARGO-YBJ experiment extending over 850 days. For an assumed value of B, we evaluate the daily GeV pair-echo flux expected from the TeV data, select the dates where this exceeds the Fermi 2σ sensitivity, compute the probability that this flux is excluded by the Fermi data for each date, and then combine the probabilities using the inverse normal method. Consequently, we exclude B < 10 −20.5 G for a field coherence length of 1 kpc at ∼4σ level, as long as plasma instabilities are unimportant for cooling of the pair beam. This is much more significant than the 2σ bounds we obtained previously from observations of Mrk 501, by virtue of more extensive data from the ARGO-YBJ, as well as improved statistical analysis. Compared with most other studies of IGMF bounds, the evidence we present here for a non-zero IGMF is more robust as it does not rely on unproven assumptions on the primary TeV emission during unobserved periods.

Schwinger effect in 4D de Sitter space and constraints on magnetogenesis in the early universe

“…The bounds should be compared with results from recent gamma ray observations, that suggest the existence of intergalactic magnetic fields of strength |B 0 | 10 −15 G, (3.33) when the correlation length is of Mpc scales or larger [28][29][30][31][32][33][34]. If the correlation length λ B is much smaller than a Mpc, the lower bound improves as λ −1/2…”

“…For example, having a field in the action that carries an electric charge of order the elementary charge and mass of order the Hubble scale or smaller, the Schwinger effect prohibits inflationary magnetogenesis from producing magnetic fields larger than 10 −30 G on Mpc scales in the current universe. The bound depends on the charges and masses of the fields in the action, however the Schwinger effect is shown to pose a major challenge for generating magnetic fields as large as 10 −15 G, which is the lower bound on the extragalactic magnetic fields suggested by the recent gamma ray observations [28][29][30][31][32][33][34].…”

Primordial magnetic fields from the post-inflationary universe

“…Focusing on super-horizon modes k aH, the electromagnetic power spectra are 5) which are the same as shown in (3.11), except for the magnetic fields on scales k a 1 H inf . The difference arise due to the super-horizon mode function u k possessing a time evolving component ∝ (aH) −1 which is a (slowly) growing mode in the MD universe, cf.…”

“…The difficulties with inflationary magnetogenesis have already been pointed out in previous studies, such as [17,22]. 5 In particular, the work [22] derived a generic upper bound on H inf without specifying the time evolution of the mode function. Compared to their result, we arrived at a more stringent bound (3.22) because we have focused on the specific case where the coupling I scales as a power-law of a.…”

“…Recent gamma ray observations suggest the existence of magnetic fields even in void regions [1,2,3,4,5,6], and the amplitude of such intergalactic magnetic fields were derived to be stronger than ∼ 10 −15 G. Although this lower bound has astrophysical uncertainties (see e.g. [7,8]), such large-scale magnetic fields hint that the magnetic fields are of cosmological origin.…”